Category: Building Science

TL;DR: Spray foam is not automatically the right answer in Idaho, and blown-in is not automatically the cheap answer. Closed-cell spray foam buys you about R-6.5 per inch plus an air seal and a vapor retarder in one pass. Blown-in cellulose or dense-pack buys you more R-value per dollar in a deep cavity. On the custom homes I build in Teton Valley and eastern Idaho, I use both — foam where air sealing and moisture control matter, blown-in where depth is free.

I have been in construction for 20 years, and insulation is the single most common place I see a custom home get quietly cheapened. Nobody walks through a finished house and says “nice cavity fill.” It is buried behind drywall three weeks after it goes in, it never shows up in a listing photo, and it is the one system you cannot fix later without tearing the house apart. So it is a great place to save money if the goal is to win a bid, and a terrible place to save money if the goal is to live in the house.

This post is the honest version of the conversation I have with owners at the design table. No brand pitch, no “foam is always better.” Just what each material does, what Idaho code actually asks for, where I spend the money, and what I would skip.

What insulation does Idaho code actually require?

Idaho enforces the 2018 IECC with state amendments, effective January 1, 2021, per the U.S. Department of Energy’s code status page. Eastern Idaho and Teton Valley sit in climate zone 6. Here is what that means in numbers, and what I build to instead.

Assembly	Idaho code minimum (zone 6)	What I spec on a custom build	Why
Ceiling / attic	R-49	R-60 blown-in, vented attic	Blown depth is nearly free. The only cost is more material and a taller baffle.
Above-grade wall	R-22, or R-13 plus R-5 continuous exterior	R-21 cavity plus R-5 to R-10 continuous exterior	Continuous exterior insulation kills thermal bridging through the studs. Cavity R-value alone does not.
Rim joist / band	Treated as wall	Closed-cell spray foam, 2 to 3 inches	Worst air leak in the house and almost impossible to seal with batts.
Floor over unconditioned space	R-30	R-30 blown-in with a hard air barrier below, or foam	Batts sag, fall and stop touching the subfloor.
Basement wall	R-19, or R-15 continuous	R-15 continuous rigid or closed-cell spray foam on the concrete	Concrete is a moisture source. Fiberglass against it is a mold experiment.
Crawl space wall	R-19, or R-15 continuous	R-15 closed-cell spray foam, sealed crawl	See the basement vs crawl space vs slab breakdown.
Blower-door air leakage	5 ACH50, tested on a 20% sample of each builder’s homes	Every house tested, target under 3 ACH50	A sample requirement is not a quality program.

Two things in that table deserve a flag. First, Idaho’s amendment softened the national wall number. The unamended 2018 IECC asks for R-20 cavity plus R-5 continuous in zone 6; Idaho will take a straight R-22 cavity with no exterior layer. That is legal and it is also the weaker assembly, because every stud is a thermal short circuit straight through the wall.

Second, the blower-door requirement is a sample. Per the same Idaho summary, beginning July 1, 2021 a minimum of 20% of each builder’s homes must be tested. So four out of five houses in this state can be code-compliant on paper without anyone ever measuring whether they leak. If your builder cannot hand you a blower-door number for your specific address, you do not know what you bought.

Spray foam vs blown-in vs batts: what is the real difference?

Insulation does two separate jobs: it slows heat conduction (that is the R-value) and it stops air movement. Most people only shop the first one. Air leakage is what actually makes a house feel cold and drafty at 10 below, and it is where spray foam earns its price.

Material	R per inch	Air seals?	Vapor behavior	Market cost per SF of surface (2026)	Where it belongs
Closed-cell spray foam	~6.5	Yes, fully	Class II vapor retarder at ~1.5 to 2 inches	$3.00 – $5.00	Rim joists, crawl walls, basement walls, unvented roofs, tight spots
Open-cell spray foam	~3.5 – 3.8	Yes	Vapor open, needs a separate retarder in zone 6	$1.50 – $3.50	Interior sound walls, deep cavities with a vapor plan
Dense-pack cellulose	~3.5 – 3.8	Slows air, does not seal	Vapor open, buffers moisture	$1.00 – $2.80	Wall cavities behind a good air barrier
Blown fiberglass (attic)	~2.2 – 2.7	No	Vapor open	$1.00 – $2.80	Vented attic floors, where depth is cheap
Fiberglass batts	~3.1 – 3.4	No	Vapor open	$1.00 – $2.60	Budget cavity fill, installed perfectly or not at all
Mineral wool batts	~4.0 – 4.3	No	Vapor open, does not burn	$1.50 – $3.80	Walls near a fire-exposed exterior, sound separation

Read that table one more time with the air-seal column in mind. A fiberglass batt and a shot of spray foam can hit the same R-value on a label and perform nothing alike, because air moving through a batt carries heat with it and the label test does not have any wind in it. Closed-cell spray foam at R-6.5 per inch, confirmed in GreenBuildingAdvisor’s spray foam primer, is also the only one on that list that air seals, blocks vapor and adds R-value in a single pass.

That is also why it costs the most. You are buying three products.

Which insulation goes where in a mountain house?

The right answer is almost never one material for the whole house. It is foam where a leak or a wet surface would cost you, and cheap R-value everywhere depth is free. Here is how I split it.

Rim joists get closed-cell spray foam, every time. The rim is where the floor framing lands on the foundation — a long, skinny, interrupted band of exposed wood on every exterior wall. In an older house it is the biggest single hole in the thermal envelope. Two to three inches of spray foam seals it and insulates it at once. I have never regretted that money.

Crawl space and basement walls get closed-cell spray foam or rigid board. Concrete is wet. It stays wet for years after the pour, and it wicks ground moisture after that. Put fiberglass against it and you have built a cold, damp sandwich. Building Science Corporation has been publishing this for two decades; their digest on vapor barriers is the plain-English version. Foam against the concrete keeps the wall warm enough that it never hits dew point on the inside.

Above-grade walls get cavity insulation plus exterior continuous insulation. This is the one most Idaho houses skip. A 2×6 wall framed at 16 inches on center is roughly 23% wood by area once you count plates, headers and jack studs. Wood is about R-1.25 per inch. So a quarter of your “R-21 wall” is actually R-7. Adding a continuous layer outside the sheathing — rigid foam or mineral wool board — is the only way to fix that, and it also keeps the sheathing warmer, which is a moisture win. If you want the framing side of this, I wrote up how stack framing works and why it matters.

Attics get blown-in, deep. In a vented attic, the only cost of more insulation is more material. Going from R-49 to R-60 is a rounding error on a custom-home budget. Blown cellulose or blown fiberglass, installed to a marked depth, with baffles at the eaves so the soffit vents stay open and the insulation does not get wind-washed back off the top plate.

Interior walls get mineral wool or open-cell spray foam for sound. Not for energy. For not hearing the laundry room from the primary bedroom. It is one of the cheapest upgrades on the upgrades-worth-the-money list and it gets cut constantly.

Does spray foam save enough on heating to pay for itself?

Honest answer: on R-value alone, usually no. On air sealing, often yes, and the payback is not only in dollars.

Here is the math problem with foam-everything. Going from R-21 to R-30 in a wall cuts that wall’s conductive heat loss by about 30%. Going from R-49 to R-60 in an attic cuts that attic’s loss by about 18%. Those are real but diminishing. Doubling the R-value never halves the bill, because walls and ceilings are only part of the load — windows, air leakage, ventilation and ground contact are the rest. Our triple pane window post runs the same kind of math on glass.

Air leakage is different. It is not diminishing in the same way, because a leaky house at 10 below is not losing heat slowly through an assembly, it is pumping warm air out the top of the building and pulling cold air in at the bottom. Tightening a house from 5 ACH50 to 2 ACH50 changes how it feels, not just what it costs: no cold floors at the exterior walls, no drafts at outlets, no ice damming from warm air leaking into the attic, and far less dust and pollen coming in through the assembly instead of through a filter.

So the way I think about spray foam is not “what is the payback.” It is “where would an air leak cost me comfort, a moisture problem or a callback.” That is a short, specific list, and spray foam goes exactly there.

One more thing worth saying out loud: a tight house needs mechanical ventilation. If you air seal a house properly and do not install a balanced ventilation system — an HRV or ERV — you have built a bottle. That is not an argument against sealing. It is an argument for doing both, and it belongs in the mechanical design, not in a change order. I cover the rest of the envelope-and-mechanical picture in which energy efficient features are actually worth paying for.

What is the catch with spray foam in a cold climate?

There are four, and a good builder talks about all of them before you sign.

1. It hides what is behind it. Once a cavity is full of closed-cell foam, nobody is ever looking at that sheathing again, and water that gets in cannot dry inward. That is fine if the water management outside is right. It is a disaster if it is not. Which is why I care more about the water-resistive barrier and flashing details than about the insulation — see the best water-resistive barrier for cold-climate Idaho builds and my honest Huber ZIP System review.

2. Installation quality varies more than with any other material. Spray foam is mixed on site, in a truck, at whatever temperature the day gives you. Off-ratio foam can shrink, pull away from framing, smell, or fail to cure. There is no way to inspect a bad batch after drywall. You are buying the installer, not the product.

3. Thickness per pass is limited, and the schedule has to respect it. Closed-cell spray foam goes on in controlled lifts with cure time between them. A crew rushing to beat the drywall truck is how you get foam that shrinks off the stud face in year two.

4. The chemistry changed, and so did the pricing. The industry moved closed-cell spray foam off HFC blowing agents onto low-global-warming HFO agents. Per the Spray Polyurethane Foam Alliance, manufacturing HFC closed-cell systems has been banned nationwide since January 1, 2025, and selling or applying older HFC stock ends January 1, 2028. The HFO products also changed spray technique, pass thickness and wait times. Practically, that means a 2026 foam quote is not comparable to a 2023 one, and an installer who has not retrained on the new formulations is a risk. If a bid looks oddly cheap, ask what product and what blowing agent.

Do I need a vapor barrier behind the drywall?

In climate zone 6, yes — some form of one. The IRC requires a Class I or Class II vapor retarder on the interior side of frame walls in climate zones 5, 6, 7, 8 and Marine 4. Kraft-faced batts and vapor-retarder primer are the common Class II answers. Sheet polyethylene is Class I, and I avoid it, because it lets the wall dry in exactly zero directions.

The useful exception: a Class III retarder — ordinary latex paint — is permitted in zone 6 if the wall has either vented cladding over the sheathing, or continuous exterior insulation of at least R-7.5 over a 2×4 wall or R-11.25 over a 2×6 wall. That is the same exterior insulation layer I argued for above, and this is the second reason to install it. Enough warmth outside the sheathing means the sheathing never gets cold enough to condense, so the wall can dry to the inside through the paint.

Where closed-cell spray foam is involved, the foam itself is the vapor retarder. At roughly an inch and a half to two inches it reaches Class II, which is why you do not put poly over it, and why a flash-and-fill assembly — a couple of inches of closed-cell spray foam against the sheathing, cavity filled with cellulose or batts — handles air sealing, vapor control and R-value in one wall without a plastic sheet in it.

A note on terms, since they get mixed up constantly. A vapor retarder slows water vapor diffusing through a material. An air barrier stops air carrying moisture through holes. They are different jobs, and air leakage moves far more water than diffusion ever will. Get the air barrier right first.

Can I insulate at the roofline instead of the attic floor?

Yes, and on a vaulted-ceiling mountain house you often have to. But an unvented roof assembly in zone 6 has a specific code requirement that gets missed, and when it gets missed the sheathing rots.

Under IRC R806.5, if you insulate an unvented roof with air-permeable insulation in the rafter bays — cellulose or batts — you must also install air-impermeable insulation directly above or below the roof sheathing, at a minimum of R-20 in climate zone 6. That R-20 is not there for energy. It is there to keep the underside of the sheathing above dew point so winter moisture does not condense on it. The code table runs R-15 in zone 5, R-20 in zone 6, R-25 in zone 7 and R-30 in zone 8. The same section adds that in zones 5 through 8, any air-impermeable insulation used has to be a Class II vapor retarder or carry a Class II coating in contact with its underside.

The simpler compliance path, and the one I use, is all closed-cell spray foam against the underside of the roof deck to the full required R-value. One material, no condensation plane, no layered calculation to get wrong. It is expensive. On a vault over a great room with 85 to 100 psf of ground snow load sitting on it, it is also the assembly I trust.

If the ceiling is flat, vent the attic and blow it deep. Unvented roofs are for geometry you cannot vent, not a default.

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

The market ranges in the comparison table above are per square foot of insulated surface — wall area, attic area, crawl wall area — not floor area of the house. That distinction trips up almost everyone pricing this themselves. A 4,000 SF house has far more than 4,000 SF of insulated surface once you count walls, attic, rim and foundation.

What those market numbers do not include:

• Air sealing before insulation. Caulking and foaming every top plate, penetration, bottom plate and window buck is labor that happens before any insulator shows up. It is a line item and it is the one that moves the blower-door number.
• Exterior continuous insulation. Rigid or mineral wool board outside the sheathing also means longer fasteners, furring strips, deeper window bucks and different trim details. It is a wall-assembly decision, not an insulation upgrade.
• Mechanical ventilation. The HRV or ERV a tight house needs, plus its ducting and commissioning.
• Baffles, dams and blocking. Eave baffles, attic access dams, fire blocking, and the barriers around chimneys and flues where foam has to stay clear of heat sources.
• Blower-door testing. Pay for it on every house, not on a 20% sample.

For whole-project budgeting: a SwagerBuilds custom home in Teton Valley and eastern Idaho starts at $400 per square foot and goes up, and in Jackson Hole starts at $1,000 per square foot and goes up. Where a specific build lands depends on the site, the design and the finish level, which is exactly what the design phase is for. Design work is covered by a fixed design fee, quoted per project. If you want the regional cost picture, it is in what you actually pay per square foot in Teton Valley.

What do I actually spec on a SwagerBuilds house?

Here is the default, which gets adjusted for the site and the design but rarely by much:

Where	What goes in	Instead of
Rim joists	2–3 in. closed-cell spray foam	Cut-and-stuff batts
Crawl / basement walls	R-15 closed-cell spray foam or rigid board on the concrete	Fiberglass against concrete
Above-grade walls	R-21 cavity plus R-5 to R-10 continuous exterior	R-22 cavity only
Vented attic	R-60 blown-in, baffled eaves, sealed penetrations	R-49 and call it good
Vaults / unvented roof	Full-depth closed-cell spray foam to code R-value	Batts with an undersized foam layer
Interior partitions	Mineral wool at bedrooms, baths, laundry, mechanical	Empty cavities
Verification	Blower door on every house, number given to the owner	Sample testing

Where a house is in a wildfire-exposed spot, mineral wool moves from the interior partitions to the exterior walls and the eaves, because it does not burn — that reasoning is in wildfire resistant home construction in Idaho. Where we are sealing a crawl space tight, the radon rough-in goes in at the same time, because both are cheap during construction and expensive after; that is the radon mitigation post. And if you are weighing certification, whether a green custom home is worth it covers what the labels actually buy.

I am involved in every phase of every build, and insulation day is one I show up for. Not because I am holding the gun — the foam crews and insulators do that work, and the good ones are better at it than I would be — but because that is the last day the wall is open. Once drywall is on, every shortcut is permanent.

How do I know the installer did it right?

Four checks, and you can do all of them yourself.

Walk the house before drywall. Batts should be cut to fit, split around wiring rather than crushed over it, and fully touching the back of the sheathing with no gaps at the edges. Blown attic insulation should be level to a marked depth ruler, not drifted. Spray foam should be trimmed flush to the studs with no voids behind it and no shrinkage gaps at the framing.

Ask for the blower-door result for your address. A number, in ACH50, on paper. If the answer is that the builder tests a sample of houses, you are not getting a measurement of yours.

Ask what the insulation inspection found. There is a code inspection for this, before cover. It is in the normal sequence I lay out in what inspections happen when you build a custom home.

Ask for the product data sheets. Brand, R-value per inch, installed thickness, and for foam, the blowing agent. A builder who cannot produce that is guessing, and guessing is how you end up with a wall that misses the vapor rule.

The broader point: insulation is a trust question, not a product question, which is the same argument I make in whether a premium custom home builder is worth it. You cannot verify it after the fact, so you verify the builder before.

Frequently asked questions about spray foam and insulation

Is spray foam worth it for a whole house in Idaho?
Usually not for every cavity. Closed-cell spray foam is worth it at rim joists, crawl and basement walls, unvented roofs and hard-to-seal details, where it buys air sealing and moisture control you cannot get another way. For open wall cavities and vented attics, blown-in insulation behind a good air barrier gets you the same R-value for less money, and the savings are better spent on continuous exterior insulation and windows.

How much insulation does Idaho code require?
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. Blower-door testing is required on a minimum 20% sample of each builder’s homes rather than every house.

Can spray foam cause moisture problems in a cold climate?
Closed-cell spray foam itself is a vapor retarder, so it does not get wet. The risk is that it prevents a wet assembly from drying inward, so if water gets past the siding and water-resistive barrier it has nowhere to go. That makes exterior water management and flashing more important on a foamed wall, not less. Open-cell foam is vapor open and needs a separate interior vapor retarder in climate zone 6.

What R-value do I need for an unvented vaulted ceiling?
Under IRC R806.5, if air-permeable insulation fills the rafter bays, you also need air-impermeable insulation above or below the roof sheathing at a minimum of R-20 in climate zone 6 for condensation control. Zone 5 is R-15, zone 7 is R-25 and zone 8 is R-30. The simpler path is all closed-cell spray foam against the roof deck to the full required R-value.

Does a tight house need mechanical ventilation?
Yes. Once a house is air sealed, it cannot rely on leaks to bring in fresh air, so a balanced heat-recovery or energy-recovery ventilator belongs in the mechanical design from the start. Air sealing without ventilation is how you get stale air and high indoor humidity. Budget both together, not as a change order.

Want this done right on your build?

If you are planning a custom home in Teton Valley, Jackson Hole or eastern Idaho and want the envelope engineered rather than bid to the lowest number, that conversation happens in the design phase — plans, structural and mechanical engineering, and the assembly decisions all together, before anyone quotes construction. SwagerBuilds handles that whole path in-house, from drawings through engineering coordination to keys. Call (208) 520-0636 or tell me about your project.

Built different. Built right. Built by SwagerBuilds.

TL;DR: Triple pane windows are worth it in Idaho on the walls you actually live against — big view glass, north and west exposures, bedrooms, and any room with a seat next to the window. On small, sheltered openings the upgrade buys you very little. The comfort gain shows up immediately; the energy savings take years. Decide it on the window schedule during design, not after the walls are framed.

I have been building in eastern Idaho and Teton Valley for 20 years, and windows are the single most argued-over line on a custom home. They are also the one thing on the drawings that is permanent. You can change a countertop in year six. You are not re-glazing a 30-foot view wall because the room turned out cold.

So here is the honest version of the triple pane question, with the numbers that actually drive it and the places where I tell clients to save their money.

What are triple pane windows, and how are they different from double pane?

A double pane window has two sheets of glass with a sealed gap between them, usually filled with argon gas. A triple pane window adds a third sheet and a second gap. That is the whole idea: one more layer of still gas between you and a January morning.

The third pane is not really about the glass. It is about the two insulating cavities and the extra low-emissivity coating, a microscopically thin metal layer that reflects heat back toward the side it came from. More cavities and more coatings mean less heat leaving the room.

The trade-off is weight. A triple pane sash can run half again as heavy as a double, which is why the hardware, the hinges and the frame have to be built for it. This is also why a cheap triple pane window from a manufacturer that mostly builds doubles is often worse than a good double.

Are triple pane windows worth it in Idaho? The short answer

Yes on the glass you live against. No on the glass you walk past.

Teton Valley, Driggs, Victor, Tetonia, Rexburg and most of the upper Snake River plain sit in climate zone 6, the second-coldest zone in the lower 48. Design temperatures here run well below zero. In that climate the difference between a code-minimum window and a good triple pane window is not an abstraction on a utility bill. It is whether the sofa under the window is usable in February.

The reason is surface temperature. A window’s interior glass surface sits somewhere between room temperature and outdoor temperature depending on how well it insulates. On a code-minimum double pane unit at 10 below, that inside surface gets cold enough that you feel your own body heat radiating toward it. People describe it as a draft. There is no draft. You are the draft.

A good triple pane window raises that interior surface temperature by a meaningful margin. The room feels even, the cold pool along the floor under the window mostly disappears, and the condensation that fogs the bottom corners on the coldest mornings stops showing up. That is the real product you are buying. The heating savings are a bonus that arrives slowly.

What do the numbers on a window sticker actually mean?

Every window carries a label from the National Fenestration Rating Council with four or five numbers on it. Manufacturers will quote you the one that flatters them. Here is what each one does.

U-factor is how fast heat escapes through the whole window, frame included. Lower is better. This is the number that matters most in a cold climate, and it is the one code regulates.

Solar Heat Gain Coefficient (SHGC) is how much of the sun’s heat comes through, on a 0 to 1 scale. In hot climates you want it low. In zone 6 you generally want it reasonably high on south-facing glass, because free winter sun is worth having, and lower on west glass, which cooks in July afternoons.

Visible Transmittance (VT) is how much light comes through. Stacking low-e coatings to chase a low U-factor can drag VT down, and a house full of gray-looking glass is a permanent mistake. I would rather give up a couple hundredths of U-factor than live behind tinted-looking windows with a Teton view on the other side.

Air Leakage is how much air the assembly passes. Lower is better and almost nobody checks it.

Condensation Resistance runs 1 to 100, higher is better, and it is the number I actually care about in Teton Valley. It predicts whether the bottom of your glass will sweat and eventually stain the stool and jamb.

What does Idaho code require for windows?

Idaho enforces the 2018 IECC with state amendments, effective January 1, 2021, per the U.S. Department of Energy’s state code status page. In climate zone 6, the prescriptive path caps window U-factor at 0.30 and does not regulate SHGC.

Read that carefully: U-0.30 is the worst window you are legally allowed to install. It is a floor, not a target. A decent double pane unit clears it. Nothing about meeting code means the house will feel good.

For comparison, ENERGY STAR Version 7.0 sets the Northern zone bar at U-0.22 or lower with an SHGC of 0.17 or higher, with equivalent-performance paths that allow a slightly higher U-factor in exchange for more solar gain. You can read the current criteria on the ENERGY STAR windows and doors page. Most cold-climate builders specifying triple pane windows land somewhere in the U-0.17 to U-0.26 range documented by GreenBuildingAdvisor.

So the honest gap is this: code says 0.30, a good triple pane window says roughly 0.20, and the difference between those two numbers is about a third of the heat loss through every square foot of glass in the house.

Where the upgrade pays for itself, and where it doesn’t

I do not spec triple pane windows across the whole house on every build, and any builder who tells you it is all-or-nothing is selling a package rather than solving your problem. Here is how I sort it.

Opening	My call	Why
Great room view wall, large fixed glass	Triple, every time	Biggest area, most heat loss, and it’s where people sit. This is the whole ballgame.
North and west elevations	Triple	North gets no solar gain to offset the loss. West takes the wind and the winter weather.
Bedrooms, especially on an exposed corner	Triple	People sleep next to this glass. Cold radiant surfaces wreck sleep more than cold air does.
South-facing glass under a deep overhang	Either, lean triple if the opening is large	Winter sun offsets some loss, so a high-SHGC double can pencil. Size decides it.
Small bath, pantry, laundry, stair windows	Double is fine	Little area, nobody lingers. Spend the money elsewhere.
Garage and unconditioned space	Double	Not heated. Don’t overthink it.
Egress windows in a basement bedroom	Size for code first, glazing second	The opening dimensions are non-negotiable. See our Idaho egress window requirements.

The pattern is simple. Triple pane windows earn their money where the glass is big, the exposure is harsh, or a human being sits still nearby. Everywhere else, a well-built double pane unit with a good frame and a clean install does the job.

Double pane vs triple pane: a side-by-side

	Code-minimum double	High-performance double	Triple pane
Typical U-factor	Around 0.30	Roughly 0.24 to 0.28	Roughly 0.17 to 0.26
Meets Idaho zone 6 code	Yes, barely	Yes	Yes, with room to spare
Meets ENERGY STAR v7 Northern	No	Sometimes, via the equivalent-performance path	Usually
Interior glass temperature at 10 below	Coldest	Noticeably better	Best by a clear margin
Condensation on cold mornings	Common at bottom corners	Occasional	Rare
Sound from the road or the wind	Baseline	Slightly better	Better, especially with uneven glass thicknesses
Sash weight and hardware demand	Standard	Standard	Heavier, needs hardware built for it
Visible light	Highest	High	Slightly lower, varies by coating package
Where I use it	Nowhere by choice	Small and sheltered openings	View walls, bedrooms, north and west

Notice the middle column. A high-performance double pane window from a manufacturer that builds serious cold-climate units will beat a bargain triple from a company that bolted a third pane onto a builder-grade frame. The label on the box is not the spec. The NFRC number is the spec.

What does the triple pane upgrade cost?

I am not going to hand you a per-window number, because it would be wrong by the time you read it and wrong again for your particular window schedule. What I can tell you is how the cost behaves.

The upgrade is priced as a percentage of the window package, not the house. Window packages scale with glass area, so a house with a 30-foot view wall and a wall of sliders feels this decision hard, and a house with conventional punched openings barely notices it. As one data point, a builder case study published on GreenBuildingAdvisor put the triple pane upgrade at roughly $4,100 on a $30,000 window order, about 14 percent. Treat that as third-party market data from another project, not a SwagerBuilds quote.

Three things move the number more than the third pane does:

• Frame material and manufacturer tier. The jump from a mid-tier line to a premium line usually costs more than the jump from double to triple within the same line.
• Glass area. Fixed picture units are cheap per square foot. Operable units, big sliders and lift-and-slide doors are not.
• Custom sizes and shapes. Every non-standard unit carries a premium, and mountain-modern elevations are full of them.

On a SwagerBuilds project the window schedule gets priced inside the design agreement, before you sign a construction contract, so the glazing decision is made against a real number instead of a guess. Our construction pricing starts at $400 a square foot and goes up on the Teton Valley and Idaho side, and starts at $1,000 a square foot and goes up in Jackson Hole. Design itself is a fixed design fee, quoted per project. For how the window allowance behaves inside a contract, see how construction allowances work and why three bids don’t compare. The window line is one of the most common places two bids quietly differ.

Does the frame matter more than the glass?

Often, yes. The U-factor on the NFRC label is a whole-window number, which means a great glass unit in a bad frame reports a mediocre result. Here is how the common frame materials behave in our climate.

Frame	Thermal performance	What to watch
Fiberglass	Excellent	Expands and contracts at nearly the same rate as glass, so seals last. My default for harsh exposures.
Vinyl	Good	Fine thermally, but wide frames eat glass area and cheap lines sag on large units. Quality varies enormously.
Aluminum-clad wood	Good	The look most Teton Valley clients want. Requires disciplined flashing, since the cladding hides what is happening underneath.
Wood, unclad	Good	Beautiful inside. Exterior maintenance in this climate is real work. I steer clients away for exposed elevations.
Thermally broken aluminum	Fair to good	Works for big modern openings where nothing else spans. Verify the thermal break is rated, not decorative.
Standard aluminum	Poor	Conducts cold straight through. Not a zone 6 product.

Frame choice also interacts with the wall. On a house with continuous exterior insulation, the window has to be set out from the structural sheathing, which changes the flashing detail and the buck construction. That is a drawing decision, not a field decision. We work it out with the wall assembly during design, alongside the weather-resistive barrier and the siding system.

Is there still a tax credit for efficient windows in 2026?

No. The section 25C energy efficient home improvement credit, which covered up to $600 a year for qualifying windows, was terminated for property placed in service after December 31, 2025. The IRS spells this out in its FAQ on the 2025 law changes. The related section 45L new-home credit ended for homes acquired after June 30, 2026.

So if you were counting on a federal credit to close the gap on triple pane windows, that math is gone. Decide the upgrade on comfort and on how long you plan to own the house, not on a rebate. We covered the same shift in our post on whether a green custom home is worth it.

What goes wrong with windows on a mountain build?

In 20 years the failures I see almost never trace back to the glass package. They trace back to four things.

Installation. A premium window set in a sloppy rough opening is a premium leak. The sill pan, the back dam, the sequencing of the flashing against the housewrap, and the air seal on the interior side all have to be right, in that order. Sequencing is what separates a dry wall from a warranty claim in year four.

Snow and grade. Window sills set too low relative to the finished grade and the drift line get buried. Buried sills stay wet. We set sill heights against the actual snow reality on that lot, not against the plan’s elevation drawing.

Interior humidity. A tight house with no balanced ventilation will condense on its windows no matter how good they are. If a house is built to modern airtightness and gets no mechanical fresh air, the glass becomes the coldest surface and takes all the moisture. That is a ventilation problem wearing a window costume. See the ventilation section in our energy efficient custom home features post.

Ordering late. Window lead times on custom sizes and premium lines routinely run months. A window order placed after framing starts is how a house sits wrapped in plastic through a Teton Valley November. The schedule is set by the longest lead item, and it is usually the windows.

How I spec windows on a custom home

The window schedule is settled during the design agreement, before anybody signs a construction contract. That is deliberate. Glazing drives the heating load, the heating load drives the mechanical design, and the mechanical design drives duct routes and soffits. Change the windows in month four of construction and you are re-engineering three other systems.

Here is the order I work in:

1. Orientation and views first. Where the glass goes is an architecture decision. We handle plan development and drawings in-house, so this gets resolved on paper with revisions in days, not weeks.
2. Size and quantity second. More glass than the house needs is the most expensive form of regret. I will push back on a window that exists only because the elevation looked bare.
3. Frame material third. Driven by exposure, maintenance appetite and the look the client is after.
4. Glazing package last. This is where triple pane windows get assigned opening by opening, using the table above rather than a blanket rule.
5. Install detail documented. The flashing sequence goes on the drawings and gets verified in the field, not left to whoever is on the wall that day.

Then it gets priced and locked into a fixed construction contract. No allowance games on the window line, because that line is too big to leave floating. If you want to see how the whole design phase runs, we wrote it up in getting a real number before you sign, and the broader upgrade question in which custom home upgrades are actually worth the money.

If you are planning a build in Teton Valley, Driggs, Victor, Rexburg, Rigby or the Idaho Falls area and want the window package sorted before it becomes a change order, start the conversation here. Built different. Built right. Built by SwagerBuilds.

Frequently asked questions

Are triple pane windows worth it in Teton Valley?
On large glass, north and west exposures, and bedrooms, yes. Those are the openings where the colder interior glass surface of a double pane unit is something you feel every winter. On small, sheltered windows the upgrade is hard to justify and the money does more good in insulation or air sealing.

How much energy do triple pane windows actually save?
Less than most people expect, and the payback on energy alone is usually measured in decades rather than years. The reason to buy them in climate zone 6 is comfort, even room temperatures and the end of winter condensation. Treat the utility savings as a small bonus.

What U-factor should windows have in Idaho?
Idaho’s 2018 IECC-based code caps zone 6 windows at U-0.30, which is a legal minimum rather than a good target. ENERGY STAR Version 7.0 asks for U-0.22 or better in the Northern zone, and most cold-climate triple pane windows land between roughly 0.17 and 0.26.

Do triple pane windows stop condensation?
They dramatically reduce it, because the interior glass stays warmer. They do not fix a house with no balanced ventilation and high indoor humidity. If a tight home has no HRV or ERV moving fresh air, moisture will still find the coldest surface in the room.

Can I mix double and triple pane windows in the same house?
Yes, and on most of my builds we do. Manufacturers build both within the same product line, so the sightlines, colors and hardware match. Put the triple pane windows where the glass is large or the exposure is harsh, and spend the savings on the openings nobody sits beside.

TL;DR: Basement vs crawl space vs slab is really a question about three things: how deep your footings have to go anyway, what the dirt under your lot will let you do, and how much usable square footage you want for the money. In Teton Valley and most of Eastern Idaho the frost line already forces the concrete most of the way down, so a basement is cheaper here, relative to the alternatives, than it is almost anywhere else in the country. That is why I build basements on most custom homes, conditioned crawl spaces when the water table or the rock says no, and slab-on-grade only in a handful of specific situations.

Basement vs crawl space vs slab: which one should you build?

Here is the short version, then the reasoning. On a custom home in Teton Valley, Driggs, Victor, Rexburg, Rigby or Idaho Falls, a full basement is usually the best dollar-per-square-foot buy on the whole project. A conditioned crawl space is the right answer when groundwater, rock or a steep lot makes a basement expensive or impossible. A slab-on-grade is the right answer on a small number of lots where the soil is good, the water is high, and the client genuinely does not want the extra space.

What makes the basement vs crawl space debate different in Idaho than it is in Texas or Arizona is that we are not choosing whether to put concrete in the ground. The code already decided that. We are only choosing what to do with the hole we are required to dig.

What does frost depth have to do with my foundation?

Frost depth is the depth the ground can freeze to in a hard winter. Footings have to sit below it, because soil that freezes swells, and soil that swells lifts concrete. Lift a footing an inch and you crack a foundation wall, rack a door, and split drywall two floors up.

Two real numbers from the counties we build in. Teton County, Idaho requires 32 inches measured from the bottom of the footing to finished grade, along with a ground snow load of 85 psf below 6,600 feet and 100 psf at 6,600 feet and above, seismic design category D-1, and 115 mph wind. The City of Idaho Falls publishes 30 inches from finished grade to the bottom of the footing, a ground snow load of 47.1 psf derived from the city’s 4,710-foot elevation, seismic design category D when soils are unknown, and a 115 mph ultimate wind speed.

Read that again with a tape measure in your head. You are already digging two and a half to three feet down and pouring a footing and a stem wall to get there. A crawl space adds a little more depth. A basement takes that same wall to roughly eight feet and gives you a floor. The concrete and the excavator are already on site either way, and that is the entire economic argument for a basement in this climate.

What does each foundation actually cost?

I am not going to hand you a per-square-foot menu for a SwagerBuilds foundation, because the foundation is not sold separately. A SwagerBuilds custom home starts at $400 per square foot and goes up in Teton Valley and Eastern Idaho, and starts at $1,000 per square foot and goes up in Jackson Hole. What the foundation does is move where you land inside that.

What I can give you is third-party market data so you can see the relative spread. These are national figures published by HomeGuide for 2026, not SwagerBuilds pricing, and they run low for mountain work with engineered footings and deep frost protection:

Foundation type	Market cost range (national, 2026)	Usable space added	Where it fits in Eastern Idaho
Monolithic slab	$6 to $14 per sq ft	None	High water table, flat lot, single-level program
Stem wall with slab	$6 to $18 per sq ft	None	Where frost depth is required but no under-floor space is wanted
Crawl space	$15 to $18 per sq ft	Mechanical and storage only	Rock, high groundwater, steep or tight sites
Full basement, unfinished	$35 to $50+ per sq ft	Full floor of the house	Most Teton Valley and Eastern Idaho custom homes
Finishing a basement later	Adds $30 to $50 per sq ft	Converts storage to living space	Common phase-two move

Here is the part most builders skip. Compare the basement line to the whole-house number, not to the crawl space line. If a finished square foot of your main floor is priced from $400 and goes up, and unfinished basement square footage lands in the range above with finishing later in its own range, the basement is the cheapest square footage you will ever add to that house. It is the only place on the project where you buy floor area at a discount instead of a premium.

For how the rest of the number comes together, see our full breakdown of what it costs to build a custom home in Idaho.

When does a basement make sense in Idaho?

Most of the time, here. The frost depth argument above does most of the work, but there are three more reasons I push clients toward a basement when the site allows it.

Mechanical space. A mountain house at 6,000 feet carries a lot of equipment: boiler or furnace, hot water, HRV or ERV, water treatment if you are on a well, a snowmelt manifold if you have one. Putting that in a basement room instead of a closet on the main floor buys back main-floor square footage you were paying full price for.

Guest and kid space you do not heat year round. A basement stays within a few degrees of ground temperature on its own. It is the cheapest space in the house to condition and the easiest to shut down when nobody is in it, which matters a lot on second homes.

Walkout potential. On any lot with grade, a walkout basement turns below-grade square footage into real daylight living space with windows and a door. That is the single best value move in mountain custom home building and it costs almost nothing extra when the slope is already there.

One rule you cannot skip: any basement bedroom needs a compliant emergency escape opening. We cover the sizes and sill heights in our guide to egress window requirements in Idaho. Plan the window wells at design, not at framing, because retrofitting one through a finished foundation wall is ugly and expensive.

When is a crawl space the right call?

Four situations put us in a crawl space instead of a basement: groundwater within a few feet of the planned basement floor, rock or hardpan that turns excavation into blasting, a lot too steep or too tight to get the dirt out economically, and a client who simply does not want the space and will not pay for it.

When we do build one, it is a closed, conditioned crawl space, not a vented one. This is not a preference, it is building science. Building Science Corporation’s work on vented versus closed crawl spaces lays out the failure mode clearly: put fiberglass batts between the floor joists and vent the space, and the underside of that insulation ends up within a degree or two of ground temperature. Humid air enters through the vents, hits those cold surfaces, and condenses. Wood has to stay below 19 percent moisture content to never rot and below 16 percent to never grow mold, and a vented crawl space in a humid stretch does not reliably hold either line.

A conditioned crawl space is built as a short basement instead. Sealed vapor barrier across the whole soil floor and up the walls. Insulation on the perimeter walls, not in the floor joists. The space is inside the building envelope and gets air from the house. It stays dry, the ductwork and pipes in it are inside conditioned space, and the floor above it is not cold in February.

A crawl space still changes how the floor system gets designed. The span, the beam layout and the post locations all shift depending on what is under them, which is why we settle it in design. See our comparison of TJI joists versus floor trusses for how that plays out.

When does a slab-on-grade actually work here?

Rarely, and I will tell you straight that I talk most clients out of it in Teton Valley. A slab makes sense when the water table is genuinely high, when the program is single-level and the client wants it that way, when the lot is flat enough that a slab does not require a mountain of imported fill, and when radiant floor heat is central to the design. A slab is a very good host for radiant tubing.

What you give up is everything below the floor. Every mechanical run, every plumbing line, every future change is now in the concrete or in the walls. Changing a drain line after the pour means a saw and a jackhammer. On a house you intend to keep for thirty years, that lack of access is the real cost, not the concrete.

And in a climate with a 30 to 32 inch frost requirement, a slab does not save you the footing. You still pour a frost wall around the perimeter. You pay for most of the depth and get none of the space.

Does the basement vs crawl space choice change my radon plan?

Yes, and this is where the basement vs crawl space decision gets practical. Eastern Idaho and Teton Valley sit in an area where elevated indoor radon is common, and the fix is dramatically cheaper before the concrete goes down than after.

On a basement, a passive radon rough-in means a layer of clean gravel under the slab, a sealed membrane, a tee in the gravel, and a pipe stubbed up through the roof. On a conditioned crawl space, the sealed soil vapor barrier you are installing for moisture control is most of the radon system already, and the pipe ties into the same space beneath it. On a slab, it is the same sub-slab approach as a basement. In all three, the rough-in happens during the foundation stage or it does not happen cheaply at all. We walk through the whole thing in our post on radon mitigation in a new Idaho home.

The same logic runs through the rest of the envelope. Foundation insulation, under-slab insulation and the air seal at the sill plate are all decisions made at this stage and expensive to revisit, which is why they show up in our list of energy efficient custom home features worth paying for.

What does the foundation do to resale?

In this market, a finished or finishable basement is an asset buyers look for and a crawl space is neutral. I have never had a buyer’s agent flag a conditioned crawl space as a problem when it is dry and properly built. I have had plenty of buyers in Teton Valley skip a house because there was nowhere to put gear, guests or a shop bench.

A slab is the one that costs you on resale here, and not because buyers dislike slabs in the abstract. It is because a $1M to $5M mountain home without any below-grade space reads as a smaller house than it is. The square footage is all priced at finished rates and there is no cheap space to absorb the overflow.

That said, do not build a basement you do not want purely for resale. If the program genuinely does not need it and the lot fights you, the right answer is the crawl space. We work through this kind of trade-off in our post on which custom home upgrades are actually worth the money.

What is not included in a foundation number?

Any time you see a foundation figure, from us or anyone else, ask what sits outside it. On a typical Eastern Idaho or Teton Valley lot, these are the items that live next to the foundation line and get forgotten:

• A geotechnical soils report where the county or the engineer requires one, and any engineered fill or over-excavation it calls for.
• Rock excavation, which is priced separately for a reason, and hauling spoils off site when there is nowhere to spread them.
• Foundation drainage: perimeter drain, filter fabric, drain rock, and somewhere for the water to actually go.
• Damp-proofing or full waterproofing on the exterior of a basement wall, which are not the same product or the same price.
• Under-slab insulation, under-slab vapor barrier, and the radon rough-in described above.
• Window wells, well covers and their drainage on any basement bedroom.
• Backfill, final grade away from the house, and the driveway and utility trenching that has to be coordinated with it.

Site work is the single most under-budgeted line on custom home projects in this valley. If your lot is raw, read our guide to how to prepare land to build a house and our numbers on well and septic costs in rural Idaho before you set a budget.

How we make this call on a SwagerBuilds build

We settle the foundation during the design agreement, before there is a construction price to sign. That order matters. The foundation drives the floor plan, the floor plan drives the structure, and the structure drives the number. Deciding this after plans are drawn means redrawing plans.

The sequence looks like this. We walk the lot and look at grade, drainage and where the water goes in spring. We get soils information when the site or the county calls for it. We coordinate the structural engineer on footing sizes for snow load and seismic category, which in Teton County means designing to 85 or 100 psf ground snow and seismic D-1. We draw the foundation with the plan set, not after it. Then we price the build against a foundation that is actually designed rather than assumed.

I have been in construction for 20 years and I am involved in every phase of every build, from that first walk on the dirt through the final walkthrough. We handle the plan development, the architectural drawings, the engineering coordination and the permitting in-house or through people I have worked with for years, under a fixed design fee quoted per project. You do not have to go assemble a team to answer this question. That is our job.

Every SwagerBuilds job also runs 24/7 site cameras, so you can watch the footings go in from wherever you are. For what happens after the concrete, see our walkthrough of the inspections on a custom home build, and if you are still at the plan stage, start with custom house plans for Teton Valley and Eastern Idaho.

Thinking about a build in Teton Valley, Jackson Hole or Eastern Idaho? Bring me your lot and your program and I will tell you straight which foundation it wants and why. Call SwagerBuilds at (208) 520-0636 or apply to work with us.

Built different. Built right. Built by SwagerBuilds.

Frequently asked questions

Is a basement cheaper than a crawl space in Idaho?

Not per square foot of foundation, but it is cheaper per square foot of house. Because Idaho frost depth requirements already push footings 30 to 32 inches below grade, most of the excavation and the concrete wall are required regardless. A basement extends that wall and adds a floor, so you buy usable square footage at a fraction of what finished main-floor space costs.

Do I need a vapor barrier in a crawl space in Idaho?

Yes. We build closed, conditioned crawl spaces with a sealed vapor barrier across the entire soil floor and lapped up the walls, with insulation on the perimeter walls rather than in the floor joists. A vented crawl space with batts between the joists is the assembly that rots floor framing in this climate.

Can you build a basement if the water table is high?

Sometimes, with engineered drainage and full waterproofing, but it gets expensive fast and the risk never goes to zero. When groundwater sits close to the planned basement floor, we usually recommend a conditioned crawl space instead. That decision is made from actual site information, not a guess, which is why we look at soils and spring drainage during design.

How deep do footings have to be in Teton Valley?

Teton County, Idaho requires 32 inches measured from the bottom of the footing to finished grade, or as determined by a soils investigation. Idaho Falls requires 30 inches. Both counties also carry significant snow and seismic loads, so footing size is engineered per project rather than taken from a table.

Does a slab foundation hurt resale value on a custom home?

In the Teton Valley and Eastern Idaho market it usually does, not because buyers object to slabs but because a home with no below-grade space offers no inexpensive storage, mechanical or guest area. Buyers in this price range expect somewhere to put gear. On a flat lot with a high water table and a single-level program, a slab can still be the right build.

Short answer: radon mitigation is not required by code in Idaho, but two out of five Idaho homes test above the EPA action level, so I rough in a passive radon system on every house I build. It is cheap while the foundation is open and expensive once the slab is poured. Test after you move in, then add a fan only if the number says to.

What is radon, and why should I care in Idaho?

Radon is a radioactive gas that comes out of the ground as uranium in soil and rock breaks down. You can’t see it, smell it, or taste it. It seeps up through soil and collects inside a house, and the only way to know your number is to test for it.

It matters because the Idaho Department of Health and Welfare reports that two out of five Idaho homes have elevated radon levels. That is not a scare statistic from a company selling fans. That is the state health department describing the ground we build on.

The EPA’s action level is 4.0 pCi/L. At 4.0 or above, you fix it. Between 2.0 and 3.9, the state says consider fixing it. Below 2.0, you’re in good shape. Those are the only three numbers you need to hold onto.

Do I need radon mitigation in a new home?

Nobody can tell you that you need radon mitigation before the house exists, because radon is measured in a finished, closed-up building. A soil test on an empty lot won’t give you a reliable answer. Anyone who quotes you a mitigation system off a vacant lot is guessing.

So the honest answer is this: you don’t know yet, and that’s exactly why you rough it in. The parts of a radon system that are painful to add later are the parts that go in while the ground is open and the slab isn’t poured. The part you might never need is a fan, and a fan takes an afternoon.

That’s the whole decision. Spend a little now so that the expensive version never becomes necessary. I’ve been in construction 20 years, and this is one of a very short list of items where the build-stage version and the retrofit version aren’t even close in cost or in quality of result.

Is radon actually a problem in Teton Valley and eastern Idaho?

Yes, and the EPA’s own mapping says so. The EPA Map of Radon Zones sorts every county into Zone 1 (highest predicted potential), Zone 2, or Zone 3. Here’s how the counties I build in land:

County	EPA radon zone	Towns I build in
Teton	Zone 1 (highest)	Driggs, Victor, Tetonia
Madison	Zone 1 (highest)	Rexburg
Bonneville	Zone 1 (highest)	Idaho Falls, Ammon
Fremont	Zone 1 (highest)	Island Park, St. Anthony
Bingham	Zone 1 (highest)	Shelley, Blackfoot
Jefferson	Zone 2	Rigby, Menan

Read that table again. Almost every county in my service area is in the EPA’s highest radon zone. Our own shop sits in Jefferson County, which is Zone 2, and I still treat Zone 2 as a reason to rough in rather than a reason to skip it.

One thing to keep straight: a zone is a prediction about geology, not a measurement of your house. Two houses on the same road can test very differently depending on how the soil sits, how the foundation was built, and how tight the house is. The zone tells you the odds are worth taking seriously. Only a test tells you your number.

What does a radon system look like in a house that isn’t built yet?

It’s simpler than most people picture. There’s no machinery in your living space and nothing you interact with day to day. A passive rough-in is four things, and all four happen in the ground and in the walls before drywall:

• A gas-permeable layer under the slab. Clean coarse gravel under the concrete, so gas can move sideways to one collection point instead of sitting under the whole floor.
• A sealed barrier. Heavy poly sheeting over that gravel, lapped and sealed, plus caulking at the joints and penetrations where the slab meets walls and pipes.
• A vent pipe from under the slab to above the roof. Usually 3 or 4 inch PVC, teed into the gravel layer, run up through an interior wall chase and out the roof, labeled as a radon pipe at every floor.
• Power roughed in at the pipe, in the attic. An electrical junction near the vertical pipe so a fan can be added later without opening finished walls.

That’s the passive system. It works on stack effect, meaning the warm air rising in the pipe pulls soil gas out from under the slab on its own, with no fan. Sometimes that alone drops a house below the action level. If it doesn’t, the fan goes on the pipe in the attic and the system becomes active.

The EPA publishes builder drawings for exactly this, including a crawlspace version, in its radon-resistant new construction resources. The current standards it points to are ANSI/AARST CCAH-2020 for reducing radon in new homes and ANSI/AARST RRNC-2020 for roughing in the components, both effective December 1, 2023.

Crawlspace instead of a slab? Same idea, different execution. The poly goes over the crawlspace floor as a sealed membrane and the pipe pulls from under it. If you’re weighing basement against crawlspace, the radon rough-in is not the deciding factor, but it is cheaper and cleaner on a slab or a basement than on a vented crawl.

What does radon mitigation cost during the build versus after?

Here are the published figures, and I want to be clear that these are market and agency numbers, not a SwagerBuilds price list. The Idaho Department of Health and Welfare puts radon-resistant features at $300 to $500 when they go in during construction. A professionally installed system in an existing house runs $1,500 to $3,500. A do-it-yourself retrofit kit lands at $300 to $600.

Approach	When it happens	Published cost range (market data)	What you’re buying
Passive rough-in during construction	Foundation and framing	$300–$500 (Idaho DHW)	Gravel layer, sealed poly, vent pipe, power at the pipe. Hidden in the structure.
Adding a fan to an existing rough-in	After testing, any time	Fan and labor, an afternoon of work	Turns the passive system active. No finished surfaces touched.
Professional retrofit, no rough-in	After the house is finished	$1,500–$3,500 (Idaho DHW)	Core through the slab, exterior pipe, fan. Visible pipe outside.
DIY retrofit kit	After the house is finished	$300–$600 (Idaho DHW)	Homeowner labor and no performance testing unless you hire it out.

The gap in that table is the entire argument. Roughing in during construction is the cheapest line on the list, and it’s also the only one that leaves no pipe running up the outside of your house. A retrofit on a finished home means coring the slab, routing pipe along an exterior wall, and living with how that looks on an elevation you paid to get right.

For context on where this sits in a whole project: SwagerBuilds custom homes start at $400 per square foot and go up in Teton Valley and eastern Idaho, and start at $1,000 per square foot and go up in Jackson Hole. A radon rough-in is a rounding error against that. It is one of the highest-return small decisions in the entire build, which is why it shows up on my list of custom home upgrades that are actually worth the money.

Passive or active: which system do I actually need?

You need the passive rough-in. Whether you need the fan is a question your test answers after you move in, and there’s no way to know sooner.

	Passive system	Active system
How it moves air	Stack effect, no fan	Inline fan on the vent pipe
Installed when	During construction	Usually added after a test
Operating cost	None	Fan runs continuously, small draw
Noise	None	Slight, and it’s in the attic
Typical reduction	Modest, sometimes enough	Reliable, and verifiable by retest
Needs a rough-in first	It is the rough-in	Far cheaper if the rough-in exists

The practical sequence is: rough in passive, move in, test, and add the fan only if your number calls for it. If you do add one, retest afterward to confirm the system actually did its job. A fan that’s running is not the same thing as a house that’s below 4.0.

Does Idaho code require a radon system?

No. Idaho’s residential code is the 2018 International Residential Code, and the state’s amendments to it cover things like footing widths, wall bracing, ventilation, and sprinklers. Radon control isn’t in them. No building department I work with in Teton, Madison, Bonneville, or Jefferson County will fail an inspection over a missing radon system, and no inspector is going to bring it up.

That’s worth understanding clearly, because it’s easy to assume that anything important is covered by code and anything not covered must be optional fluff. Code is a floor, not a standard of care. Plenty of what makes a house good to live in sits above it. If you want the full picture of who inspects what and when on an Idaho build, I wrote that up in what custom home inspections actually happen, and the county-by-county permitting paths are on our Idaho building permits by county page.

So radon mitigation in eastern Idaho is a builder-and-owner decision, not a code compliance item. I make it the default on my builds rather than a line item you have to know to ask about. Most people don’t know to ask, and I’d rather not have that conversation in year three.

When do I test, and what happens if the number comes back high?

Test after you move in and the house is being lived in and closed up. Winter is the better season here, because the house is shut tight and the stack effect is strongest, which tends to show you your realistic high end rather than a flattering summer number.

Short-term tests run 2 to 90 days and give you a fast read. Long-term tests run past 90 days and better reflect a true annual average. If a short-term test comes back near or above 4.0, confirm it with a second test before you spend money. Idaho’s health department offers residents a free test kit every two years, and there’s a library lending program with free kits at participating public libraries, though availability moves around.

If your confirmed number is at or above 4.0 and you have a rough-in, this is an easy day. A fan goes on the existing pipe in the attic, and you retest to verify. No slab work, no exterior pipe, no patching. If your number is between 2.0 and 3.9, the state says consider mitigating, and with a rough-in already in place that decision costs you very little to act on.

What’s not included in that number?

Fair question, and one I’d want answered too. The $300 to $500 construction figure covers the rough-in materials and labor. It does not include a fan, the testing itself, or a post-mitigation retest. None of those are large, but they’re real, and I’d rather name them than let you find them later.

Radon also isn’t the same job as sealing your house for comfort and energy, even though the two overlap. Air sealing and a proper water-resistive barrier are their own scope, and I covered those in which energy efficient features are worth paying for. A tight house with no radon path is great. A tight house sitting on unvented soil gas without a pipe is tighter around the wrong thing.

And if your build is rural, radon sits alongside the other below-grade decisions that get made early, like well and septic and the site prep work covered in how to prepare land to build a house. These all land in the same window of the schedule, which is why they belong in design rather than in a panic during excavation.

What I do on every house I build

I rough in a passive radon system on every home, in every county listed in that table, without waiting to be asked. It gets drawn into the plans during the design phase, priced into the construction contract, and installed at the foundation stage. Then I tell the owner to test the first winter they’re in the house, and what to do with the result.

I’m involved in every phase of every build, which in practice means I’m the one making sure the gravel is clean, the poly is actually lapped and sealed rather than thrown down, and the pipe is labeled so a future electrician doesn’t cut into it wondering what it is. A radon system that was installed carelessly is a pipe, not a system, and you won’t find out which one you got until you test.

The reason this is a design-phase item and not a field decision is simple: the pipe needs a chase to run in, and chases are a plan question. We handle plan development and design and engineering in house, so the radon pipe gets a home in the drawings before framing starts instead of being shoehorned into whatever wall is closest. You don’t need to go hire that out. What that phase looks like and what gets locked in it is in the design phase explained, and design is covered by a fixed design fee, quoted per project.

If you’re planning a build in Teton Valley or anywhere in eastern Idaho and you want to know how involved you’ll need to be in decisions like this one, here’s what that actually looks like. Radon is one of maybe forty of these decisions, and handling them so you don’t have to is the job.

Building science items like this rarely make the highlight reel, and they’re the ones that quietly separate a house that performs from one that just looks finished. Same category as wildfire resistant construction and what radiant floor heat really costs, both of which get decided in the same early window.

Want the radon rough-in and the rest of the below-grade decisions handled on your build? Get in touch or call (208) 520-0636 and we’ll walk through your lot and your plans.

Frequently asked questions

Can you test for radon before the house is built?
Not usefully. Radon is measured inside a finished, closed-up building, so a reading on a vacant lot doesn’t predict your house. Soil gas testing exists but isn’t reliable enough to base a decision on. That uncertainty is the argument for roughing in a passive system rather than waiting.

Does a radon system in a new home need a fan?
Not necessarily. A passive rough-in uses stack effect to vent soil gas with no fan, and sometimes that’s enough to stay under 4.0 pCi/L. You test after moving in, and if the number is at or above the action level, a fan goes on the existing pipe in the attic. The rough-in is what makes that easy.

How much does radon mitigation cost in Idaho?
The Idaho Department of Health and Welfare publishes $300 to $500 for radon-resistant features installed during construction, $1,500 to $3,500 for a professional retrofit on an existing home, and $300 to $600 for a DIY kit. Those are agency figures, not a builder’s quote, and the construction-stage option is the cheapest by a wide margin.

Is radon worse in a basement than on a slab?
Basements often test higher because they’re below grade with more soil contact and stronger stack effect. That doesn’t make a slab house safe. Slab-on-grade homes test above the action level regularly in Zone 1 counties, so both get the same rough-in on my builds.

Do I have to disclose radon when I sell an Idaho home?
If you have test results, buyers and their inspectors will ask, and radon testing is common during Idaho home inspections. A documented passive system with a test showing you’re under the action level is an asset in that conversation. No result at all just means the buyer runs their own test and you find out on their timeline.

TL;DR: A wildfire resistant home is not a bunker and it is not a product you buy. It is a stack of unglamorous details — a Class A roof, ember-resistant vents, five clean feet around the foundation, and cladding that does not hold a flame — that together decide whether a house survives an ember storm. I have been building in Eastern Idaho and Teton Valley for 20 years, and almost none of this is required by the code we build under here. It is worth doing anyway, and right now there is a second reason: Idaho’s homeowners insurance market has gotten hard on wildfire-exposed property, and the choices you make at framing are the ones an underwriter reads five years later.

What a wildfire resistant home actually means

Start with how houses actually burn in a wildfire, because the mental picture most people carry is wrong. The picture is a wall of flame coming through the trees and consuming everything in its path. That happens, and when it happens nothing you built is going to matter much. But it is not how most homes are lost.

Most homes are lost to embers, and that single fact is what a wildfire resistant home is designed around. A wind-driven fire throws burning material a mile or more ahead of the front. Those embers land on and around houses by the thousands. They collect in a gutter full of pine needles. They get sucked into an attic through a soffit vent. They land in the bark mulch somebody spread against the foundation, smolder for twenty minutes, and then light the siding. The fire front passes. The house burns down two hours later from something the size of a matchhead.

That is the whole design problem, and it is why a wildfire resistant home is a detail problem rather than a materials problem. You are not trying to make a fireproof house — there is no such thing. You are trying to eliminate the places where a small burning object can land, stay lit, and find something to feed on. That is a framing-stage and finish-stage conversation, which is exactly why it has to happen while we are still drawing, not after you have signed a contract for a house with wood decking wrapped around three sides of it.

The national standard worth knowing is the Insurance Institute for Business & Home Safety’s Wildfire Prepared Home designation, which comes in a base level and a Plus level and was updated in June 2025 — its first revision since the program launched in 2022. The NFPA’s home ignition zone framework covers the same ground in plainer language. Neither is law in Teton County. Both are the best free checklist you will find.

Why this became a Teton Valley conversation

For most of my career this topic did not come up. Clients asked about snow load, frost depth, and whether the driveway would hold a concrete truck in April. Wildfire hardening was something you read about in California.

Two things changed it. The first is obvious if you have watched a summer here lately — the fire seasons are longer and the smoke is a given. The second is insurance, and it is the one that actually moves people.

The numbers are not subtle. According to reporting on Idaho Department of Insurance data, average statewide homeowners premiums rose roughly 37% between 2022 and 2024, from a little over $1,300 to about $1,800. Over the same window the number of homeowner policies in force statewide fell from about 464,000 to about 424,000 — roughly a 9% drop — and Idaho’s insurance director has said publicly that the fear is tens of thousands of people simply going without coverage. Those are market figures, not SwagerBuilds numbers, and they are statewide rather than valley-specific. But they describe a market that has gotten selective about wildfire-exposed property.

Here is the practical consequence for somebody building a custom home. Underwriting on a new build looks at the roof, the vents, the deck, and the vegetation clearance. Those are all things we set during construction and that are expensive or impossible to change later. A composition roof and wood decking are a decision you make once and live with for thirty years. I would rather have that conversation with you at schematic design than have you call me in 2031 asking what it would take to re-roof a house to get insurable.

This lands hardest on out-of-state buyers, who are a big share of who we build for. If you are managing a build from out of state, you will not be walking the site in August noticing that the neighbor’s slash pile is forty feet from your future great room. That is my job.

The roof is most of the fight

If you do one thing, do the roof. It is the largest horizontal surface on the building, it is where the most embers land, and a roof failure puts fire inside the structure where nothing else you did matters.

On a wildfire resistant home you want a Class A roof assembly. That is the top fire rating, and the good news is that the roofs people already want here mostly qualify. Standing seam metal is a Class A assembly over the right underlayment, and it is already the default on a lot of our builds for snow-shedding reasons — I wrote up the real numbers in our metal roof cost guide for Idaho. Concrete and clay tile qualify. Class A asphalt shingle exists and is common.

The failure is almost never the field of the roof. It is the edges and the penetrations. Specifically:

• The gutters. A gutter packed with needles and leaves is a fuel trough bolted to the most vulnerable edge of the building. Metal gutters with covers, and an actual plan for cleaning them, or no gutters at all with proper grade and drip protection.
• The eaves and soffits. Open eaves catch embers and hold heat against the underside of the roof deck. Boxed, enclosed soffits in a noncombustible or ignition-resistant material take that away.
• The valleys and roof-to-wall intersections. Anywhere debris naturally collects, embers collect with it.
• Skylights and solar penetrations. Every hole is a detail somebody has to get right.

None of this costs meaningfully more if it is in the drawings. All of it costs a fortune as a retrofit.

Vents: the cheapest fix nobody makes

On a wildfire resistant home, attic and crawlspace vents are, dollar for dollar, the highest-leverage thing on this whole list, and they are the thing that gets value-engineered out because nobody sees them.

A standard vent is a hole in your building envelope covered by screen that was chosen to keep out squirrels. In an ember storm, wind drives burning material straight through it into an attic full of insulation and framing. The house burns from the inside out while the exterior looks untouched.

The minimum fix is noncombustible metal mesh at 1/8-inch or finer — coarser screen passes embers, and plastic screen is not a fire product. The better fix is a listed ember-resistant vent tested to ASTM E2886, which is engineered to block both embers and flame intrusion rather than just being a finer hole. Market pricing on the hardware runs roughly $15 to $50 per vent depending on type; that is third-party market data, not our number. On a typical custom home that is a rounding error against the build, and it is the single change I would make first on any house going up in timber.

The same logic applies to anything else that opens into the structure: dryer vents, combustion air intakes, crawlspace access. Make a list at framing and handle them as a set.

Zone 0 — the first five feet

Zone 0 is the five-foot ring around the perimeter of the house, and it is the part of this that costs almost nothing and gets ignored almost universally — because it is landscaping, and landscaping shows up at the end of a project when everybody is tired and over budget.

The rule is simple: within five feet of the structure, nothing that burns. That means:

• No bark or wood mulch against the foundation. Gravel, decomposed granite, or bare mineral soil instead.
• No shrubs or ornamental grasses planted against the wall, particularly not under windows or against a deck.
• No firewood stacked against the house. I know. Everybody does it. Move it thirty feet out.
• No combustible fence attached to the building. A wood fence run into the side of a house is a fuse with a house on the end of it. Use a metal or masonry section for the last several feet where it meets the structure.
• Nothing stored under the deck or in the crawlspace hatch.

Beyond that ring, NFPA’s intermediate zone runs five to thirty feet and the extended zone runs thirty to a hundred, and both are about breaking up continuous fuel — spacing trees, limbing up, keeping grass cut, not letting a ladder of vegetation run from the ground into the crowns.

A lot of this gets decided when you buy the lot rather than when you build on it. A heavily treed parcel with a single-track access road is a different fire proposition than an open meadow lot, and it is one of the things I look at on a walk before we talk about anything else — see our field guide to lots in Driggs and the site work covered in how to prepare land to build a house.

Siding, windows, and decks compared

These are the three assemblies where a wildfire resistant home forces a tradeoff between what clients want aesthetically and what performs. Here is how they actually compare.

Assembly	Weakest common choice	Strong choice	What actually drives the decision
Siding	Untreated wood or cedar shake, especially with open joints	Fiber cement, stucco, metal, or masonry veneer; noncombustible at the bottom several inches where ground fuels touch	Most clients want the look of wood somewhere. You can usually get it on protected elevations and keep the exposed faces noncombustible.
Windows	Single-pane, or dual-pane with no tempered lite	Multi-pane with a tempered exterior pane; metal or metal-clad frames	Glass fails from radiant heat before the wall does, and a failed window lets fire straight in. Tempered is a small upcharge on a window package you are already speccing.
Decks	Wood decking with open framing and storage underneath	Noncombustible or ignition-resistant decking, enclosed or screened underside, nothing stored below	This is the one people fight me on. A deck is a horizontal ember collector attached to the house at the worst possible height.

On siding specifically, the cold-climate durability argument and the fire argument point the same direction here, which makes the conversation easier — I went through the long-term performance side in best siding for Idaho winters.

One note on the envelope: tightening a house for fire and tightening it for energy are complementary but not identical. Sealing and detailing overlap; venting strategy does not. Both get worked out together, and the efficiency side is covered in energy efficient custom home features.

What Idaho code actually requires

Here is the part I would rather you hear from me than find out later. Teton County, Idaho adopted the 2018 family of I-Codes in March 2021 — including the 2018 International Residential Code and the 2018 International Fire Code, per the county building department’s published code policies. What that package does not include is a separately adopted wildland-urban interface code with ignition-resistant construction mandates.

Translated: almost everything in this article is optional here. Nobody at the county is going to red-tag your build for a wood deck, bark mulch against the foundation, or standard vents. You can build a house in timber in this valley that meets every code we enforce and is, from an ember standpoint, a pile of kindling with a kitchen in it.

I am not arguing the county is wrong. I am telling you that “it passed inspection” and “it will survive an ember storm” are two unrelated statements, and that your insurer is going to evaluate you against the second one. Inspections here cover the things covered in our Teton County permits and timeline guide — they do not cover this.

Subdivisions sometimes fill the gap. Some CC&Rs have fire-related provisions, and a few HOAs push the other way by requiring wood shake or heavy vegetation screening. That conflict is worth catching during design rather than at architectural review.

What a wildfire resistant home costs to build

The honest answer: on a custom home at our level, most of what makes a wildfire resistant home is inside the noise.

Our custom home pricing in Teton Valley and on the Idaho side starts at $400 per square foot and goes up, based on construction cost and conditioned square footage. On a house in that range, upgrading vents, specifying a tempered exterior pane, enclosing soffits, and putting gravel instead of mulch in the first five feet does not move the number in a way you will notice. These are specification choices, not scope additions.

The items that do carry real cost are the ones that were going to be real decisions anyway: roof material, siding material, and decking material. If you were already planning a standing seam roof and fiber cement siding, you are most of the way to a hardened house and it costs you nothing extra. If your heart is set on cedar shake and a wraparound wood deck, the fire-resistant version of that vision costs more — and that is a design conversation, not a line item I can quote you cold.

What I will not do is give you a hardening budget before we have a design. Every number I put in front of a client comes out of the design agreement, which carries a fixed design fee, quoted per project. Site conditions drive too much of this — a meadow lot and a lodgepole lot are different projects.

What I would do on your build

Twenty years in, here is the order I would spend in on a wildfire resistant home, highest return first:

1. Ember-resistant vents everywhere. Cheapest meaningful protection that exists. Do it on every build regardless of exposure.
2. Class A roof with enclosed soffits and covered metal gutters. Largest target, worst consequence of failure.
3. Five clean feet. Costs almost nothing. Requires that your landscape plan and your builder are talking to each other, which is on me.
4. Tempered exterior pane in the window package. Small upcharge inside a spec you are already writing.
5. Noncombustible siding on exposed elevations, with wood used deliberately where it is protected rather than everywhere by default.
6. Deck strategy. Material, enclosure, and a rule that nothing gets stored underneath.

The thing I want you to take from this is that none of it is exotic. There is no wildfire package to buy. It is a series of ordinary decisions made in the right direction, which is why it only works if somebody is tracking it from schematic design through the final walkthrough. I am involved in every phase of every build we do, and this is exactly the kind of detail that falls through when nobody owns it — the same way punch items do if the warranty and punch list process is not run deliberately.

If you are planning a build in Teton Valley or Eastern Idaho and want this handled as part of the design rather than bolted on after, that is a conversation worth having early. You can see how we work on our custom homes page or get in touch.

Frequently asked questions

Is a wildfire resistant home required by code in Teton Valley, Idaho?

No. Teton County, Idaho adopted the 2018 I-Codes in March 2021, including the 2018 IRC and 2018 International Fire Code, but has not separately adopted a wildland-urban interface code with ignition-resistant construction requirements. Everything described here is voluntary in this jurisdiction, which is precisely why it has to be a deliberate decision during design rather than something the inspection process catches for you.

Will building a wildfire resistant home lower my insurance premium?

It can help, and more importantly it can affect whether you are offered a policy at all on an exposed parcel. But carriers weigh things differently, rate by their own models, and change their appetite year to year, so nobody — including me — can promise you a specific premium or a specific outcome. What I can tell you is that roof class, vent type, deck construction and vegetation clearance are the items underwriters consistently ask about, and all four are set during construction.

What is the single most cost-effective wildfire upgrade?

Ember-resistant vents. Attic and crawlspace vents are how burning material gets inside an otherwise sound house, and upgrading them is inexpensive hardware on a new build — market pricing on the hardware itself generally runs in the range of $15 to $50 per vent. Second place is clearing the first five feet around the foundation of anything combustible, which costs essentially nothing if it is in the landscape plan from the start.

Can I still have a wood deck and cedar siding?

Usually yes, but placed deliberately rather than everywhere. The workable approach is noncombustible materials on the elevations and surfaces most exposed to ember and radiant heat, with wood used where it is sheltered and where the detailing supports it. Where clients run into trouble is treating wood as the default finish for the whole exterior and then trying to harden it afterward.

Does any of this matter if my neighbor’s property is overgrown?

Yes, and it matters more, not less. Home hardening is specifically about surviving embers and radiant heat from surrounding fuel you do not control. You cannot make your neighbor limb their trees, but you can make sure that when embers land on your property they have nothing to catch. The structure work is entirely within your control; that is the argument for doing it.