Published
The energy code on an Ashton build: two ways to comply, and the sheet the county uses
The county gives you a choice, and says so plainly
Most of the Fremont County permit file is a list of things you must do. The energy code is one of the few places the county offers a genuine alternative:
“There are two different ways to comply with the Energy Code. (1) would be to complete a res-check at www.energycodes.gov and submit to our office. (2) would be to use the Prescriptive Path and Insulate your house to the values listed below.”
— Fremont County, Idaho, Pre-Construction Check List for Building Permits, revised 17 April 2026 (county-published PDF, downloaded and re-read 14 August 2026)
Those are not equivalent routes dressed up as a choice. REScheck trades components against each other — it computes whether the whole envelope performs well enough, so a weaker wall can be bought back with a better ceiling or better windows. The prescriptive path does not trade: every listed value has to be met on its own. On a corridor build with big glazing facing the river or the Tetons, that distinction decides whether the design you want is compliant.
The county's prescriptive values, reproduced
Fremont County prints one climate zone row, Zone 6, and these are its values:
| Component | Required R-value | What it means on the drawings |
|---|---|---|
| Ceiling | R-49 | Needs depth over the exterior wall, which is a truss decision |
| Wood frame wall | R-20, or R-13 + 5 | Either a deeper cavity or insulated sheathing outside the frame |
| Floor | R-30 | Applies over unconditioned space, not over a heated basement |
| Basement and crawl space wall — continuous | R-10 | Rigid insulation against the concrete |
| Basement and crawl space wall — cavity | R-13 | The framed alternative to the continuous route |
| Unheated slab | R-10, to a depth of 4 feet | Vertical at the slab edge; heated slabs add R-5 |
The county explains its own shorthand in a footnote worth reading before anyone orders sheathing: “13+5” means R-13 cavity insulation plus R-5 insulated sheathing. Where structural sheathing covers 25 percent of the exterior, the county's note says R-5 sheathing is not required where that structural sheathing is used; above 25 percent, the structural sheathing must be supplemented with insulated sheathing of at least R-2. A second note adds R-5 to the required slab edge value for heated slabs.
The two things that sheet does not tell you
Read the table again and notice what is absent.
- There is no window number. The county's prescriptive sheet lists insulation only — no fenestration U-factor, no skylight figure, no solar heat gain value. On a house whose whole point is glass facing a view, that is the single largest envelope decision, and it is not on the handout.
- The values do not obviously match the code edition people quote. Fremont County has adopted the 2018 family of codes, but the wall and foundation figures on its prescriptive sheet are less demanding than the Climate Zone 6 numbers commonly published for the 2018 energy code. The ceiling, floor and slab values line up; the wall and basement values do not.
This page is not going to resolve that second point by quoting a code table at you, because the only renderings available to check against were state-amended versions rather than the base code, and a number that cannot be pinned to its source does not belong here. What can be said with confidence is more useful anyway: the sheet the county hands out is the sheet the county's plan reviewer works from, and if your design is being drawn to a different table, that is a conversation to have with the building department before drawings are final rather than after they are submitted.
Which route to take
| REScheck report | County prescriptive path | |
|---|---|---|
| How it works | Models the whole envelope and reports pass or fail | Each component must meet its listed value |
| Trade-offs allowed | Yes — that is the point of it | No |
| Suits | Large glazing areas, unusual assemblies, SIP or log walls | Conventional framing with modest window area |
| Effort | Someone has to build the model and submit it | None beyond specifying the insulation |
| Where windows are handled | Inside the model, explicitly | Not addressed on the county's sheet |
| Failure mode | A late model failure forces a redesign | A missed value is caught at inspection |
For most corridor houses the honest answer is REScheck, and the reason is glass. A design with a big south or west wall of windows is exactly the case the prescriptive path handles worst, because it cannot let a superb ceiling pay for an ordinary window. It is also the design most people building here actually want.
Where this collides with everything else on the file
The energy code does not sit on its own. Three collisions are worth planning for:
- Roof structure versus ceiling insulation. R-49 at the ceiling needs depth, and the roof above it is already sized for the district's ground snow load. Raised-heel trusses solve it cleanly, but they have to be drawn that way from the start — the truss designs are part of the permit submission, not an afterthought.
- Slab edge insulation versus frost depth. The county requires R-10 to four feet at an unheated slab, and separately requires footings at a 32-inch minimum frost depth. They are different rules answering different questions, and both apply.
- Wall assembly versus the structural package. The R-13 + 5 route puts insulated sheathing on the outside of the frame, and the interaction with structural sheathing is governed by the county's own 25 percent note. On an engineered corridor build, the engineer and the energy path need to agree before either is finalised.
The structural side of that is covered on the Ashton snow load page, the footing depth on the frost depth page, and where the whole submission sits in sequence on the permit path page.
What to do with this before you draw
Pick the compliance route deliberately and early, because it changes the drawings rather than following them. Ask your designer which route the plans assume, and ask the building department at (208) 624-7200 to confirm what it expects to see for your specific file — particularly on windows, where the county's handout is silent. Doing that costs one phone call. Discovering it at plan review costs a redraw and a place at the back of a four-to-six-week queue. What the envelope decisions do to a budget is on the cost page, and how different wall systems behave here is on the log, timber and SIP page.
Common questions
What are the two ways to comply with the energy code in Fremont County?
The county's checklist names both: complete a REScheck at www.energycodes.gov and submit it to the office, or use the prescriptive path and insulate the house to the values the county lists. They are genuine alternatives, and which one you choose changes what the drawings have to show.
What insulation does Fremont County require?
Its prescriptive sheet gives Climate Zone 6 values: R-49 ceiling, R-20 or R-13 + 5 wood frame wall, R-30 floor, R-10 continuous or R-13 cavity for basement and crawl space walls, and R-10 to a depth of four feet at an unheated slab. Heated slabs add R-5 at the edge.
What does R-13 + 5 mean?
The county spells it out: R-13 cavity insulation plus R-5 insulated sheathing. Where structural sheathing covers 25 percent of the exterior, R-5 sheathing is not required where that structural sheathing is used; above 25 percent, structural sheathing must be supplemented with insulated sheathing of at least R-2.
Does the county's sheet specify a window U-factor?
No, and that is worth knowing before you design a wall of glass. The prescriptive sheet lists insulation values only — no fenestration U-factor, no skylight figure, no solar heat gain coefficient. Confirm with the building department how it expects windows to be documented on your file.
Which climate zone is Ashton in for energy code purposes?
Fremont County's prescriptive sheet prints a single row, Climate Zone 6, and applies it across the county. That is the county's own statement of the zone, and it is the row a plan reviewer works from when the prescriptive path is used rather than a REScheck report.
Is REScheck or the prescriptive path better for a corridor home?
For most custom homes here, REScheck — because of glazing. REScheck models the whole envelope and lets a strong component offset a weaker one, while the prescriptive path allows no trade-offs at all. A design with large windows facing a view is the case the prescriptive path handles worst.
Do the county's insulation values match the 2018 energy code?
Not obviously, and this page will not pretend otherwise. The ceiling, floor and slab figures line up with commonly published Climate Zone 6 values, but the wall and basement numbers on the county's sheet are less demanding. The county's own sheet is what its reviewer uses — ask the department if your design assumes a different table.
How does ceiling insulation interact with the roof structure?
R-49 needs depth at the ceiling, and the roof above it is already sized for the district ground snow load. Raised-heel trusses are the usual answer, giving full insulation depth over the exterior wall, but they have to be specified from the outset because truss designs form part of the permit submission.
Is slab insulation the same thing as frost depth?
No — they are separate requirements that both apply. The energy provisions call for R-10 slab insulation to a depth of four feet at an unheated slab, while the county's building ordinance sets a 32-inch minimum frost depth for footings. One is about heat loss, the other about frost heave.
When should I decide which compliance route to use?
Before drawings are finished, because the route changes the drawings rather than following them. Deciding late is how a design ends up needing a redraw and a fresh place in a review queue the county says runs roughly four to six weeks.