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TL;DR: Whole house ventilation is required in every new Idaho home, and in Idaho it is not conditional the way it is in most of the country. The state deleted the national code’s leak-rate trigger and replaced it with a flat requirement: every dwelling unit gets a whole-house mechanical ventilation system. On a tight mountain house I install a balanced system with a recovery core, and west of the Tetons that core is almost always an ERV rather than an HRV.

Why does a brand-new house even need whole house ventilation?

Because a house built to today’s standards does not leak enough to air itself out. That is the whole answer, and it surprises almost every client I walk through it.

The house your parents grew up in breathed through its own defects. Gaps at the sill plate, uninsulated rim joists, leaky windows, a chimney pulling air up out of the living room all day. It was drafty and expensive to heat, but it was never stuffy. Air changed over constantly whether anybody wanted it to or not.

Then we got good at air sealing. A house I frame and seal today, with a taped sheathing or fluid-applied air barrier and foam at every penetration, will typically test somewhere between 1.5 and 3 air changes per hour at 50 pascals on a blower door. That is tight. It is why the heating bill is low, why the floors are not cold, and why you cannot hear the wind. It is also why the air inside has nowhere to go.

Everything that happens inside a house puts something into the air. Cooking throws off grease, moisture and combustion byproducts. Showers and laundry add water vapor. People exhale carbon dioxide all night in a closed bedroom. New cabinets, new carpet, new paint and new furniture off-gas for months. A dog adds dander. Firewood stacked by the hearth adds mold spores. In a leaky house that load got diluted by accident. In a tight house it accumulates until you deliberately move it out.

That is what whole house ventilation is: a small, continuous, intentional air change that replaces the accidental one we engineered out of the building. The EPA’s own position is that inadequate ventilation raises indoor pollutant levels because not enough outdoor air comes in to dilute what is being emitted inside, and that outdoor air is one of the dilution paths a house needs. You can read their overview of improving indoor air quality if you want the public-health framing.

I will put it in plainer terms. I have walked into two-year-old homes in this valley with condensation running down the windows, a musty smell in the mechanical closet, and a homeowner who thinks the builder did something wrong with the windows. The windows were fine. Nobody gave the house a way to exhale.

Is whole house ventilation actually required by code in Idaho?

Yes, and Idaho is stricter about it than the national code is. This is the single most useful thing in this article, and most people building here have never been told.

The national model code, the 2018 International Residential Code, makes whole-house ventilation conditional. Its Section R303.4 only kicks in where a dwelling tests at 5 air changes per hour or less at 50 pascals. The logic was that a leaky house ventilates itself, so only tight houses need a fan.

Idaho threw that condition out. Under IDAPA 24.39.30, Section 600, the state adopts the 2018 IRC but deletes R303.4 and replaces it with one sentence: “Dwelling units shall be provided with whole-house mechanical ventilation in accordance with Section M1505.4.” No leak-rate trigger. No test result required to make it apply. Every new dwelling unit in Idaho gets a whole-house system, period.

That matters more than it looks, and here is why. Idaho also amended the energy code so that blower-door testing is only required on a 20 percent sample of each builder’s new single-family homes, with the other 80 percent allowed to pass on visual inspection instead. Idaho enforces the 2018 IECC with state amendments effective January 1, 2021, which you can confirm on the Department of Energy’s Idaho code status page. So under the national rule, four out of five Idaho houses would never produce the test number that triggers the ventilation requirement. Idaho closed that loophole by making the requirement unconditional.

The sizing rules themselves come from IRC Section M1505.4, in Chapter 15 of the 2018 IRC. Teton County, Idaho adopted the 2018 I-Codes in March 2021, so that is the book being enforced on a Driggs, Victor or Tetonia permit.

Two practical notes from permit sets I have run through this county. First, “required” does not mean “inspected well.” The inspector will look for the equipment and the controls; nobody is putting a flow hood on every register. Whether the system actually moves the air it was designed to move is on your builder, not the county. Second, a cheap compliance fan satisfies the letter of this rule. So does a balanced recovery system. The code sets a floor, not a target. I will come back to that.

How much fresh air does my house actually need?

Less than people expect. The code rate is a trickle, not a wind.

IRC Equation 15-1 sets the continuous whole-house rate at 0.01 times the floor area in square feet, plus 7.5 cfm for each bedroom plus one, with a 30 cfm floor. The “bedrooms plus one” part is the code’s stand-in for occupancy: a three-bedroom house is assumed to hold four people.

Run that on a real custom home. A 3,500 square foot, four-bedroom house needs 35 cfm for the floor area plus 37.5 cfm for five assumed occupants, which rounds to about 73 cfm continuous. That is roughly the output of one good bathroom fan, moving all day. Here is the code table for the sizes I actually build:

Conditioned floor area	2 bedrooms	3 bedrooms	4 bedrooms	5+ bedrooms
2,001 – 2,500 sq ft	50 cfm	55 cfm	65 cfm	70 cfm
2,501 – 3,000 sq ft	55 cfm	60 cfm	70 cfm	75 cfm
3,001 – 3,500 sq ft	60 cfm	65 cfm	75 cfm	80 cfm
3,501 – 4,000 sq ft	65 cfm	70 cfm	80 cfm	85 cfm
4,001 – 4,500 sq ft	70 cfm	75 cfm	85 cfm	90 cfm
4,501 – 5,000 sq ft	75 cfm	80 cfm	90 cfm	95 cfm

The code also lets the system run intermittently instead of continuously, as long as you multiply the rate by a run-time factor and the system runs at least half of every four-hour block. Run it 50 percent of the time and you double the airflow; run it 75 percent and you multiply by 1.3. I generally do not chase this. A small fan running continuously is quieter, gentler on the house and easier to balance than a big fan cycling on and off.

One thing worth knowing: the engineering world does not agree that the code rate is enough. ASHRAE Standard 62.2 uses the same per-occupant term but triples the floor-area coefficient to 0.03 cfm per square foot. On that 2,500 square foot three-bedroom house, the IRC asks for 55 cfm and 62.2 asks for about 105 cfm. Building Science Corporation’s own review of the two standards argues the higher 62.2 rate often produces worse outcomes in cold and dry climates, because you end up over-drying the house in winter for no measured health benefit. Their recommended winter indoor humidity floor is above 25 percent.

Where I land: size the equipment so it can hit the 62.2 number if you ever want it, then commission it at or slightly above the IRC rate and give the homeowner a boost switch. That is a design decision, not a product decision, and it costs nothing to get right on paper.

What are my three options, and which one do builders really install?

There are exactly three ways to ventilate a house, and the difference between them is which direction the air is pushed.

Exhaust-only runs a quiet fan that pulls air out continuously, usually from a bathroom or a central hallway. Fresh air comes in wherever the house leaks. It is the cheapest way to satisfy the Idaho rule and it is what most production builders install.

Supply-only does the opposite: a small duct brings outdoor air into the return side of the furnace or air handler, and the house pushes stale air out through its leaks. You get filtered, slightly pressurized incoming air, but you are also pulling outdoor air in through a cold duct in winter.

Balanced uses two fans in one box, one pushing out and one pulling in, through a core that transfers heat between the two air streams. That is an HRV or an ERV. You know where the air comes from, you know where it goes, and you keep most of the heat.

	Exhaust-only	Supply-only	Balanced (HRV / ERV)
How fresh air gets in	Through leaks, wherever they are	Through one duct into the furnace return	Through a dedicated, filtered, ducted path
Heat recovered in winter	None	None	Typically 60–90% of the heat
Pressure effect on the house	Negative, pulls air in through walls, crawl space, garage	Positive, pushes moist indoor air into wall cavities	Neutral
Filtration of incoming air	None	Furnace filter only, when the fan runs	Yes, at the unit
Market installed cost (2026 market data, not SwagerBuilds pricing)	A few hundred dollars	Roughly $300–$2,000	Roughly $1,200–$8,000 on new construction
What it does in a tight mountain house	Depressurizes it, the worst habit in cold-climate building	Works, but risks pushing moisture into cold assemblies	The right answer
My verdict	Code minimum only	Acceptable on a modest budget	What I spec

Why I will not put exhaust-only in a house I build here: a negative-pressure house draws its makeup air from wherever the holes are. In Teton Valley that often means the crawl space, where the soil gas lives, or the attached garage, where the cars live. A house that is actively sucking air out of a crawl space is a house working against its own radon mitigation, and one actively pulling from a garage is a carbon-monoxide problem looking for a cold morning. On a house with an engineered crawl space or basement and a sealed envelope, you have spent real money building a controlled box. Depressurizing it on purpose throws that away.

Supply-only is defensible. I have no quarrel with it on a tighter budget. But it pushes warm, moist interior air outward through the wall assembly in winter, which is exactly the direction you do not want moisture moving in climate zone 6. It also leaves you running the big furnace blower to get ventilation, which costs more electricity than a 40-watt dedicated fan.

Balanced ventilation with a recovery core is what belongs in a $1M-and-up custom home in this climate. It is a small line item on a build of that size and it is the difference between a house that merely passes inspection and a house that actually works in February.

HRV or ERV: which one belongs in a Teton Valley house?

An ERV, almost always, and the reason is water, not heat.

Both units do the same basic job. Two air streams pass each other through a core and trade energy without mixing. An HRV, a heat recovery ventilator, transfers only heat. An ERV, an energy recovery ventilator, transfers heat and some moisture.

That moisture transfer is the whole decision. Cold air holds almost no water. When it is 5 degrees outside, the outdoor air you are bringing in is bone dry even at high relative humidity, and once you warm it to 70 degrees indoors its relative humidity collapses into the single digits. Ventilate a tight house all winter with an HRV and you will dry it out: cracking trim, gapping hardwood, static, split lips, instrument cases that will not hold tune. An ERV hands part of the indoor moisture back to the incoming stream, so the house stays in a livable range instead of sliding toward desert conditions.

Allison Bailes makes the same argument in his piece on why you probably need an ERV, not an HRV: ventilating a cold climate without moisture exchange produces very low winter humidity, because cold air is dry air. He also notes that the frost problems that gave early ERVs a bad reputation in very cold places have been engineered out, so the old objection no longer holds.

	HRV (heat recovery ventilator)	ERV (energy recovery ventilator)
What the core transfers	Heat only	Heat plus some moisture
Winter effect on indoor humidity	Drops it, often hard	Holds it closer to a livable range
Best fit	Houses that run too humid in winter: high occupancy, indoor pool, lots of plants	Cold, dry climates: Teton Valley, Rigby, Idaho Falls, Jackson
Summer behavior here	Fine	Fine, and limits humid-day moisture gain
Frost control in deep cold	Mature	Mature on current equipment
Cost difference	Baseline	Usually a small premium
What I spec west of the Tetons	Only for a specific humidity problem	Default

The exception is real, so I will name it. If you are putting six or seven people in a 2,400 square foot airtight house, or you want an indoor hot tub, or somebody keeps forty houseplants, the house makes its own moisture and an HRV’s drying effect becomes a feature. Some units take a swappable core so you can run an ERV core in summer and an HRV core in winter. I have not needed to do that here yet, but it exists.

What I will not do is let a mechanical sub pick this by habit. “We always use HRVs” is not a climate analysis. On our builds the ventilation strategy gets decided during design, alongside the heating system and the insulation assemblies, because all three interact.

Where does the equipment go, and how much room does it take?

A residential unit is roughly the size of a carry-on suitcase, hung from joists or wall-mounted. The box is not the problem. The ducting is.

A balanced system needs four duct paths: an intake from outdoors, an exhaust to outdoors, a supply distribution to the rooms where people sleep and sit, and a return pickup from the rooms that make the stale air. On top of that it needs a condensate drain, because a recovery core makes water.

Here is what I want in a set of plans before framing starts:

• A dedicated mechanical room or closet with real service clearance, not a unit buried above a soffit where nobody will ever change a filter.
• Exterior intake and exhaust hoods at least ten feet apart, so you are not re-breathing your own exhaust, and placed where drifting snow will not bury them. In this valley that means figuring snow accumulation into the elevation, not just the gable.
• Intake away from the driveway, the garage door, the dryer vent and the water heater flue.
• Short, straight runs in smooth metal duct. Flex duct crushed around a truss is how a 75 cfm system becomes a 30 cfm system.
• Insulated and vapor-sealed intake and exhaust ducts inside the conditioned space, or they will sweat and drip in January.
• A condensate path to a drain, with a trap and freeze protection.
• Supply into bedrooms and the main living area; return from bathrooms, laundry and the kitchen area, but not inside the range hood’s territory.

That list is why whole house ventilation is a design-phase item and not a purchase. The hardest ventilation retrofits I have been asked to look at were not hard because of the equipment. They were hard because the house was already finished and there was no honest path from the mechanical closet to the far bedrooms. On our projects the drawing set, the structural coordination and the mechanical layout all come out of the design agreement, so the duct path and the truss layout get resolved on paper by the same people instead of being fought out in the field.

What does a whole house ventilation system cost, and what is not in that number?

Third-party 2026 market data puts a residential HRV or ERV unit at roughly $700 to $2,500 for the equipment alone in the 50 to 200 cfm range, and roughly $1,200 to $6,000 installed on a house under 3,000 square feet, with complex jobs or all-new ductwork running $8,000 to $12,000. A simple fresh-air intake with an inline fan and damper comes in around $300 to $2,000 installed. Those are market figures from HVAC cost surveys, not SwagerBuilds numbers.

Now the part nobody prints. Here is what is usually not in a quoted ventilation price:

• The ducting itself, if the quote is equipment and hookup only. On a sprawling single-level floor plan the duct run is the expensive half.
• Exterior penetrations and hoods, flashed and sealed into the air barrier properly. Two more holes in a weather-resistive barrier that has to still work in twenty years.
• Commissioning, meaning somebody actually measures and balances the airflow instead of trusting the nameplate. This is the line item most often skipped and the one that decides whether the system works.
• The condensate drain and its freeze protection.
• Electrical: a dedicated circuit and a controller location somebody will actually use.
• Filters, forever. Budget for two sets a year. A clogged filter is the number one reason a working system quietly stops working.
• Range hood makeup air, if the kitchen hood is big. See below, because this one bites.

In the context of a custom home here, ventilation is a rounding error on the build and one of the highest-return dollars in the mechanical package. SwagerBuilds custom homes start at $400 per square foot and go up on the Idaho side of the pass, and start at $1,000 per square foot and go up in Jackson Hole, quoted on construction cost and conditioned square footage. Against that, a properly ducted and commissioned balanced system is a small fraction of one percent of the job. I have never had a client regret it. I have had clients regret a $90,000 kitchen in a house that smells like last night’s salmon at breakfast.

If you want the fuller cost picture, I lay out what drives the number on our cost per square foot to build in Teton Valley page, and the things that surprise people on energy efficient features worth paying for.

Will running it wreck my heating bill?

No, and a balanced system with a recovery core is cheaper to run than the alternatives, which is the part that gets lost.

Think about what you are paying for. Ventilation costs you two things: the electricity to turn the fans, and the heat that leaves with the outgoing air. A good residential ERV moving 75 cfm draws roughly as much power as a couple of LED bulbs. The heat is the real money, and that is precisely what the core is for: current equipment recovers something like 60 to 90 percent of it, depending on the unit and the conditions.

Compare that to exhaust-only. Every cubic foot you push out gets replaced by outdoor air at outdoor temperature, and you heat 100 percent of it from scratch. Running 75 cfm of 5-degree air up to 70 degrees, all winter, is not free. Exhaust-only is the cheap option to install and the expensive option to own.

There is a second-order benefit on our builds. Because the ventilation is ducted and balanced, I can size the heating equipment to the real load instead of padding it to cover a mystery. That matters whether you end up with a heat pump or a furnace, and it matters on a radiant floor house, where the slab is slow and a ventilation load you did not account for shows up as a room that never quite catches up.

One honest caveat: a recovery core only recovers when it is clean and when the airflows are actually balanced. An ERV with a plugged filter and an 80 cfm supply against a 40 cfm exhaust is pressurizing your house and recovering very little. That is a commissioning and maintenance problem, not an equipment problem, and it is why I put the filter schedule in the homeowner packet at closeout.

What about the range hood, the bath fans and the dryer?

They are a different job, and confusing the two is the most common mistake I see in mechanical plans.

Whole house ventilation is the slow, continuous background air change. Local exhaust is the fast, targeted removal of a specific mess at a specific moment. The code treats them separately, and so should your plan set. Under IRC Table M1505.4.4(1), kitchens need 100 cfm intermittent or 30 cfm continuous, and bathrooms need 50 cfm intermittent or 20 cfm continuous. Your 70 cfm ERV does not satisfy the bathroom requirement, and your bath fan does not satisfy the whole-house requirement.

Then there is the range hood, which is where tight houses get dangerous. Everybody wants the 1,200 cfm professional hood. In a house that tests at 1.5 ACH50, a 1,200 cfm hood cannot find 1,200 cfm of makeup air, so it depressurizes the house hard and starts pulling air down the nearest available chimney or flue. IRC Section M1503.6 requires makeup air for hoods exhausting more than 400 cfm, and the real-world trigger for danger is any naturally drafted combustion appliance inside the envelope: an atmospheric water heater, a gas fireplace, a wood stove.

How I handle it: either a powered makeup-air damper interlocked with the hood, so opening the hood opens a tempered air path, or a mechanical package with no atmospheric-draft appliances in the house at all, meaning direct-vent or sealed-combustion everything. The second is cleaner and is what I spec on new work. It also plays well with the fact that Teton County has no piped natural gas, so everything gas here is propane and worth treating with the same care.

Two more small things that bite. The clothes dryer is a 150 to 200 cfm exhaust fan that nobody counts, and it runs during the exact hours the house is most occupied. And a bath fan on a timer is not a whole-house system no matter how long the timer runs, because it is still just depressurizing the house without a controlled inlet. Inspectors sometimes accept it. I do not spec it.

Does ventilation fix radon, wildfire smoke or a musty crawl space?

Partly, and the honest answer is different for each one. People try to make ventilation do three jobs it was not designed for.

Radon: no. Diluting soil gas after it is already in the house is a losing strategy, and an exhaust-only system actively makes it worse by pulling harder on the soil under the slab. Radon gets solved under the slab with a passive or active sub-slab depressurization system, which is the whole point of putting the rough-in in during foundation work. A balanced ERV does help a little, because it does not depressurize, but the radon system is the fix and ventilation is not a substitute.

Wildfire smoke: yes, if you planned for it. This is a real summer issue here, and a balanced system is the only one of the three strategies that gives you a usable answer. Because the incoming air comes through one known duct at one known filter, you can put real filtration on it and you can shut the intake on a bad smoke day while keeping the house slightly positive. A leaky house with an exhaust fan has no such option. If you are already thinking about wildfire resistant construction, treat the ventilation intake as part of that package, not as an afterthought.

Musty crawl space or basement smell: no, and stop trying. That smell is a moisture problem with a source. Ventilating the house just moves crawl space air through your living room on the way out. Fix the source: ground vapor barrier, sealed and conditioned crawl, drainage, roof water taken away from the foundation. I cover the assembly choices in the basement versus crawl space versus slab breakdown.

Same logic applies to the one people never ask about: ventilation does nothing for hard water or for ice dams, even though both get blamed on “air” at some point in every build. Dilution is not a substitute for source control. That is Building Science Corporation’s framing and it is right.

What goes wrong when this gets done badly?

I would rather tell you the failure modes than the brochure. Every one of these is something I have seen or been called in to look at.

• The compliance fan. A bath fan wired to run continuously, nothing else. Satisfies the inspector, depressurizes the house, pulls crawl space and garage air into the living space, recovers no heat.
• Never commissioned. The unit is rated 90 cfm and delivers 38 because of flex duct, two hard elbows and a crushed run. Nobody measured, so nobody knows.
• Out of balance. Supply exceeds exhaust by 40 percent, so the house runs positive and pushes humid interior air into cold wall cavities all winter. This shows up as a moisture stain two years later and gets blamed on the siding.
• Intake in the wrong place. Six feet from the dryer vent, or under a snow drift, or next to where the truck idles on a January morning.
• No condensate plan. The core makes water, the water has nowhere to go, and in a cold mechanical space the drain freezes.
• Controls nobody understands. A unit with a dial in an attic nobody visits. If the homeowner cannot find the boost switch, the system is decoration.
• Filters never changed. The quiet killer. A plugged filter turns a well-designed balanced system into an expensive paperweight.
• A giant range hood with no makeup air. Covered above. This is the one that can actually hurt somebody.

Notice how few of those are equipment failures. Almost all of them are design and verification failures, which is exactly why I treat ventilation as a drawing-set item and a closeout-measurement item rather than a box on a materials list. The same thinking applies to the inspections that happen on your build: passing is the floor, not the goal.

When do we decide this on a SwagerBuilds build?

During design, before anything is framed. Not at mechanical rough-in, and definitely not at drywall.

The sequence on our projects looks like this. We lock scope, site conditions and finish direction in the design agreement. The drawing set gets developed with the mechanical layout coordinated against the structural layout, so the duct paths and the truss and joist layout are reconciled on paper. Our structural, civil and mechanical engineering relationships get looped in where the build needs them, which on a Teton Valley or Jackson Hole house it almost always does, because stamped snow-load engineering and setbacks drive the schedule. Interior selections, millwork from our Rigby shop, lighting and tile get sequenced so they do not blow the schedule. Then we bid and sign a fixed construction price.

Clients are sometimes surprised that all of that is in-house or handled through people I have worked with for twenty years. You are not supposed to go assemble an architect, an engineer, a mechanical designer and an interior designer yourself and then bring me the pieces. Design is sold as a fixed design fee, quoted per project, and if you want the detail on what that phase actually produces, I wrote it up in the design phase and the real number before signing.

I am involved in every phase of every build, and there are 24/7 site cameras on every job, so when I say the ventilation intake got placed where the drawing said, I can show you. If you are planning a custom home in Teton Valley, Driggs, Victor, Tetonia, Rigby, Idaho Falls or Jackson Hole and you want this decided properly instead of discovered later, call me at (208) 520-0636 or start with how we build in Driggs.

Built different. Built right.

Frequently asked questions about whole house ventilation

Is whole house ventilation required in Idaho?

Yes. Idaho deletes the 2018 IRC’s conditional Section R303.4 and replaces it under IDAPA 24.39.30, Section 600 with a flat requirement that dwelling units be provided with whole-house mechanical ventilation per IRC Section M1505.4. Unlike the national code, there is no blower-door threshold that has to be met before the requirement applies. Every new Idaho dwelling needs a system.

Should I get an HRV or an ERV in a cold climate?

In a cold, dry climate like Teton Valley or Eastern Idaho, an ERV is the better default. It transfers some moisture along with the heat, so it does not dry the house out in winter the way an HRV does. Choose an HRV only if the house has a specific excess-humidity load, such as high occupancy or an indoor pool. Frost control on current ERV equipment is mature.

How many CFM does my house need for whole house ventilation?

Under IRC Equation 15-1, the continuous rate is 0.01 times the conditioned floor area, plus 7.5 cfm per bedroom plus one, with a 30 cfm minimum. A 3,500 square foot four-bedroom house works out to about 73 cfm. ASHRAE 62.2 would ask for roughly three times the floor-area portion, so size equipment with headroom and commission at or just above the code rate.

Can a bathroom exhaust fan satisfy the whole-house requirement?

It can satisfy an inspector, but it is not a system I will spec. A continuous bath fan depressurizes the house, which pulls makeup air out of the crawl space, the attic and the attached garage, and recovers none of the heat that leaves. Local exhaust and whole-house ventilation are separate code requirements with separate airflow rates.

Does a whole house ventilation system increase my heating bill?

A balanced system with a recovery core is the cheapest of the three strategies to operate. The fans draw very little power and the core recovers roughly 60 to 90 percent of the heat leaving the house. Exhaust-only is the opposite: it is the cheapest to install and the most expensive to run, because every cubic foot of outgoing air is replaced by outdoor air you have to heat from scratch.

TL;DR: Most people building here do need a water softener, and in a lot of Idaho new homes the code already makes me plumb for one. City water in Idaho Falls tests at 14 grains per gallon, which is “very hard” on the standard scale. Teton Valley wells run all over the map, so you test first and size second. Budget the softener, then test separately for arsenic, nitrate, uranium and bacteria, because a softener does not touch any of those.

I have been in construction about twenty years, and water treatment is one of the few line items where I have watched the same conversation go badly over and over. A client gets a bid, sees a few thousand dollars for equipment nobody can see, and cuts it. Two winters later the glass shower door is etched, the dishwasher is spotting everything, and the tankless water heater they paid a premium for is running worse than the day it went in.

So here is the straight version. What the numbers mean, what the code already requires, what a softener fixes, what it absolutely does not fix, and what I do on my own builds.

How hard is the water where I’m building?

You do not know until you test. That is not me dodging the question, it is the whole problem with water in this part of Idaho.

Hardness is just dissolved calcium and magnesium, measured either in milligrams per liter or in grains per gallon. One grain per gallon is about 17 milligrams per liter. The scale everybody works from looks like this, and the Department of Energy’s Pacific Northwest National Laboratory uses the same breakpoints in its report on water quality and residential water heating:

Classification	Milligrams per liter	Grains per gallon	What you actually notice
Soft	0–60	Under 3.5	Soap lathers easily, no scale, nothing to fix
Moderately hard	61–120	3.5–7	Faint spotting on glass, slow scale in a water heater
Hard	121–180	7–10.5	Visible spotting, soap scum, scale on fixtures
Very hard	Over 180	Over 10.5	Crusted aerators, etched glass, appliances aging fast

For a reference point on the valley floor, the City of Idaho Falls publishes its own hardness number in its water quality report: 14 grains per gallon, which works out to roughly 240 milligrams per liter. That is very hard water, on a municipal system, out of a treated public supply. Rigby, Ammon and Rexburg all draw from the same basalt and carbonate geology of the Eastern Snake River Plain, so nobody building around Rigby or Idaho Falls should be surprised by a hard-water result.

Teton Valley is a different animal. Driggs, Victor and Tetonia sit on glacial and alluvial fill, and most of the homes I build out there are on a private well rather than city water. Hardness on a private well is a property-by-property number. I have seen two lots within a mile of each other come back with meaningfully different water. Which is exactly why I will not quote a treatment package off a map.

One practical wrinkle worth knowing before you call a lab: the labs that serve the Idaho side of Teton Valley mostly run the health panel — bacteria, nitrate, arsenic, fluoride, uranium — and hardness is not automatically in it. The well water testing guide published by Friends of the Teton River lists the regional labs, and the one that routinely reports total hardness is on the Wyoming side and does not serve Idaho residents. So when you order your test, ask for hardness, iron and manganese by name. Otherwise you will get a clean health panel back and still have no idea how to size equipment.

Do I need a water softener, or is that just an upsell?

You need a water softener if your tested hardness is above roughly 7 grains per gallon, and you almost certainly want one above 10. Below about 5 grains I would not spend the money. Between 5 and 7 it is a comfort call, not a damage call.

That threshold is not something I made up to sell equipment. The PNNL report notes that softening or other treatment is commonly recommended above about 140 milligrams per liter, which is a hair over 8 grains per gallon. New Hampshire’s environmental agency, in its published guidance on iron and manganese in drinking water, recommends softeners when hardness runs over 150 milligrams per liter and there are aesthetic problems to go with it. Two independent sources, both landing in the same band.

Here is the honest part most water-treatment salespeople leave out: hardness is not a health problem. There is no federal limit on it, and the Environmental Protection Agency does not regulate it as a contaminant. Hard water will not hurt you. It will cost you money in appliances, water heaters, soap and plumbing fixtures, and it will make your brand-new tile and glass look old before its time. That is a maintenance-and-money argument, and it is a good one. It is not a safety argument, and I do not pitch it as one.

What changes the math is the rest of the house. If you are putting in a tankless water heater, hardness stops being a comfort question and becomes an equipment-protection question. Same with radiant floor heat, high-end plumbing fixtures, a steam shower, or any of the other things that show up on a custom home’s selection list. The more you spend downstream, the less sense it makes to skip the water softener upstream.

What does hard water actually do to a house?

It builds scale on anything that heats water, and it leaves mineral deposits on everything water touches and dries on. The second part is cosmetic. The first part costs real money, and there is good data on it.

From the PNNL work for the Department of Energy:

• A gas tankless water heater in hard water dropped from 80 percent efficiency to 72 percent in about 1.6 years. Deliming brought it back to 77 percent — better, but not all the way back.
• A gas storage tank tested at roughly 444 milligrams per liter fell from 70.4 percent to 67.4 percent over the equivalent of two years, and the literature shows losses as high as 10 percent in extreme cases.
• Scale thickness and efficiency loss track together almost linearly. A layer six hundredths of an inch thick on a heat exchanger cut efficiency by about 11 percent on average.
• At 170 milligrams per liter, roughly ten grains, that much scale can accumulate in about four years at a 140-degree setpoint — and take almost thirty years at 120 degrees. Tank temperature matters enormously.
• Citing New Mexico State University, the report puts the hit to appliance life at 25 to 40 percent.
• In extremely hard water, tankless units can fail in under two years without frequent and expensive deliming.

The nuance I appreciate in that report, and that I pass along to clients, is that it does not recommend a softener in every case. For an electric storage water heater it says a softener is not needed for efficiency reasons, because the elements keep working until sediment buries the bottom one. For a gas storage tank it calls a softener close to a last resort, since softened water may shorten tank life. For tankless, it says a softener may be the only viable option in hard water. That is a more useful answer than “soften everything,” and it maps directly onto which water heater you picked.

On the cosmetic side: etched shower glass is permanent. Once hard water has pitted the surface, no amount of cleaning brings it back, and the glass gets replaced. If you are spending real money on tile and glass in a master bath, that alone justifies the equipment.

Does Idaho code require a water softener loop in a new home?

In a lot of new homes, yes. This is the part almost nobody knows, and it is the single best reason to decide about a water softener during design rather than after drywall.

Idaho adopts the 2015 Uniform Plumbing Code as the Idaho State Plumbing Code, with state amendments, under IDAPA 24.39.20. One of those amendments, Section 610.2, requires that new one- and two-family homes built slab-on-grade, or with a finished basement at final inspection, have a pre-plumbed water softener loop. The same amendment is specific about the kitchen: one hot soft line, one cold soft line, and one cold hard line at the sink, so you can drink and cook with unsoftened water. And outdoor cold hose bibbs used for irrigation have to be piped with hard water, which is the right call anyway — there is no reason to run softened water onto a lawn.

There are a couple of related items in the same rule set. Section 805.41 says softener discharge has to terminate at an approved location and the drain line has to be at least three-quarters of an inch, and it allows a dual-outlet washer box if it is on the same floor as the softener or one floor below. And a footnote to Table 611.4 says a softener serving more than four bathroom groups gets sized to the manufacturer’s standards rather than the table.

Practically, that means the decision point is on the plans. The loop, the drain, the clearances and the hard-water branch to the kitchen and the hose bibbs all get drawn before anyone pours. We handle plan development and the engineering coordination in house, so this is a line on a sheet during design, not a change order during rough-in. If you are reading this with a set of plans from somewhere else, check them for the loop. I have seen plenty that do not have it.

Salt-based softener or salt-free conditioner: what’s the difference?

A salt-based water softener removes hardness. A salt-free conditioner does not remove anything — it changes the minerals so they are less likely to stick as scale. Both can be the right answer. They are not the same product and they should not be priced against each other as if they were.

A salt-based unit is ion exchange. Water passes through a resin bed that trades calcium and magnesium for sodium. When the bed fills up, the unit regenerates with a brine rinse and flushes the hardness down a drain. Your test result goes from 14 grains to effectively zero, and you get every benefit: no scale, no spotting, soap works, appliances last.

A salt-free conditioner, usually template-assisted crystallization, pulls hardness minerals out of solution into tiny crystals that pass through without plating onto a heat exchanger. Your hardness number does not change. You still measure 14 grains at the tap. What you get is a lot less scale. What you do not get is the slick feel, the soap performance, or the end of spotting on glass.

	Salt-based water softener	Salt-free conditioner	Do nothing
Removes hardness	Yes, to near zero	No — same test result	No
Stops scale in a tankless heater	Yes	Mostly	No
Ends spotting on glass and tile	Yes	No	No
Soap and detergent use drops	Yes	No	No
Adds sodium to the water	Yes, small amount	No	No
Needs a drain	Yes, 3/4 in. minimum	No	No
Ongoing cost	Salt plus regeneration water	Media change every few years	None up front, plenty later
Brine to septic question	Yes — see below	No	No
Works with high iron	Poorly — needs a filter ahead of it	Poorly	—

Where I land: if the house is on a septic system and hardness is moderate, a salt-free conditioner is a legitimate choice and it sidesteps the brine problem entirely. If hardness is above 10 grains, or the client cares about how the water feels and how the glass looks, it is a salt-based water softener and we solve the drain question properly.

One more thing on the salt itself. The Water Quality Association’s best-practice guide says a well-set unit should remove about 4,000 grains of hardness per pound of salt, and that under the NSF/ANSI 44 standard regeneration should use no more than five gallons of water per 1,000 grains removed. It also says to buy demand-initiated regeneration — a unit that regenerates based on actual water use — rather than a time-clock model that cycles on a schedule whether you used water or not. On a well, with a septic system downstream, that difference matters. Ask for it by name.

What else is in well water besides hardness?

This is where a water softener conversation usually needs to get bigger. A softener is a hardness appliance. It does not remove arsenic, nitrate, uranium or bacteria, and in most cases those are the only things in your water that are actually a health question.

The testing schedule that Idaho’s public health guidance recommends for private wells, as laid out in that Friends of the Teton River guide, is worth writing on the inside of a cabinet door:

What to test for	How often	Treat at	What actually fixes it
Bacteria (total coliform, E. coli)	Every year	Any detection	Shock chlorination, then UV or chlorination; find the source
Nitrate	Every year	10 mg/L or higher	Reverse osmosis, anion exchange, or a different source
Nitrite	Every year	1 mg/L or higher	Same as nitrate
Arsenic	Every 3–5 years	0.01 mg/L or higher	Adsorptive media, anion exchange, or reverse osmosis
Uranium	Every 3–5 years	30 µg/L or higher	Anion exchange or reverse osmosis
Fluoride	Every 3–5 years	4 mg/L or higher	Activated alumina or reverse osmosis
Hardness	Once, before you build	About 7 gr/gal	Water softener or conditioner
Iron	Once, before you build	0.3 mg/L (EPA aesthetic limit)	Oxidation filter ahead of the softener
Manganese	Once, before you build	0.05 mg/L (EPA aesthetic limit)	Oxidation filter with a strong pre-oxidant

A few local notes on that list. Parts of Teton County have elevated nitrate, tied to septic systems, fertilizer and animal waste, and pockets of arsenic and fluoride show up around Idaho because of local geology. None of that is a reason to panic. It is a reason to test before you design the mechanical room, so the equipment you buy matches the water you actually have.

Iron and manganese deserve their own warning, because this is the most common way a water softener install goes sideways. Iron and manganese are covered by the EPA’s secondary drinking water standards — 0.3 milligrams per liter for iron and 0.05 for manganese. Those are aesthetic guidelines, not enforceable limits, and they are set where staining and metallic taste start. Iron stains fixtures orange. Manganese stains them black.

Here is the trap: a water softener will strip some iron out of the water, so a salesperson can tell you it handles it. New Hampshire’s guidance is blunt that softeners are “highly inefficient” at removing iron and manganese, and recommends an oxidation filter instead — with a strong pre-oxidant like chlorine or potassium permanganate if manganese is present, and simple air injection if it is iron only. The Water Quality Association’s efficiency numbers assume source water under 2 parts per million iron and under 750 parts per million total dissolved solids, and warns that hardness leakage shows up above 30 grains per gallon, above 750 TDS, or above 2 ppm iron. Translation: if you have real iron, you put a filter in front of the softener. You do not ask the softener to do both jobs.

Do I need whole-house filtration too, or just a water softener?

It depends entirely on your test results, and that is the only honest answer. Here is how I sequence it.

Sediment first. Any well gets a sediment filter at the point of entry. It is cheap, it protects everything behind it, and on a new well it earns its keep in the first year while the formation settles down.

Iron and manganese second, if the test calls for it. Oxidation filter, sized to the flow rate of the house, ahead of the softener.

Softening third. Clean water into the resin bed means the resin lasts and the salt efficiency numbers hold up.

Point-of-use last, for the health contaminants. If arsenic, nitrate or uranium came back above the treatment thresholds, the fix is usually reverse osmosis at the kitchen sink, or a whole-house adsorptive or anion-exchange system if the levels warrant it. For most houses, treating the two or three taps people actually drink from is more sensible than treating every gallon that flushes a toilet.

What I will not do is sell a client a whole-house filtration package before the lab report comes back. I have seen “whole home purification systems” quoted at five figures on water that needed a softener and a sediment cartridge. Test, then buy.

Can water softener brine go into my septic system?

Not by default in Idaho, and this is a real design constraint on a rural build, not a technicality.

Idaho’s subsurface sewage disposal rules, IDAPA 58.01.03, Section 004.03, list water softener brine along with backwash and backflush water among the substances that cannot be discharged into a septic system unless that discharge is approved by the Director. The same rule set, in Section 004.04, makes it unlawful to add wastewater sources that push the system past its approved design flow. So the two questions you have to answer before you buy a salt-based water softener on a septic lot are: is the brine discharge approved, and is the system sized for the extra volume?

There are four ways this typically gets resolved, and we work through them during design:

1. Get the discharge approved and size the system for it. That means the softener volume is in the permit application from the start, which Section 005.04 requires you to document anyway.
2. Use a high-efficiency, demand-initiated softener so the discharge volume is as small as it can be. The WQA’s five-gallons-per-1,000-grains benchmark is the number to hold a vendor to.
3. Discharge somewhere else if the health district will approve an alternative termination point under Section 805.41 of the plumbing code.
4. Go salt-free. A conditioner has no brine, no drain and no septic conversation. On a marginal lot, that simplicity is worth a lot.

While we are in the same rule book: a well has to be 100 feet from the drainfield and from the septic tank’s own required separations, and larger systems need more. If you are still shopping lots, the interaction between well placement, drainfield and setbacks is one of the first things I look at — see well and septic costs in rural Idaho and how to prepare land to build a house.

What does a water softener cost in 2026?

Market data, not my rate card: national cost guides put an installed residential water softener at roughly $1,200 to $3,800 with an average around $2,500, with the outer edges running from about $800 to $8,000 depending on capacity and brand. By type, the same 2026 HomeGuide market cost guide lists salt-based ion exchange at $1,200 to $3,800 installed, salt-free conditioners at $1,500 to $4,500, dual-tank ion exchange at $1,700 to $5,000, and whole-house reverse osmosis at $4,800 to $10,000. Salt or potassium runs $50 to $150 a year.

Now the part those guides leave out, which is the question every client asks next: what is not in that number?

• The water test. A full panel through a regional lab, and the sample bottles, before anything gets specified.
• An iron or sediment filter, if the test calls for one. That is separate equipment with its own footprint and its own backwash.
• Point-of-use reverse osmosis for arsenic, nitrate or uranium. Separate again, and it is the one people forget when they assume a softener covers health contaminants.
• The rough-in, if the plans did not include it. Adding a loop, a 3/4-inch drain and a dedicated receptacle after the slab is poured or the basement is finished is the expensive way to do this.
• Septic permitting if brine discharge has to be approved and the system sized for it.
• Floor space, which is free on the plans and costs real money later.

On my builds the equipment, the rough-in and the test all sit inside the construction budget as their own line, not buried in a plumbing allowance where it can quietly vanish. Vague allowances are how clients end up surprised — which is the same pattern behind most of the hidden costs of building a custom home and most construction allowance fights. If you want to see how this fits a whole-project budget, start with what a custom home costs in Idaho.

For what it is worth on return: a water softener is not an investment that pays you back in resale. It pays you back in water heater efficiency, appliance life, and not replacing etched shower glass. Compare that against the rest of the list in which custom home upgrades are actually worth the money and it holds up better than most.

Where does the equipment go, and what do you rough in?

The mechanical room, and the time to figure out how big it needs to be is during design — not when the plumber shows up with a 48-inch-tall brine tank and nowhere to set it.

What a full treatment train needs:

• Floor space. A softener plus a brine tank is roughly a 2-by-4-foot footprint. Add an iron filter and you are closer to 2 by 6. Leave room to pull the resin tank.
• A drain, three-quarters of an inch minimum per Section 805.41, terminating at an approved location. On a basement install that often means a floor drain or an ejector pit; on a slab it means planning the termination before the pour.
• A power receptacle within reach of the control head.
• The loop and the bypass, so the house can run on hard water if the unit is down or being serviced.
• The hard-water branches — cold hard to the kitchen sink and to every irrigation hose bibb, per Section 610.2.
• Salt access. Somebody carries 40-pound bags to this room. If it is at the bottom of a basement stair at the far end of the house, that gets old. I try to put it near the garage door.
• Conditioned space. Equipment and brine do not belong anywhere that can freeze, which rules out an unheated garage unless you are heating it. That ties into whether to heat the garage at all.

How the foundation choice plays in: a full basement gives you the easiest answer, because the floor drain is usually already there and the space is cheap. A crawl space is the hardest, because the equipment has to live somewhere on the main floor and that is square footage you are paying for. Slab-on-grade puts the loop and the drain termination on the critical path before the pour. If you are still deciding, that trade-off is laid out in basement vs crawl space vs slab.

And the sequencing point I will keep hammering: the plumbing rough-in inspection is where a missing softener loop gets caught, and by then the framing is up. The inspection schedule is not the place to discover a design decision nobody made.

How do you size a water softener?

Daily grain load. You multiply the people in the house by the gallons each one uses, then multiply by your tested hardness. The WQA’s guide uses a household of four at about 200 gallons a day as its sizing starting point, so 50 gallons per person per day is a reasonable planning number for a custom home with efficient fixtures.

Run it for a four-person house on Idaho Falls-grade water at 14 grains per gallon: 200 gallons times 14 grains is 2,800 grains a day. You want the unit to regenerate every three to seven days rather than nightly, so you are shopping somewhere around a 24,000 to 32,000-grain unit, then you check it against the flow rate the house needs rather than just the capacity.

Flow rate is the thing people get wrong. A big house with two or three full baths, a soaking tub, multiple showerheads and a laundry running at once can pull more gallons per minute than a small softener can pass without a pressure drop you will feel. That is exactly what the Idaho amendment to Table 611.4 is getting at when it says a softener serving more than four bathroom groups gets sized to the manufacturer’s standards instead of the code table. On a bigger custom home, size for peak flow first and capacity second.

Two more sizing realities worth knowing before you sign a proposal:

• If your hardness is above 30 grains per gallon, or total dissolved solids is over 750, or iron is over 2 parts per million, the WQA notes you should expect some hardness leakage through the bed. That is an argument for pretreatment, not for a bigger softener.
• If you use potassium chloride instead of sodium chloride — the usual reason being a sodium-restricted diet — plan on 13 to 15 percent higher concentration to regenerate, and watch for salt bridging in the tank. It works. It costs more and needs more attention.

What I do on a SwagerBuilds build

Here is the actual sequence, and it is the same every time.

During the design agreement, before any construction price is signed, we pull a water test. On a well lot that is a full panel: bacteria, nitrate, nitrite, arsenic, uranium, fluoride, plus hardness, iron and manganese called out by name. On a city lot I pull the municipality’s own water quality report and we still test at the tap if anything looks off. The treatment approach is a design decision with a dollar figure attached, and I would rather have it on the table in the design phase than in a change order.

On the plans, the mechanical room gets drawn to fit the equipment the test says we need, with the softener loop, the 3/4-inch drain and its termination, the receptacle, the bypass, the cold hard branch to the kitchen, and hard water to the irrigation bibbs. We do the plan development and the drawings in house, and we coordinate the engineering, so a change here is a revision, not a six-week wait on somebody else’s calendar.

At rough-in, the loop gets plumbed and verified before insulation and drywall. On a slab, the drain termination is settled before the pour.

At the septic permit, if the lot is on a septic system and the client wants salt-based, the brine volume goes in the permit application and the system is sized for it — or we specify demand-initiated regeneration to hold the volume down, or we go salt-free. We decide that in design, not after the system is installed.

At closeout, the client gets the lab report, the equipment manuals, the actual settings, and the retest schedule on paper. A softener that nobody salts and nobody retests is a decoration. Everything I build carries a one-year SwagerBuilds workmanship warranty on top of standard Idaho coverage, and we have 24/7 cameras on every job so you can see the rough-in happen whether you are in Victor or in California.

I am involved in every phase of every build, which is why I care about a two-thousand-dollar appliance in a room nobody tours. It is the thing that decides whether the rest of the house looks new in ten years.

Thinking about a build in Teton Valley, Jackson Hole or Eastern Idaho? Bring me the lot and I will tell you what I would test for before you design the mechanical room. Start with how I build, or go straight to the custom homes page and reach out. Design is sold as a design agreement with a fixed design fee, quoted per project — no guessing, and no numbers before we know what we are building.

Frequently asked questions

Does a water softener remove arsenic or nitrate?

No. A water softener is a hardness appliance — it trades calcium and magnesium for sodium and does nothing for arsenic, nitrate, uranium or bacteria. Idaho public health guidance says to treat arsenic at 0.01 mg/L and nitrate at 10 mg/L, and the fix for those is adsorptive media, anion exchange or reverse osmosis, usually at the kitchen sink. Test for them separately and budget for them separately.

Is softened water safe to drink?

For most people, yes. Ion exchange adds a small amount of sodium in proportion to the hardness it removes, so very hard water means more sodium added. If anyone in the house is on a sodium-restricted diet, the two normal answers are a cold hard-water line at the kitchen sink — which Idaho’s plumbing code amendment already requires on new slab and finished-basement homes — or regenerating with potassium chloride instead.

How hard is the water in Driggs and Victor?

It depends on your well, and that is not a dodge. Most Teton Valley homes I build are on private wells, and hardness varies lot to lot across glacial fill. For a nearby reference, the City of Idaho Falls reports 14 grains per gallon on its municipal supply, which is very hard. The only number that matters for sizing your equipment is the one from your own well, and hardness is usually not in the standard health panel, so ask the lab for it specifically.

Will a water softener wreck my septic system?

It can create a permitting problem before it creates a performance problem. Idaho rule IDAPA 58.01.03 Section 004.03 does not allow softener brine or backwash into a septic system unless the discharge is approved, and Section 004.04 bars pushing a system past its approved design flow. The workable paths are getting the discharge approved and sizing the system for it, specifying a high-efficiency demand-initiated unit to minimize discharge, terminating the drain somewhere else with approval, or using a salt-free conditioner with no brine at all.

Do I need a water softener if I have a tankless water heater?

Almost certainly, in hard water. PNNL’s testing for the Department of Energy showed a gas tankless unit falling from 80 percent to 72 percent efficiency in about 1.6 years in hard water, recovering only to 77 percent after deliming, and units in extremely hard water failing in under two years without frequent descaling. That same report says a softener may be the only viable option for tankless in hard water — while calling it unnecessary for an electric storage tank. Which water heater you pick changes the answer.

TL;DR: A heated garage is worth it in Idaho if you build the box before you buy the heat. Insulate under and around the slab, air seal the wall and ceiling between the garage and the house, then pick a heat source sized for the space. I hold most heated garage builds at 45 to 55 degrees instead of 68, because the goal is a floor that dries, tools that work and a vehicle that is not buried in frost, not a second living room.

Do it in that order and a heated garage costs less to run than people expect. Skip the envelope and you are paying to warm the outdoors for the next thirty years. The bonus room over that garage is a separate problem, and it is the one that gets built wrong most often.

Is a heated garage worth it in Idaho?

Yes, for most of the houses I build, and the reason has nothing to do with comfort. A heated garage in Teton Valley or eastern Idaho earns its money by keeping the slab above freezing. Snow comes off the vehicle, melts, and drains instead of turning into a sheet of ice you walk on every morning from November to April. Road salt and mag chloride rinse off the floor instead of sitting on your concrete all winter. Paint, caulk, adhesives and batteries stay above their minimum working temperature. A water line or a hose bib in that wall is no longer something you worry about during a cold snap.

The comfort is a bonus. The durability is the point.

Where I push back is when a client wants the garage held at 68 degrees so it feels like the house. That is a different building, with a different price and a different mechanical system, and nine times out of ten the real need is 50 degrees and a dry floor. I would rather spend that budget on the envelope and get a garage that holds 50 cheaply and forever than on a bigger heater that fights a leaky box every day for thirty years.

What does “heated” actually mean in a garage?

There are three different buildings people call a garage, and mixing them up is where budgets go sideways. “Heated garage” is not one spec, it is a range, and the honest conversation starts by picking which one you want.

Level	Envelope	Heat source	Winter temp it holds	What it is good for
1. Unheated, insulated	Insulated walls and ceiling, insulated doors, no slab insulation	None, borrows heat from the house wall	Roughly 35 to 45 degrees in a normal Idaho winter	Keeping a vehicle out of the wind and the worst of the frost. No freeze protection you can rely on.
2. Tempered, the one I build most	Insulated walls and ceiling, under-slab and slab-edge foam, air sealed, insulated overhead doors	One right-sized heater on a setback thermostat	Held at 45 to 55 degrees	Dry floor, freeze protection, tools and finishes stay usable, cheap to run. The default.
3. Fully conditioned	Same as level 2, plus continuous exterior insulation and a tighter air barrier	Heat and cooling, usually a cold-climate heat pump, sometimes in-slab radiant as well	Held at living-space temperature year round	A garage that doubles as a gym, a hobby space or a place you actually spend hours. Costs the most to build and to run.

When someone tells me they want a heated garage, my first question is which of those three they are describing. The answer changes the slab detail, the door spec, the heat source and the number.

How do you build a garage that can actually hold heat?

You insulate under the concrete. That is the one that gets skipped, and it is the one that decides whether a heated garage is cheap or expensive to run for the life of the house.

A garage slab sits on dirt. Dirt in Teton Valley is cold year round, and it will pull heat out of a warm slab continuously. If you are going to heat that slab, or heat the air above it, you put rigid foam under the whole slab and at the slab edge first. I run foam under the slab and a vertical strip of foam at the perimeter so the concrete is not in direct contact with the frost-depth soil and the exterior grade. Skipping the edge is worse than skipping the middle, because the edge is where the slab touches the cold outside.

Here is the whole list, in the order it gets built:

• Under-slab and slab-edge rigid foam. Not optional on a heated garage. The edge detail matters more than the field.
• A vapor barrier under the slab. You are about to warm that concrete, and warm concrete drives ground moisture up through it.
• Insulated, air-sealed walls. The garage walls facing outdoors get the same treatment as a house wall. Idaho’s energy code does not require it for an unconditioned garage, so it gets value-engineered out constantly. Specify it.
• An air-sealed, insulated ceiling. Whether there is a room above or just attic, warm air leaves through the top.
• Insulated overhead doors with good weatherstripping. The doors are the biggest hole in the box. An R-18 sectional door with a fresh bottom seal and side seals does more for a heated garage than a bigger heater.
• Air sealing at the house wall and the ceiling. This one is about safety as much as energy. More on that below.

The sequence matters because every one of those items is cheap during framing and brutal afterward. Under-slab foam costs very little when the gravel is open and is effectively impossible once the concrete is poured. This is the same lesson as the rest of the envelope, which I wrote about in detail in spray foam or blown-in insulation: the assembly is a system, and the air sealing is usually worth more than the R-value.

For reference, Idaho enforces the 2018 IECC with amendments, effective January 1, 2021. In climate zone 6, which covers eastern Idaho and Teton Valley, that is R-49 ceilings, R-22 walls or R-13 plus R-5 continuous, R-30 floors, and R-19 or R-15 continuous at basement and crawl space walls. You can confirm the current adopted edition at the Department of Energy’s Idaho code status page. Those numbers apply to conditioned space. A garage is not conditioned space, which is exactly why the garage envelope has to be specified on purpose rather than assumed.

What are my options for heating the garage?

Five options get used in this climate. I spec different ones for different garages, and the deciding factor is almost always how many hours a week somebody is actually out there.

Option	How it works	Market cost range (installed)	Best for	The catch
In-slab hydronic radiant	PEX tubing in the concrete, warm water from a boiler or a water heater	Market data: hydronic radiant generally runs $6 to $20 per square foot installed	A heated garage you want warm, dry and quiet. The best floor-drying system there is.	Has to be planned before the pour. Slow to respond, so it is a hold-a-temperature system, not an on-demand one. Needs the under-slab foam to make sense.
Electric in-slab radiant	Heating cable or mat in the slab or in the topping	Market data: electric radiant generally runs $10 to $15 per square foot installed	Smaller garages, or a zone at the overhead door where the snow melts off	Cheaper to install than hydronic, more expensive to run on Idaho electric rates over a big area.
Ductless mini-split heat pump	One wall or ceiling head, outdoor unit, no ductwork	Varies by capacity and head count; priced per project	A garage that is also a shop, gym or hobby space. Gives you cooling in August too.	Blows air, so it warms the room faster than the slab. Specify a cold-climate model rated for Idaho design temperatures.
Gas unit heater, hung	Sealed-combustion gas heater hung from the ceiling, ducted to outdoors for intake and exhaust	Lowest install cost of the ducted options; priced per project	Fast recovery after the overhead door has been open. Big garages and high bays.	Noisy, stratifies heat at the ceiling, and must be sealed combustion. I do not install unvented gas heaters, period.
Electric unit heater	Resistance coil and a fan	Cheapest to install of anything on this list	Backup heat, or a garage used a few hours a month	Most expensive thing on this list to run. Fine as freeze protection, painful as primary heat.

For a level 2 tempered heated garage, which is where most of my clients land, the answer is usually in-slab hydronic tied into the home’s heating water, or a single sealed-combustion gas unit heater if hydronic is not already in the house. If the home is getting a heat pump anyway, adding a head in the garage is often the cheapest path to a garage that is both warm in January and cool in July. I walked through how cold-climate heat pumps actually perform here in heat pump or furnace in Idaho.

If the house already has radiant floors, extending a zone into the garage slab is the easy win. That decision gets made at the same time as the rest of the hydronic design, and I covered the cost side of that system in radiant floor heat cost in Idaho.

Can you just run a duct from the house furnace into the garage?

No. Code does not allow it, and the code is right.

The IRC requires that ducts in the garage, and ducts penetrating the wall or ceiling that separates the house from the garage, be a minimum of No. 26 gauge sheet steel with no openings into the garage. That means no supply register, no return grille, nothing. You can read the dwelling-garage separation requirements in IRC chapter 3, building planning.

The reason is simple. A return grille in the garage would put the garage air, which has exhaust, gasoline vapor, solvents and whatever else is out there, directly into the air you breathe in the bedrooms. A supply register alone pressurizes the garage and pushes that air back into the house through every gap in the common wall. The garage is on the other side of a fire and air separation from the living space, and it needs to stay that way. A heated garage gets its own heat source, isolated from the house system. If it is hydronic, the water crosses the wall, not the air.

Why is the bonus room over the garage always freezing?

Because it is the hardest room in the house to build right, and almost nobody treats it that way.

A bedroom over the garage is sitting on an exterior floor. The space under it is not heated, so that floor is doing the same job an exterior wall does, except it is horizontal, gravity is working against the insulation, and it is full of framing, plumbing and ducts. Then the room usually has knee walls and a sloped ceiling, so it has five or six exterior surfaces instead of two. Then the ducts that serve it run the longest distance from the air handler, often through an unconditioned attic. Then it is on the same thermostat as the main floor, so the hallway hits temperature and the system shuts off while that room is still cold.

Every one of those is fixable. Together they are why the room over the garage is the number one comfort complaint in American houses. Allison Bailes has written extensively about how this failure traces back to missing enclosure design rather than bad equipment, and his piece on building enclosure design is worth reading if you want the physics.

Here is what I do differently:

• Treat the garage ceiling as the air barrier and make it continuous. The floor over the garage gets sealed at the subfloor, at every rim and band joist, and at every penetration. Air leakage through that floor, not R-value, is what makes the room cold.
• Fill the floor cavity completely, with insulation that stays put. Batts dropped into a floor cavity sag away from the subfloor and leave an air gap, and an air gap under the floor makes R-30 perform like R-10. Closed-cell foam against the underside of the subfloor solves the air sealing and the contact in one step.
• Keep the ducts inside the conditioned envelope. If a duct has to run through the garage ceiling or a vented attic to reach that room, the heat is gone before it arrives. I would rather reroute than insulate my way out of it.
• Give the room its own zone, or its own head. A separate zone with its own thermostat, or a dedicated mini-split head for the bonus room, ends the fight with the hallway thermostat permanently.
• Insulate and air seal the knee walls as walls. A knee wall with batts and no sheathing on the attic side is an exterior wall with nothing holding the insulation against it. It needs a rigid air barrier on the back.
• Size it room by room. A room-by-room load calculation, done to the ACCA Manual J standard, is how you find out that room needs more airflow than the plan gave it. Rule-of-thumb sizing is how it ends up with one undersized supply.

None of that is exotic. It is detailing, and it has to be decided during design and drawn, not negotiated on site in February.

What does the floor over the garage need?

In climate zone 6, a floor over unconditioned space is an R-30 assembly under Idaho’s adopted energy code. But the number is the easy part and it is not where these rooms fail.

What that floor actually needs is all three of these at once:

1. Continuous air sealing at the subfloor, the rim, the band and every hole through it. This is first in importance and last in attention.
2. Insulation in full contact with the subfloor, with no gap. Sagging batts are the single most common defect I find in a room-over-garage assembly.
3. A 5/8-inch Type X gypsum ceiling in the garage, which code already requires where there are habitable rooms above. Installed tight and sealed, it is also a competent air barrier. Installed with gaps at the walls and around can lights, it is neither.

And one more thing I insist on: no HVAC equipment, no water heater and no plenum in the garage ceiling cavity under a bedroom if I can avoid it. Every one of those becomes a thermal hole and a maintenance access problem later. Where it is unavoidable, it gets sealed and insulated as a deliberate detail on the drawings.

What does a heated garage cost to build and to run?

SwagerBuilds custom homes start at $400 per square foot and go up on the Teton Valley and Idaho side, and start at $1,000 per square foot and go up in Jackson Hole. That number is construction cost on conditioned square footage only. I do not pad it with garage or covered-porch area, which is why you cannot take the per-square-foot number, multiply it by the garage, and get the garage.

The garage is its own line item, and a heated garage is that line item plus four specific additions: under-slab and slab-edge foam, the insulated and air-sealed envelope, the heat source, and the controls. For the heat source itself, third-party market data puts in-slab hydronic radiant at roughly $6 to $20 per square foot installed and electric radiant at roughly $10 to $15 per square foot installed. Those are market figures, not my pricing. What your garage costs depends on size, door count, whether the house already has a boiler, and how much slab you are heating, and you get a real number inside the design agreement rather than a guess over the phone.

Operating cost is the part people get backwards. A heated garage is not expensive to run because it is heated. It is expensive to run because of how it was built and what you set it to. Two things drive the bill:

• The setpoint. Holding a garage at 50 degrees instead of 68 cuts the heat loss roughly in half in an Idaho January, because heat loss scales with the temperature difference across the walls. This is the single biggest lever you have, and it is free.
• The envelope, especially under the slab and at the doors. An uninsulated slab on cold ground and two uninsulated overhead doors will out-lose everything else in the building combined. Fix those and the running cost of a heated garage stops being a line you notice.

That is also the honest case against level 3. A fully conditioned garage held at living-space temperature year round costs meaningfully more to build and more every month to run, and most people who ask for it are happy at 50 degrees once they have used it for one winter.

What is not included in that number?

Whenever a builder quotes you a garage, ask what is outside the line. On a heated garage the usual omissions are:

• The driveway and the approach. Grading, base, and the concrete or asphalt outside the door are site work, not garage.
• Snowmelt in the driveway or the apron. Heating the slab inside the garage and melting the apron outside it are two different systems with two different price tags. If you want the apron, say so during design.
• A floor drain and where it goes. A heated garage melts snow, and that water has to leave. A drain means a plumbing route and, depending on the county, a specific discharge path. This is a permit conversation, not an afterthought.
• Finish inside the garage. Drywall beyond what code requires, paint, epoxy or polished floor, cabinets, a sink, a utility sink hot water line.
• Electrical capacity for what you plan to plug in. An EV charger, a welder, a compressor and a lift are panel and circuit decisions. Roughing in conduit during framing costs almost nothing. Adding it later costs real money.
• Bigger mechanical to serve a bonus room above. If that room gets its own zone or its own head, that is equipment and controls, and it belongs in the mechanical scope.

Surprises like these are the entire reason I write a detailed scope before a construction price exists. I went through the pattern in the hidden costs of building a custom home, and the garage is one of the most reliable places for it to show up.

Is a heated garage safe? What about carbon monoxide?

A heated garage is safe when it is detailed right, and the detail that matters is the air separation between the garage and the house.

Vehicles in attached garages are a recognized route for carbon monoxide to get into homes. The EPA’s guidance on carbon monoxide and indoor air quality lists them among the common sources. Heating the garage does not create that risk, but it does change the air pressures, so it is worth doing deliberately. Here is how I handle it:

• Air seal the common wall and the garage ceiling as a plane. Top plates, bottom plates, every penetration, every can light, the rim at the floor above. This is the barrier.
• No ducts open to the garage. Covered above, and it is code.
• Sealed-combustion equipment only. Any gas heater in a garage draws its combustion air from outdoors and vents to outdoors through a sealed pipe. No unvented heaters, which dump both combustion byproducts and a surprising amount of water vapor into the space.
• Carbon monoxide alarms inside the house. The IRC requires CO alarms outside the sleeping areas in dwellings with an attached garage or fuel-fired appliances. This is not optional and it is cheap.
• A self-closing door, where the code edition or the county requires it. The 2021 IRC puts the self-closing and self-latching requirement back on the garage-to-house door. Idaho jurisdictions are on different editions, so your building official is the authority on which applies to your permit. I install self-closing hardware either way, because the failure mode of a propped-open garage door is exactly what the separation exists to prevent.
• Do not idle a vehicle in a closed garage. A warm garage makes this more tempting, not less. It is still the thing not to do.

What does code require between the garage and the house?

This comes up on every build with an attached garage, and the requirements are specific. Under the IRC’s dwelling-garage separation provisions:

• Walls between the garage and the residence, and the garage and attics: not less than 1/2-inch gypsum board, applied to the garage side.
• Ceilings where habitable rooms are above the garage: not less than 5/8-inch Type X gypsum board. This is the one that applies to a bonus room over the garage.
• Structure supporting that floor-ceiling assembly: not less than 1/2-inch gypsum board. The posts and beams carrying the room above get covered too, which surprises people on a garage with a timber element.
• The door between the garage and the house: solid wood not less than 1 3/8 inches thick, solid or honeycomb-core steel not less than 1 3/8 inches thick, or a 20-minute fire-rated door. It cannot open directly into a sleeping room.
• Ducts: minimum No. 26 gauge sheet steel, with no openings into the garage.
• The garage floor: an approved noncombustible surface, sloped so liquid drains toward the main door or a drain.

Those are the minimums. A heated garage with a bedroom above it is a place where I build past the minimum on the air sealing, because the code separation is written for fire and smoke, and the comfort of the room above depends on an air barrier that code does not explicitly require. Which inspections catch what, and how that sequence runs in each county, is covered in custom home inspections.

How do I decide what my garage needs?

Four questions, in this order:

1. How many hours a week will somebody actually be out there doing something? Under two, you want level 2 tempered. Ten or more, look hard at level 3 with cooling.
2. Is there a habitable room above it? If yes, the floor assembly and a dedicated zone for that room move to the top of the list, ahead of the garage heat itself.
3. Is the house getting hydronic or a heat pump anyway? If yes, extending that system is usually the cheapest good answer, and it has to be decided during mechanical design.
4. What is going to get plugged in? EV charger, lift, compressor, welder. This drives the panel, and the panel is sized long before you think about it.

Answer those four during design and the heated garage decision makes itself. Leave them until framing and you are choosing between a compromise and a change order. The way that sequence is supposed to run is laid out in the steps to building a custom home, and the cost of deciding late is in change orders on a custom home build.

It is also worth being honest about where a heated garage sits against other places to put the same money. I ranked the upgrades I think actually pay in which custom home upgrades are worth the money, and the garage envelope lands high while the garage heat source lands in the middle. Under-slab foam is cheap and permanent. A bigger heater is neither.

Who figures all this out, me or the builder?

I do. That is the point of the design phase.

The slab detail, the heat source, the load calculation, the duct routing, the zone layout and the electrical capacity for the garage all get decided and drawn before there is a construction price. I coordinate the drawings and the structural and mechanical engineering in house, and I am involved in every phase of every build, so the person who decided where the ducts run is the same person standing there when they get installed. You are not hiring an architect, then a mechanical designer, then an electrician, and refereeing between them.

That work happens under a design agreement with a fixed design fee, quoted per project. Nobody gets a real garage number before that, because a real number requires the slab detail, the door count and the mechanical approach to exist first. What a preliminary budget looks like and why it comes after scope, not before, is in the design phase and getting a real number before signing.

If you are planning a build in Teton Valley, Driggs, Victor, Rigby or the Idaho Falls area and the garage matters to you, say so early. It is a cheap conversation in design and an expensive one in February. Here is how I build custom homes, and you can get in touch when you are ready to talk about a specific lot and a specific plan.

Frequently asked questions

How warm should I keep a heated garage in Idaho?

I set most heated garage spaces to hold 45 to 55 degrees. That is warm enough to melt snow off a vehicle, dry the floor, protect plumbing in the wall and keep paint, caulk and batteries usable, and it costs roughly half what holding 68 degrees costs in an Idaho January because heat loss scales with the temperature difference. If you are going to spend hours out there working, put a mini-split head in and bump it up while you are using it, then let it fall back. Setback is free performance.

Does a heated garage count toward my square footage or my price per square foot?

No. SwagerBuilds quotes construction cost on conditioned square footage only, with no garage or covered-porch padding, so the garage is always a separate line item. That is also true of the published anchor: custom homes start at $400 per square foot and go up on the Idaho and Teton Valley side, and start at $1,000 per square foot and go up in Jackson Hole, and neither number includes the garage. How that per-square-foot figure is built up is in cost per square foot to build in Teton Valley.

Do I need to insulate under the garage slab if I am not heating it?

If you might ever heat it, yes, put the foam in now. Under-slab and slab-edge rigid foam is inexpensive while the gravel is open and effectively impossible to add after the pour, so it is the cheapest option you will ever buy on a garage. If you are certain the garage will never be heated, you can skip the field insulation, but I still run the slab-edge foam, because that edge is also a thermal bridge into the house foundation. The same logic applies to the foundation itself, which I compared in basement vs crawl space vs slab in Idaho.

Will heating the garage cause ice dams or condensation problems?

Not if the ceiling is air sealed. A heated garage with a leaky ceiling pushes warm, moist air into the attic or into the floor cavity above, and warm air in a cold roof assembly is exactly how ice dams start and how you get frost on the underside of the sheathing. The fix is the same one that governs the rest of the roof: seal the plane and keep the heat below it. I wrote the full version in how to prevent ice dams in Idaho.

Can I add a heated garage to a house that is already built?

You can add a heat source, and you can usually improve the walls, ceiling and doors. What you cannot practically add is the under-slab insulation, which is the item that decides whether the space is cheap or expensive to hold at temperature. That is why this belongs in design on a new build. For reference, SwagerBuilds builds new custom homes rather than taking on retrofit work, so on an existing house this is a conversation with your HVAC contractor about the heat source and your insulation contractor about the envelope. The envelope upgrades that actually move the needle are ranked in which energy efficient custom home features are worth paying for.

TL;DR: A tankless water heater is worth it in an Idaho custom home when you have gas, you plan to own the house a long time, and your hot water demand is spread across the day instead of stacked into one hour. It is not automatically the better choice. Our groundwater comes out of the ground cold enough in winter that a tankless water heater here does roughly twice the work the same unit does in Phoenix, which means fewer gallons per minute, bigger gas lines, and a real conversation about propane. On a 4,000-square-foot house with five bathrooms, two well-placed tanks and one properly sized tankless bank will both deliver. What decides it is fuel, mechanical room space, water quality, and whether anyone is actually going to maintain the thing. Here is how I work through it on every build.

What are my water heater options, in plain English?

Four, and they are easy to tell apart once somebody stops using trade shorthand.

A storage tank heats 40 to 80 gallons and holds it hot until you use it. A tankless water heater holds nothing; it fires a burner the moment you open a tap and heats water as it flows past. A heat pump water heater is a tank that steals heat out of the surrounding air instead of making heat with an element, like a refrigerator running backward. An indirect tank is a storage tank with no burner of its own, heated by a boiler you already have for something else.

That last one gets skipped constantly, and it shouldn’t. If your plans already include radiant floor heat, you are installing a boiler anyway. Hanging an indirect tank off that boiler is frequently the cheapest, quietest, longest-lived hot water in the whole house, and almost nobody asks for it by name.

Is a tankless water heater worth it in an Idaho custom home?

Yes, when three things are true: you have gas service or a propane tank big enough to feed it, you are building the house to keep, and your family does not run every hot water fixture inside the same 40 minutes. Under those conditions a tankless water heater will outlive two storage tanks, free up a sizable chunk of mechanical room floor, and never leave anybody cold at the end of a long weekend with a full house.

No, when you are all-electric on a 200-amp rural service, when the mechanical room sits in an unheated space, when nobody is going to flush the unit once a year, or when the house is built on a budget that is already tight in places that matter more. A 50-gallon tank that gets replaced in year eleven is not a failure. It is a reasonable decision.

What I will not tell you is that a tankless water heater is an upgrade in the way a better window or thicker insulation is an upgrade. It changes how hot water arrives, not how much the house costs to run in any way you will notice on a bill. If you are ranking which custom home upgrades are worth the money, this one sits in the middle of the list, not the top.

Why does Idaho’s cold groundwater change the math?

Because a tankless water heater is rated on temperature rise, and ours is brutal.

Temperature rise is just the difference between the water coming in and the water you want coming out. In midwinter, well water in Teton Valley and across eastern Idaho comes in around 40 to 45 degrees. If you want 120-degree water at the tap, that is a 78-degree rise. In a warm climate with 70-degree incoming water, the same tap is a 50-degree rise. Same house, same shower, completely different load.

Here is the arithmetic a spec sheet hides. Gallons per minute equals usable BTU divided by rise, divided by 500. Take a big residential unit, 199,000 BTU input at roughly 95 percent efficiency, so about 189,000 usable BTU:

• At a 50-degree rise: 189,000 ÷ (50 × 500) = 7.6 gallons per minute
• At our 78-degree rise: 189,000 ÷ (78 × 500) = 4.8 gallons per minute

That is the whole story in two lines. The box on the pallet says 9 GPM. In a Driggs winter that unit is a 4.8 GPM appliance. Now put real fixtures against it: a shower head runs 1.8 to 2.5 GPM, a kitchen faucet 1.5 to 2.2, and a freestanding tub filler, the kind that shows up on every set of plans crossing my desk, runs 4 to 7 GPM by itself. One tub filler can consume an entire tankless unit while somebody upstairs is waiting on a shower.

This is why I size for January, not for the average. A tankless water heater that is perfect in September and embarrassing in February was sized by somebody reading the front of the brochure.

Do you even have gas? In Teton Valley, probably not piped

This single question kills more tankless plans than the physics does, and it comes down to geography.

Intermountain Gas lists Idaho Falls, Ammon, Iona, Rexburg, Menan, Firth, Basalt, Lewisville and Parker among the towns it serves. Teton County, Idaho is not on that list — no Driggs, no Victor, no Tetonia. So if you are building on the Teton Valley side of the pass, “gas” does not mean a meter on the wall. It means propane in a tank on your lot, delivered by truck, on a road that has to stay plowed in February.

That matters in three concrete ways. A 199,000 BTU unit wants a three-quarter to one-inch gas line and its own sealed vent, not a tee off whatever is already there. Two units stacked for a big house is close to 400,000 BTU, which routinely means a larger regulator and a step up from a 250-gallon to a 500-gallon propane tank. And propane is a commodity you buy by the truckload, so the operating cost conversation is different from a metered utility.

None of that is a dealbreaker. It is a design decision, and it belongs in the drawings before anybody digs. We coordinate the mechanical layout, the gas sizing and the propane tank location during the design agreement, alongside the well and septic work and the rest of the site plan — you do not go assemble that team yourself.

What about a heat pump water heater?

I like them, and they need to be placed deliberately in a cold climate or they will disappoint you.

A heat pump water heater pulls heat out of the air around it, so it runs two to three times more efficiently than a standard electric tank. The catch is in the install conditions. ENERGY STAR calls for roughly 1,000 cubic feet of air space around the unit and an interior space that stays between 40 and 90 degrees year-round, and notes that in colder spaces the unit may drop into backup electric-resistance mode, which is exactly the expensive mode you bought it to avoid.

Translate that to an Idaho house. A heat pump water heater does not belong in an unheated garage, and it does not belong in a vented crawl space. It belongs inside the conditioned envelope, in a mechanical room with real air volume or with ducted intake and exhaust. The other thing nobody mentions: it cools and dries the room it sits in. Put it next to a finished basement media room and somebody is going to complain in January. Put it in a mechanical room that runs warm, and it gets more efficient.

Where your mechanical room ends up is partly a foundation decision, which is one more reason the basement versus crawl space versus slab question gets settled early rather than late.

Tank vs. tankless vs. heat pump, side by side

This is the comparison I actually walk through at the table, with Idaho conditions baked in rather than national averages.

Factor	Storage tank (gas or electric)	Tankless water heater (gas)	Heat pump water heater
Hot water supply	Finite. 50 gallons, then recovery time	Endless, but capped at gallons per minute	Finite, with slower recovery than gas
Cold-climate penalty	Minor. Longer recovery in winter	Significant. Output drops about 35% at our winter rise	Significant if the space is cold or small
Floor space	Largest. Needs real footprint plus drain pan	Smallest. Wall-hung, frees the floor	Largest, plus about 1,000 cu ft of air around it
Fuel needed	Gas, propane or standard electric	Gas or propane, upsized line and dedicated vent	Electric only, but modest draw
Maintenance	Flush occasionally, replace anode rod	Annual descaling is not optional on well water	Clean the air filter, keep the condensate clear
Typical service life	8–12 years (market data, Fixr 2026)	15–20 years (market data, Fixr 2026)	Longer than standard electric; varies by install
Best fit here	Simple layouts, tighter budgets, all-electric lots	Gas or propane, long-term owner, spread-out demand	Conditioned mechanical room, all-electric house

Read the “cold-climate penalty” row twice. Everything else on that chart is the same conversation you would have in Texas. That row is the Idaho-specific one, and it is the row most quotes ignore.

How big does it need to be for my house?

Not by square footage. By simultaneous fixtures, which is a different question and the only one that matters.

The method is simple. List the fixtures that could realistically run at the same time on your worst morning, add up their flow rates, then size to that number at a 78-degree winter rise. Not the average rise. The winter one.

House profile	Realistic simultaneous demand	What I generally spec
2,500 SF, 2.5 baths, no large tub	About 4 GPM (two showers plus a sink)	One 199K tankless, or a single 50-gal gas tank
3,500 SF, 4 baths, one freestanding tub	About 7 GPM (tub filler plus a shower)	Two tankless units banked, or a 75-gal gas tank
5,000 SF and up, 5 baths, tub plus multi-head shower	9–12 GPM	Two to three banked units, or boiler plus indirect tank
All-electric lot, any size	Whatever the electrical service allows	Heat pump water heater, often two, inside the envelope

On that last row, here is why whole-house electric tankless almost never pencils in Idaho. Four gallons per minute at a 78-degree rise is 156,000 BTU per hour, which is about 46 kilowatts, which is roughly 190 amps at 240 volts. On a house with a 200-amp service, that is the entire service for one appliance. I have never specified one on a custom home and I do not expect to.

Why does hot water take two minutes to reach the far bathroom?

Because hot water has to physically travel from the heater to the tap, and in a big house that is a long pipe full of cold water that has to be pushed out first. This complaint has nothing to do with which heater you bought, and a tankless water heater can actually make it feel slightly worse, because the burner has to light before the first hot water even starts moving.

There are three fixes, and the right time to choose one is during plumbing rough-in, not after drywall.

A dedicated return line is the real answer: a small pipe that loops back from the far end of the house so hot water is always close by. A crossover valve at the farthest fixture is the retrofit version, cheaper and less effective because it uses the cold line as the return. Demand control is the part people get wrong — a button, motion sensor, or smart control that runs the pump only when someone is actually about to use water. An always-on recirculation loop on a big house burns energy around the clock and short-cycles a tankless unit for no good reason.

Roughing in a return line while the walls are open costs a fraction of what it costs to add later, when it becomes a demolition project. It is one of the cheap decisions that quietly separates a house that feels expensive from one that just cost a lot, and it is the same logic behind rough-ins for smart home features and leak detection.

What does this cost, and what is not in that number?

Market data first, so you have a reference point that is not mine. Fixr’s 2026 figures for a typical home put installed cost at roughly $600 to $2,500 for a conventional tank, $1,000 to $4,000 for tankless and $1,200 to $4,600 for a heat pump unit. Those are market numbers for an average house, not SwagerBuilds pricing.

Now the part the market data leaves out, because on a custom home it is most of the cost. Those figures do not include upsizing a gas line, a dedicated sealed vent through a roof assembly, a larger propane tank and regulator, a second unit when one will not carry the fixture count, a recirculation return line, a condensate drain and neutralizer with a pump if the unit sits below grade, isolation valves for annual descaling, or the water treatment that well water demands. A five-bath house with two banked units, recirculation and softening lands above the top of every one of those ranges, and anybody quoting you the bottom of the range is quoting a water heater, not a hot water system.

For reference on the whole project: SwagerBuilds custom homes start at $400 per square foot and go up on the Teton Valley and Idaho side, and start at $1,000 per square foot and go up in Jackson Hole. Design is sold as a fixed design fee, quoted per project. Mechanical selections like this one are made inside that design work, before a construction price is signed, which is the entire point of getting to a real number before you sign instead of discovering it later. If you want the broader picture, we keep an honest list of the hidden costs of building a custom home in Idaho and how construction allowances hide decisions like this one inside a single line item.

Will the 2029 water heater rule affect my house?

Not the unit you install in 2026. It will affect the one that replaces it.

The Department of Energy finalized amended efficiency standards for water heaters that take effect in 2029. The headline change is on the electric side: DOE expects more than half of newly manufactured electric storage water heaters to use heat pump technology, up from about 3 percent today. Under the final rule, electric storage units above 35 gallons are headed toward heat pump technology, and gas storage sees a more moderate efficiency bump. Gas tankless was largely left alone in that rulemaking.

The practical takeaway for somebody building right now is small but worth doing: if there is any chance a future owner puts an electric unit in that mechanical room, give the room the air volume, the clearance and the floor drain a heat pump water heater needs. That is free on paper today and expensive in 2031.

One more timing note, because it changes the arithmetic people did two years ago. The federal Energy Efficient Home Improvement Credit under Section 25C, which paid up to $2,000 toward a heat pump water heater, applies to property placed in service before December 31, 2025. If you are reading a 2024 blog post that leans on that credit to justify an upgrade, the credit is no longer there. Spec the equipment because it is right for the house, not because of a credit that expired. The same correction applies to a lot of the older math behind whether a green custom home is worth it.

What does a well and hard water do to a water heater?

It shortens its life, and it does it faster to a tankless water heater than to a tank.

Most of eastern Idaho and Teton Valley sits on hard water. Hard water means dissolved minerals, and when you heat it, those minerals come out of solution and plate onto the hottest surface they can find. In a tank, that means sediment on the bottom, which is annoying and slow-acting. In a tankless unit, it means scale inside a narrow heat exchanger, which is the single most common reason a tankless water heater dies early.

Three things prevent it, and all three are cheap at rough-in. Install isolation valves so the unit can be descaled in 45 minutes without cutting pipe. Put a whole-house softener or conditioner in the plan rather than treating it as an afterthought. Then actually flush the unit once a year. That last one is a homeowner commitment, and if the honest answer is that nobody is going to do it, a storage tank is the smarter specification. I would rather hear that at the design table than warranty it out in year six.

Water quality is a well decision as much as a plumbing decision, which is why it gets tested and planned alongside the rest of the build sequence rather than after the house is dried in.

What I spec, and how we decide it on your build

Twenty years in construction and I have never once treated the water heater as a default line item, because it is one of the few mechanical choices a homeowner notices every single day.

My general starting points, which get overridden by the specific house every time: if there is piped natural gas and the fixture count is moderate, a single properly sized tankless water heater wall-hung in the mechanical room. If there is a boiler for radiant floor heat, a boiler plus indirect tank, nearly always. If the lot is all-electric, a heat pump water heater inside the conditioned envelope with the air volume it needs. If the house has five baths and a tub filler somebody is going to use, two banked units and a demand-controlled return line. And if the budget is working harder somewhere else in the house, a well-placed gas tank with no apology.

This gets settled during the design agreement, on paper, with the mechanical room drawn and the gas and electrical loads sized — not negotiated in the field when the plumber is standing there. I am involved in every phase of every build, and there are 24/7 cameras on every job, so when the mechanical room gets roughed in you can watch it happen. That is the same reason the rest of the envelope decisions get made early too: insulation, windows, the heating system, the roof assembly, and the energy-efficiency features worth paying for are one coordinated set of decisions, not six separate arguments.

Built different. Built right. Built by SwagerBuilds.

Frequently asked questions about tankless water heaters

Does a tankless water heater really give you unlimited hot water?

Unlimited in duration, limited in flow. A tankless water heater will run a single shower all day, but it can only heat so many gallons per minute at once. At our winter incoming water temperature, a large 199,000 BTU residential unit delivers roughly 4.8 gallons per minute, which is two showers or one big tub filler, not both.

How long does a tankless water heater last in Idaho?

Market data puts tankless service life at 15 to 20 years versus 8 to 12 for a storage tank. On untreated hard well water with no annual descaling, I have seen that advantage disappear entirely. The maintenance is what buys the extra decade.

Can I put a heat pump water heater in my garage in Idaho?

Not in an unheated one. ENERGY STAR specifies an interior space that stays between 40 and 90 degrees year-round with about 1,000 cubic feet of air around the unit. An unheated Idaho garage fails the temperature requirement for months at a time, so the unit falls back to electric-resistance heating and the efficiency you paid for disappears.

Is electric tankless an option if my lot has no gas?

Practically, no. Delivering 4 gallons per minute at a 78-degree winter rise takes about 46 kilowatts, roughly 190 amps at 240 volts, which is the entire service on most houses. On an all-electric lot the right answer is a heat pump water heater, or two.

Should I run a hot water recirculation line in a large custom home?

Yes, rough it in. A dedicated return line with demand control costs a fraction during plumbing rough-in compared to adding it later, and it is the actual fix for waiting two minutes for hot water at the far end of the house. Skip the always-on pump; use a button or sensor.

Planning a build in Teton Valley or eastern Idaho?

Hot water is one of a few hundred decisions that are cheap on paper and expensive in the field. We settle them during design, in writing, before a construction price gets signed — drawings, engineering coordination, interior selections, permitting and construction all run through us.

If you are building in Driggs, Victor, Tetonia, Rexburg, Rigby, Idaho Falls or Jackson Hole, start a conversation with us or call (208) 520-0636. If you are still a year out and just reading, that is fine too — start with cost per square foot to build in Teton Valley, cost-plus versus fixed price, and how involved you actually have to be.

TL;DR: Ice dams are not a snow problem, they are a heat problem. Warm air leaking out of the house melts snow on the upper roof, the water runs down to a cold eave and freezes there, and the dam that builds up pushes water back under the roofing. You stop ice dams in Idaho by air sealing the ceiling plane, insulating it heavily and evenly, and venting the roof deck so it stays as cold as the outside air. Heat cables and roof rakes treat the symptom.

What actually causes ice dams on a roof

I have been in construction for twenty years, most of it in a valley where 85 to 100 pounds per square foot of roof snow load is the design number, and I am involved in every phase of every build we run. In that time I have looked at a lot of roofs with ice dams along the eave and a stain on the ceiling inside. The homeowner almost always describes it the same way: bad winter, too much snow, unlucky roof.

It is almost never the snow. Ice dams form when one part of the roof is warm enough to melt snow and another part is not. Heat from inside the house reaches the underside of the roof deck, snow against that deck melts from below, the meltwater runs down the slope, and when it reaches the overhang, where there is no heated space underneath, it refreezes. That ridge of ice is the dam, and ice dams grow with every melt and freeze cycle. Once it is a few inches tall it is holding standing water behind it, and standing water finds the laps in your roofing and goes under.

The Building Science Corporation digest on the subject is blunt about the ranking of causes. Air leakage from the house into the attic is, in their words, “the most common, most damaging, and often the most obvious problem to solve.” Conduction through thin or compressed insulation is second. Ductwork and hot water piping run above the insulation is third, because those are literally heaters installed in a space you are trying to keep cold. And the fourth one surprises people: solar gain. Bare shingles or dark metal in February sun can reach well above ambient air temperature, which melts snow even on a roof that is otherwise built correctly.

That last one matters because it means there is no assembly that makes ice dams physically impossible on every roof in every winter. What good detailing does is remove the causes of ice dams that you control and shrink the ones you do not. On the builds I run, that is the standard: remove the heat, then protect the eave anyway.

How to prevent ice dams in Idaho: three things, in this order

Every conversation I have about how to prevent ice dams in Idaho eventually turns into a conversation about products. Which membrane. Which vent. Which foam. Those are real decisions, but they come third. The order is what determines whether the roof works, and the order is air sealing, then insulation, then ventilation.

The reason the order matters is that ventilation is the weakest of the three. Building Science is explicit that ventilation removes residual heat only after insulation and air sealing have been handled, and that it cannot offset poor insulation or a big air leak on its own. If you build a leaky ceiling and then try to vent your way out of it, you are pushing warm indoor air up into the attic faster, not cooling the deck. I have seen that exact outcome: ridge vent added to a house with a wide open mechanical chase, and the ice got worse.

Cause of ice dams	What fixes it	When it has to happen
Air leaking from the house into the attic	Continuous sealed ceiling air barrier: top plates, can lights, plumbing stacks, chases, attic hatch	Before insulation, at rough-in. Nearly impossible to fix well later.
Thin or compressed insulation, especially over exterior walls	High-heel (raised-heel) trusses plus full-depth insulation carried to the wall line	At truss order. This is a design-phase decision, not a field fix.
Ducts, air handlers and hot water lines above the insulation	Keep mechanicals inside the conditioned envelope, or bring the roof plane inside it	Design phase, with the mechanical layout.
Warm roof deck with nothing moving the heat away	Vented attic or a vented air space above the insulation, with balanced intake and exhaust	Framing and roofing. Hard to retrofit on a cathedral ceiling.
Solar melting on the upper roof	Nothing structural. Accept it, and protect the eave with membrane.	Roofing, every time.
Water already backing up behind a dam	Self-adhering ice barrier membrane run well past the wall line	Dry-in, before any roofing goes on.

Notice how much of that column on the right says design phase. This is the part people do not expect about ice dams: most of the real decisions are made before anyone is on the roof. That is one of the reasons we sell a design agreement before a build contract, and it is covered in more detail in our write-up on design-build versus architect-then-bid and on the design and engineering page.

Step one: air seal the ceiling plane before anything else

The ceiling is the air barrier that keeps your heat out of the attic. On a production house it is drywall with a hundred holes in it. On a house built to resist ice dams it is a deliberate, inspected plane.

What gets sealed, in the order we hit it:

• Top plates. Every interior and exterior top plate joint, and the gap between plate and drywall, gets sealed. On a long wall this is the single biggest leak by area.
• Chases and soffits. Dropped soffits, plumbing walls and the chase around a furnace flue are open shafts straight into the attic unless somebody caps them with rigid material and seals the edges. Building Science recommends caulk for small gaps and expanding foam for any opening larger than about an inch.
• Can lights. We specify airtight, IC-rated fixtures, or we move the light. A row of leaky cans down a hallway under a cold roof is a reliable ice dam generator.
• Plumbing and electrical penetrations. Each stack, each wire bundle, sealed individually.
• The attic hatch. Weatherstripped, latched and insulated to the same level as the ceiling. An uninsulated pull-down is a hole in your R-49.
• Bath and range fans. Ducted to the outside, not into the attic, with an insulated duct and a damper that actually closes.

Idaho ties this to a number. The state enforces the 2018 IECC with state amendments, effective January 1, 2021, per the Department of Energy’s state code status page, and the envelope air leakage target is 5 air changes per hour at 50 pascals. Since July 1, 2021 Idaho requires blower door testing on only a 20 percent sample of each builder’s homes, which means a builder can legally never test the house you are standing in. We test, and I would ask any builder you are interviewing whether they do. Building science writers have made the same point for years, that air sealing and insulation are what prevent ice dams, not gadgets. That question is on the list in our post on questions to ask a Teton Valley custom home builder, and the air sealing itself shows up in the inspection sequence we walk through in what custom home inspections happen.

Step two: get the insulation right, especially at the truss heel

Idaho’s prescriptive ceiling number in climate zone 6 is R-49. Teton County is zone 6B. Hitting R-49 in the middle of an attic is easy. The place ice dams start is the three feet of ceiling directly above your exterior wall, where a standard truss heel pinches the insulation down to a few inches right at the coldest, most vulnerable part of the roof.

The fix is a raised-heel truss, sometimes called an energy heel. It lifts the top chord above the wall plate so full-depth insulation runs all the way out to the wall line with an air channel still above it. It costs almost nothing at truss order and cannot be added later. If you remember one line from this article, make it that one.

Building Science’s recommended minimums for keeping a roof cold enough to resist ice dams are higher than a lot of people assume, and they are stated by assembly type, not as one number: at least R-30 below a ventilated attic roof, R-35 below a ventilated cathedral ceiling, and R-40 below an unventilated cathedral ceiling, with increases for very cold climates. We are in one of those climates, and Idaho’s R-49 ceiling requirement already clears the vented attic figure comfortably. The place you have to pay attention is the cathedral ceiling, where the cavity is shallow and the code minimum and the ice dam minimum are not the same number.

Insulation choice interacts with this. A dense-pack cellulose or blown fiberglass attic needs a separate, well-executed air barrier underneath it, because neither material stops air on its own. Closed-cell spray foam is its own air barrier, which is why it shows up on unvented roof assemblies. We break down that tradeoff in detail in spray foam or blown-in insulation.

Step three: vent the roof so the deck stays cold

Ventilation’s only job here is to carry away the heat that got past the first two steps, plus any moisture that migrated up from the house. In a cold climate that is genuinely useful, and it is the reason a well-built vented attic is still the most forgiving roof you can put on a house in a snow country.

The code ratio is net free ventilating area of 1/150 of the attic floor area, reduced to 1/300 when the venting is split between high and low and a vapor retarder is present. Building Science notes the published ratios across codes run from 1:150 to 1:600 depending on which code you consult, with 1:300 being the most common. What matters more than the ratio is that intake and exhaust are both real. A ridge vent with blocked soffits does not ventilate, it just gives warm air a place to collect.

On a vented cathedral ceiling, code allows a one inch airspace above the insulation. Building Science recommends a minimum of two inches of clear airspace between the underside of the deck and the top of the insulation. We build to the two inch figure, with site-built baffles rather than the thin plastic ones that collapse when the insulation goes in.

Two details that get missed on vented roofs in high snow country:

• Buried soffit vents. If the soffit vent is a foot above grade-level snowdrift, it is not an intake in February. Intake has to be placed with the real snowline in mind.
• Ridge vents under drifted snow. A ridge vent buried under two feet of wind-packed snow is closed. On a roof with a dominant wind direction we plan exhaust accordingly instead of assuming the ridge always breathes.

Vented attic vs. vented cathedral vs. unvented roof

There is no universal right answer here, and I distrust anyone who tells you there is. The assembly follows the house you are designing. A simple gable with a flat ceiling and a big attic is a different problem than a vaulted great room with a ridge beam.

Assembly	Ice dam resistance	Where it fits	What it demands
Vented attic, flat ceiling, raised-heel trusses	Highest, and the most forgiving of small mistakes	Gable and hip roofs with an accessible attic. Our default when the plan allows it.	A genuinely sealed ceiling, R-49 carried to the wall line, balanced intake and exhaust, no ducts in the attic
Vented cathedral ceiling (vented airspace above cavity insulation)	Good, but the margin is thin	Vaulted rooms where the rafter depth can carry both insulation and a real airspace	Deep rafters or TJIs, two inch minimum airspace, continuous baffles, intake at every rafter bay, R-35 or better below
Unvented roof, closed-cell foam above and below the deck	Good if the above-deck R-value is right; poor if it is not	Complex rooflines, scissor trusses, valleys and dormers where venting every bay is not realistic	In zone 6, 2018 IRC Table R806.5 wants at least R-20 of air-impermeable insulation above the deck for condensation control, and the air-impermeable layer has to act as a Class II vapor retarder
Unvented roof plus a vented over-roof above the insulation	Highest of the unvented options	Extreme snow exposure on a complex roof, where you want a cold cladding over a warm assembly	Two layers of framing and sheathing, more money, careful detailing at eaves, ridges and penetrations

That last row is worth knowing about, because Building Science specifically recommends it for our conditions: in extreme snow regions, they say it is necessary to add a vented air space between the roof cladding and the rigid insulation to avoid ice damming. It is the belt-and-suspenders approach on a roof that has to be unvented for geometric reasons. We have used it on builds where the roof had four valleys and a shed dormer and there was no honest way to vent every bay.

If you are weighing a vaulted ceiling against a flat one, the framing side of that choice is covered in our posts on scissor trusses in Idaho and TJIs versus floor trusses.

Ice barrier membrane: what the 2018 IRC requires at the eave

Every roof we build gets a self-adhering ice barrier membrane at the eaves, because even a correctly built roof will see solar melting. This is the layer that decides whether a dam becomes a leak.

Section R905.1.2 of the 2018 IRC requires the ice barrier to extend from the lowest edges of all roof surfaces to a point not less than 24 inches inside the exterior wall line of the building. The important word is inside: the 24 inches is measured horizontally, from the wall line inward, not up the slope. That has a consequence most people miss. On a house with a 12 inch overhang and 2×6 exterior walls, the distance actually measured up the roof slope works out to roughly 47 inches at a 6:12 pitch, about 50 and a half inches at 8:12, and about 59 inches at 12:12. A single standard 36 inch roll of membrane does not reach the code line on any of those roofs. Two courses is the honest answer, and on steep pitches it still deserves a look at the math rather than an assumption.

We go past the minimum. Building Science frames the right height differently and more usefully: the membrane should rise far enough to cover 6 to 8 inches of standing water height above the edge of the wall insulation below, which on a low-slope roof means running it much further up than on a steep one. Low pitch, more membrane. That is the rule I use.

Membrane also belongs in the places where water concentrates regardless of pitch: valleys, around chimneys and skylights, at wall-to-roof intersections, and over any low-slope porch or dormer roof that a main roof dumps onto. Those are the spots where I have seen the most expensive water damage, and the extra material is trivial next to the repair. Our broader approach to keeping water out of the shell is in the builder’s guide to exterior waterproofing.

Snow load, roof pitch, and where ice dams still form

Teton County, Idaho publishes its design criteria, and the numbers set the context for every roof decision here: roof snow load of 85 psf plus dead load plus drift below 6,600 feet of elevation, and 100 psf plus dead load plus drift at 6,600 feet and above, with a 32 inch frost depth, 115 mph design wind speed and seismic category D-1. Those figures are about structure, not ice dams, but they tell you how much snow is sitting on the assembly all winter.

Pitch helps and does not solve. A steeper roof sheds more snow and gives meltwater less time to sit, which is why most of the homes here have steep primary roofs. But ice dams do not care about the average pitch, they care about the cold spots and the places where water slows down:

• Valleys. Two roof planes dumping into one channel, often shaded, often where the ice is thickest.
• Roof-to-wall intersections. A lower roof running into an upper wall traps snow against the wall and holds it.
• Dormer cheeks and the low-slope roof below a dormer. Reliably the first place I look.
• Over entries, porches and garage doors. Usually a shallower pitch, usually over an unconditioned or partly conditioned space, and the one place where falling ice is a safety problem rather than a maintenance problem.
• North-facing eaves. They never get the solar help that clears the rest of the roof.

This is a design-phase conversation, not a roofing-phase one. A roof with fewer valleys and no low-slope saddle over the front door is a roof that will not need rescuing in year three, because the places ice dams start were taken out of the drawing. On our builds that tradeoff gets discussed while the plan can still change, which is the point of the process described on our process page.

Metal roofs and snow retention: shedding is not the same as solving

Standing seam metal is common here and it is a good roof. It sheds snow well, has no laps for water to creep under across the field of the roof, and holds up to the sun and the freeze-thaw cycling better than asphalt. A metal roof genuinely reduces how often ice dams form.

Two honest caveats. First, metal gets hot in winter sun, and solar melting on a metal roof is more pronounced, not less. A metal roof with a leaky ceiling below it will still produce ice at the eave, and the meltwater will still find the one spot where the detailing is weak. Second, shedding creates a different problem: a roof that releases a winter’s worth of snow in one slide onto a walkway, a deck, a heat pump or a car. That is why snow retention devices, properly engineered and attached to the seams, go on over the places people and equipment actually are, and why we locate mechanical equipment with the shed path in mind from the start.

Metal also does not excuse you from the ice barrier. If anything the eave detail matters more, because once water gets behind a panel it can travel a long way before it shows up. Material choice and pricing for metal roofing in this market is covered in our post on metal roof cost in Idaho, and the same logic about enduring an Idaho winter applies to the walls in best siding for Idaho winters.

Heat cables and roof rakes: why I treat them as a failure signal

Heat cables work. Run in a zigzag at the eave and down the gutter and downspout, they keep a drain path open and prevent the standing water that causes the leak. I am not against them, and there are roofs where they are the right call: an existing house with a complex roof and no realistic way to fix the assembly, or one stubborn north valley on an otherwise sound roof.

What I will not do is design a new custom home that needs them. On a new build, a heat cable is a line item that exists because something upstream was not detailed. It costs money to install, it costs money to run every winter, the cables have a service life and fail, and they do nothing about the moisture that is still being driven into your attic by the air leak that caused the problem.

Same with roof rakes. Pulling snow off an eave every few weeks is a maintenance routine, and it carries real risk: a homeowner on a ladder in winter, or a torn membrane from a metal rake edge. If a new house needs to be raked to avoid ice dams, the house has a defect, and the defect is almost always air sealing.

The one accessory I do specify freely is attention to the mechanical layout, meaning keeping ducts and water lines out of the cold attic entirely. That is not an accessory, it is just the right drawing.

What detailing against ice dams costs on a custom home

Here is the part people want a number for, and here is the honest answer: nearly everything in this article is already inside a correctly priced custom home, and the pieces that are extra are small.

Raised-heel trusses are a minor adder at truss order. A second course of ice barrier at the eaves is a roll of material and an hour. Airtight can lights cost a little more than the cheap ones. Site-built baffles are labor, measured in hours per roof, not days. A blower door test is a single line item. The expensive items on this list are not materials at all, they are the design decisions: how many valleys the roof has, whether the mechanicals are inside the envelope, whether a vaulted ceiling has enough depth to be vented. Those cost nothing to get right on paper and a great deal to fix in the field.

Our custom home pricing starts at $400 per square foot and goes up on the Teton Valley and Idaho side, and starts at $1,000 per square foot and goes up on the Jackson Hole side, quoted on construction cost and conditioned square footage, fixed price after design lock. Everything in this article is inside that, because I do not treat a roof that keeps water out of the house as an upgrade. Design is a separate fixed design fee, quoted per project. How we get from an anchor number to a real fixed price is laid out in cost per square foot to build in Teton Valley and the cost reality page.

What I would be suspicious of is a bid that is cheaper than the others and silent on all of this. Resistance to ice dams is invisible on a bid sheet and invisible on a finished house in July. It shows up in February of year two. That is exactly the kind of difference our post on why three custom home bids don’t compare is about, and it is why we run 24/7 site cameras on every job, so the work that gets buried is on record.

Ice dams FAQ

What causes ice dams on a roof?

Heat escaping from the house into the attic or roof assembly melts snow on the upper roof. That meltwater runs down the slope and refreezes when it reaches the overhang, where there is no heated space below. The ridge of ice that builds up holds standing water, which works its way under the roofing. Air leakage through the ceiling is the single biggest contributor, followed by thin or compressed insulation, mechanical equipment located in a cold attic, and winter sun on dark roofing.

How do you prevent ice dams on a new house?

In this order: seal the ceiling air barrier completely, insulate the ceiling heavily and carry full depth insulation out over the exterior walls using raised-heel trusses, and vent the roof so the deck stays near outside temperature. Then install a self-adhering ice barrier membrane at the eaves and in every valley as protection against the solar melting you cannot design out. Keep ducts and water lines out of the cold attic.

Does a metal roof prevent ice dams?

No, it reduces them. Metal sheds snow well and has no laps in the field of the roof for water to creep under, so ice dams form less often and do less damage when they do. But metal heats up in winter sun, so solar melting is more pronounced, and a metal roof over a leaky ceiling will still ice up at the eave. Metal also needs engineered snow retention over walkways, decks and mechanical equipment, because a full slide is its own hazard.

How far up the roof does ice and water shield have to go?

Section R905.1.2 of the 2018 IRC requires the ice barrier to run from the lowest roof edge to at least 24 inches inside the exterior wall line, measured horizontally. Because the measurement is horizontal, the distance up the slope is longer than people expect: with a 12 inch overhang and 2×6 walls it is roughly 47 inches at 6:12 and about 59 inches at 12:12, so one 36 inch roll does not satisfy the code. On lower-slope roofs it should go further still, far enough to cover 6 to 8 inches of standing water height above the wall insulation below.

Are heat cables worth it for ice dams?

On an existing house with a roof that cannot realistically be fixed, yes, they keep a drain path open and prevent the standing water that causes leaks. On a new custom home, no. A heat cable on a new build means something upstream was not detailed, usually the ceiling air barrier. It costs money to install, money to run every winter, eventually fails, and does nothing about the moist indoor air still being pushed into the attic.

Built different. Built right. Built by SwagerBuilds. If you are planning a custom home in Teton Valley, eastern Idaho or Jackson Hole and want the roof assembly settled while the plan can still change, start with the custom homes page or the FAQ.