Colorado Climate Zones and Code R-Values

The zone follows the county, not the elevation of your particular site — which catches people out, because a job at 9,000 feet in Eagle County is in a colder-sounding place than the county's assignment suggests.

Zone 5B — Denver, Boulder, Mesa (Grand Junction), Garfield (Glenwood Springs, Carbondale, Rifle), Montrose, La Plata (Durango), Delta.

Zone 6B — Eagle (Vail, Avon, Edwards), San Miguel (Telluride), Archuleta (Pagosa Springs).

Zone 7 — Pitkin (Aspen, Snowmass), Summit (Breckenridge, Frisco, Dillon), Routt (Steamboat Springs), Gunnison (Crested Butte).

Vail and Telluride are 6B, not 7. It is the most common zone mistake we see on a submittal, because both towns feel like the highest thing on the map and neither sits in a zone 7 county.

This is the part that catches people out. Under the 2021 IECC residential table the wood-frame wall requirement is the same in zones 5, 6 and 7: R-30 in the cavity, or R-20 plus R-5 continuous, or R-13 plus R-10 continuous, or R-0 plus R-20 continuous. Grand Junction and Breckenridge want the same wall.

What changes is the ceiling. Zone 5 asks for R-49. Zones 6 and 7 ask for R-60. So going from Glenwood Springs up to Vail does not change your wall order — it changes the depth of blow in the attic.

Steel-frame walls are a separate column and read differently, because the stud conducts straight through and the table compensates with continuous insulation. Commercial buildings run off a different chapter again. Send the wall section rather than working from the residential table.

Batt between studs insulates about three-quarters of the wall. The studs conduct straight through. Continuous insulation — rigid board across the whole face — covers the studs too. That is why the tables read "R-20 + 5ci" instead of one number, and why polyiso and XPS are on the rack next to the batt.

Colorado is mid-transition, and three answers are live at once. Since 1 July 2026, a jurisdiction updating any building code has to adopt the state's Model Low Energy and Carbon Code — the 2024 IECC with Colorado amendments. Between July 2023 and June 2026 the requirement was the 2021 or 2024 IECC plus the electric-ready and solar-ready code. A jurisdiction that has not updated since is still on whatever it adopted then.

So the honest answer to "what does the code want" is that it depends on when your building department last updated, and somebody has to ask them. Local amendments can go above the state minimum but never below it — Aspen and Pitkin County are the usual examples in the high country.

Call the building department, or call us and we will call them. We do it often enough that it is not an imposition.

The R-value calculator takes the county and the assembly and gives the prescriptive minimum, with the product on the shelf that meets it. Or send us the wall section.