North Idaho sits in IECC climate zone 6 — the same bracket as northern Minnesota and much of Vermont. That single fact drives every insulation requirement on your plans, and it is why an insulation package designed for Boise or Spokane Valley is not automatically the right one in Kootenai or Bonner County.
Here is what the code asks for, what the Idaho amendments changed, and where builders most often get caught.
Before you build to anything on this page: Idaho enforces the 2018 IECC residential provisions with state amendments, and local jurisdictions can adopt their own. Requirements change. Confirm the current numbers with your building department — or ask us and we will confirm them with you for your address.
What’s in this guide
Which climate zone are you in?
Northern Idaho falls in climate zone 6. That includes Kootenai County — Hayden, Coeur d’Alene, Post Falls, Rathdrum — and Bonner County, including Sandpoint and Priest River. Southern Idaho sits mostly in zone 5, which is why insulation specs pulled from a Boise-area plan set come up short here.
Zone assignment is by county, so it is not a judgment call. If your plans were drawn out of the area, check that the energy compliance page was done for zone 6 before it reaches your inspector.
The prescriptive R-values
The prescriptive path is the simplest way to comply: hit each of these component values and you are done, no energy modeling required. For zone 6 under the 2018 IECC, the targets look like this:
| Component | Typical zone 6 requirement | What that usually means in practice |
|---|---|---|
| Ceiling / attic | R-49 | Blown-in over a properly air-sealed lid, or foam at the roof deck in an unvented assembly |
| Wood-frame wall | R-20 + continuous, or R-22 cavity under the Idaho amendment | 2x6 framing with a high-density cavity fill, or a hybrid assembly |
| Floor over unconditioned space | R-30 | Or insulate the crawl space walls instead and bring the crawl inside the envelope |
| Basement wall | R-15 continuous / R-19 cavity | Closed-cell against the concrete is the common approach |
| Crawl space wall | R-15 continuous / R-19 cavity | Requires a sealed crawl with a ground vapor barrier |
| Slab edge | R-10, 4 ft deep | Has to happen during construction — there is no retrofit for this |
| Windows | U-0.30 maximum | Idaho amendment; a fenestration spec, not insulation, but it lands on the same compliance page |
Treat the table as orientation, not as your permit set. The values above reflect the 2018 IECC as amended; your jurisdiction's current adoption governs.
What Idaho changed
Idaho's amendments to the model code are the part out-of-area designers miss most often. Two that matter for insulation:
- The continuous exterior insulation requirement was removed, and the wall requirement was raised in exchange — R-22 in exterior walls in zone 6. Practically, that means you can meet the wall requirement inside the cavity rather than adding exterior foam board, which changes how you get there but not the performance target.
- Fenestration U-factor is capped at 0.30 in zone 6.
There are also amendments covering duct insulation, visual inspection options for the air-sealing checklist, and a separate envelope table for log homes — relevant here, since log construction is common in this part of the state.
Air sealing is a code requirement too
Homeowners tend to focus on R-values because they are printed on the bag. The code also requires the building envelope to be sealed, verified either by a blower door test or by an inspected air-barrier checklist.
This is where spray foam has an inherent advantage: it is insulation and air barrier in one application, so the cavity work and the sealing work happen at the same time. With batts, the air sealing is a separate scope that someone has to actually do — caulk, foam at penetrations, sealed top plates, attention at rim joists and can lights.
The number that predicts comfort is not the R-value on the ceiling — it is how much outside air moves through the house on a windy January night. Two homes built to identical R-values can be a world apart on that measure.
How spray foam gets you there
- Walls: a 2x6 cavity filled with closed-cell lands well past the wall requirement; open-cell or flash-and-batt can meet it with careful specification. Which one is right depends on cost and on the drying strategy for the assembly — see closed-cell vs. open-cell.
- Unvented roof decks: foam at the deck brings the attic inside the envelope, which is often what you want when ductwork or mechanicals live up there. In zone 6 this assembly has to be detailed correctly for vapor control.
- Crawl spaces: insulating the crawl walls and sealing the floor is usually more effective and more durable than insulating the floor above, and it stops the stack effect pulling crawl-space air into the house.
- Rim joists: a code detail and one of the largest air leaks in most homes. Closed-cell handles both jobs at once.
Building or remodeling in North Idaho?
We work with builders and homeowners across Hayden, Coeur d’Alene, Post Falls, Sandpoint and Harrison, and we handle the energy-code side of the insulation package as part of the job — the assembly, the R-value it delivers, and getting it through inspection.
Read more about new construction insulation and retrofit work, or send us your plans and we will tell you what the envelope needs.
Frequently asked questions
What climate zone is North Idaho in?
Northern Idaho is in IECC climate zone 6, which includes Kootenai County (Hayden, Coeur d’Alene, Post Falls, Rathdrum) and Bonner County (Sandpoint, Priest River). Southern Idaho is mostly zone 5, so insulation specifications drawn for the Boise area fall short of what is required in the north.
What R-value does insulation need to be in North Idaho?
Under the 2018 IECC prescriptive path for zone 6, the targets are roughly R-49 ceilings, R-20 plus continuous or R-22 cavity walls under the Idaho amendment, R-30 floors over unconditioned space, R-15 continuous or R-19 cavity for basement and crawl space walls, and R-10 slab edge insulation four feet deep. Idaho also caps window U-factor at 0.30. Confirm current requirements with your local building department, since adoptions and amendments change.
Which energy code does Idaho use?
Idaho enforces the 2018 IECC residential provisions with state amendments. Local jurisdictions can adopt amendments of their own, so the authority having jurisdiction over your address is the final word.
Is air sealing required by code?
Yes. The energy code requires the building envelope to be sealed, verified either by a blower door test or through an inspected air-barrier checklist. Spray foam has an advantage here because it insulates and air-seals in one application, whereas batt insulation requires air sealing as a separate scope of work.
Need a number for your building?
NW Spray Foam & Insulation is a family-owned, owner-operated crew in Hayden serving Coeur d’Alene, Post Falls, Sandpoint, Harrison and the surrounding North Idaho communities. Tell us what you are building or fixing and we will spec it properly and put it in writing.
Request an estimate Call (208) 818-4170More insulation guides
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