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Colorado REScheck Requirements (2026)

Key facts:

  • Home-Rule State — New Statewide Floor as of 7/1/2026

  • Climate Zones 4, 5, 6 & 7

  • 3 ACH50 Blower Door

  • REScheck Confirmed in Colorado Springs (Pikes Peak)

 

Colorado has historically been a home-rule state — no single statewide energy code, with each city or county adopting its own IECC edition. That changed on July 1, 2026: the state's new Model Low Energy and Carbon Code (MLECC), based on the 2024 IECC, is now a mandatory statewide floor that every jurisdiction must meet or exceed the next time it updates its building code. REScheck is explicitly named as an accepted compliance method by the Pikes Peak Regional Building Department (Colorado Springs area); Denver's own posted forms reference REScheck as well. Boulder is the notable exception — its 2024 code is an ERI-based, all-electric net-zero standard where REScheck doesn't apply the same way. Always confirm the specific pathway with your local building department.

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Colorado jurisdictions require REScheck and energy compliance documentation under whichever IECC edition that city or county has adopted — Colorado is a home-rule state, so this has varied significantly by location. As of July 1, 2026, a new statewide floor applies: any jurisdiction updating its building code must adopt at least the Colorado Model Low Energy and Carbon Code (MLECC) or something stricter. This is a genuinely new development, so expect more Colorado jurisdictions to shift onto the MLECC over the next few code cycles.

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Colorado's New Statewide Energy Code Floor

⚠ This is brand new: Colorado's Model Low Energy and Carbon Code (MLECC) took effect July 1, 2026 — just months ago. It's based on the 2024 IECC with Colorado-specific modifications, and it sets a mandatory minimum every jurisdiction must meet or exceed the next time it updates its building code (local amendments are allowed, but they can't reduce the code's efficiency). Before this, from July 2023 through June 2026, the statewide floor required jurisdictions updating their codes to adopt at least the 2021 or 2024 IECC. Because MLECC only applies when a jurisdiction updates its code, plenty of Colorado cities and counties are still enforcing an older IECC edition adopted before the requirement existed — don't assume MLECC is already in effect everywhere.

  • Historically: home-rule state, no single statewide energy code

  • July 2023 – June 2026: jurisdictions updating codes had to adopt 2021 or 2024 IECC (or stricter)

  • July 1, 2026 onward: Colorado MLECC (2024 IECC-based) becomes the mandatory statewide floor

  • MLECC applies only when a jurisdiction updates its building code — older adoptions remain in effect until then

  • Local jurisdictions may exceed MLECC; they cannot weaken it

 

MLECC's Home-Size-Tiered Requirements

One distinctive feature of Colorado's new statewide floor: it scales stringency by home size rather than applying one flat standard. Homes under 5,000 square feet meet a baseline efficiency standard. Homes between 5,000 and 7,499 square feet face stricter requirements. Homes at 7,500 square feet or larger must offset their total energy use — through on-site solar or a community solar subscription. The code also sets equal energy-performance footing between all-electric heat pump systems and mixed-fuel systems, rather than favoring one over the other.

 

Colorado Climate Zones

Colorado spans four IECC climate zones, largely following elevation and geography — the high mountain counties sit in the coldest zones, while the Front Range corridor and eastern plains sit in the milder zones.

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  • 4:  Baca, Las Animas, Otero   —   Southeastern plains — mildest zone in the state

  • 5:  Most of the remaining counties   —   Includes Denver, Colorado Springs, Boulder, and Fort Collins along the Front Range

  • 6:  Alamosa, Archuleta, Chaffee, Conejos, Costilla, Custer, Dolores, Eagle, Moffat, Ouray, Rio Blanco, Saguache, San Miguel   —   San Luis Valley and Western Slope counties

  • 7:  Clear Creek, Grand, Gunnison, Hinsdale, Jackson, Lake, Mineral, Park, Pitkin, Rio Grande, Routt, San Juan, Summit   —   High mountain counties — includes resort areas like Aspen, Breckenridge, and Steamboat Springs

 

Prescriptive Requirements (2021 IECC, Zones 5-7)

The 2021 IECC — still the operative code in many Colorado jurisdictions that haven't yet updated to MLECC — raised the prescriptive ceiling requirement to R-60 across Zones 5 through 8, a full tier above the 2018 IECC. Where a jurisdiction has since moved to the 2024 IECC or MLECC, confirm current numbers, since the 2024 edition adjusted some of these values back down.

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  • Ceiling:  R-60 (R-49 with raised-heel truss)   —   R-60 (R-49 with raised-heel truss)   —   R-60 (R-49 with raised-heel truss)

  • Wood Frame Walls:  R-30, or R-20+R-5ci, or R-13+R-10ci, or R-20ci   —   R-20+R-5ci, or R-13+R-10ci, or R-20ci (continuous insulation effectively required)   —   R-20+R-5ci, or R-13+R-10ci, or R-20ci

  • Floor:  R-30   —   R-30   —   R-38

  • Basement Walls:  R-15ci, or R-19 cavity, or R-13+R-5ci   —   R-15ci, or R-19 cavity, or R-13+R-5ci   —   R-15ci, or R-19 cavity, or R-13+R-5ci

  • Fenestration (Windows):  U-0.30, SHGC 0.40 (U-0.32 above 4,000 ft elevation)   —   U-0.30   —   U-0.30

 

Air Leakage and Blower Door Requirements

Colorado follows the standard IECC blower door threshold: 3 ACH50 maximum air leakage, tested at 50 Pascals, for Climate Zones 3 through 8 — which covers the entire state. This has been the stable IECC baseline since the 2012 edition and applies regardless of which specific IECC edition a given jurisdiction has adopted.

 

Is REScheck Accepted in Colorado?

It depends on the jurisdiction, but confirmed acceptance exists in at least one major metro area. The Pikes Peak Regional Building Department — which covers Colorado Springs and El Paso County — explicitly names RESCheck as an accepted method under its Total UA Alternative compliance path, stating builders can use "manual calculations, or RESCheck calculations using the Department of Energy RESCheck software" to demonstrate thermal envelope compliance. Denver's own building department has a posted document titled "RESCheck Interim Submittal Requirements," indicating REScheck plays a role in Denver's process as well, though the specific submittal details should be confirmed directly with Denver Community Planning & Development.

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⚠ Boulder is the clear exception. Its 2024 City of Boulder Energy Conservation Code (CoBECC) is an ERI-based, outcome-verified performance code — new construction must be all-electric, and homes 3,000+ square feet must hit a net-zero ERI score of 0. REScheck, which is built around the IECC's prescriptive and UA-tradeoff paths, isn't the compliance tool for Boulder's code structure. If you're working on a Boulder project, confirm the applicable compliance path with the city before assuming REScheck applies.

 

Common Permit Requirements

  • REScheck compliance report — confirmed accepted in Colorado Springs/Pikes Peak; referenced in Denver's own submittal documentation

  • Manual J HVAC load calculations

  • Window and door schedule (U-factor and SHGC)

  • Insulation R-value specifications matching the applicable climate zone

  • Blower door test results (3 ACH50)

  • ERI/HERS rating documentation — required in Boulder and other performance-path jurisdictions instead of REScheck

 

Common REScheck Failure Reasons in Colorado

Most Common Errors

  • Wrong climate zone selected — the Zone 5/6/7 boundary follows elevation and county lines, not simple geography

  • Using an outdated IECC edition — many jurisdictions are mid-transition between their prior code and the new MLECC floor

  • Assuming REScheck applies in a jurisdiction running a performance/ERI-based code, such as Boulder

  • Ceiling R-value pulled from the 2024 IECC when the jurisdiction is still enforcing the 2021 edition (or vice versa) — the two editions differ on this value

 

Documentation Issues

  • Submitting a standard REScheck report to a jurisdiction that requires ERI/HERS Index performance documentation instead

  • Missing the home-size-tiered MLECC requirements (5,000 sq ft and 7,500 sq ft thresholds) on larger residential projects

  • Basement wall insulation using the wrong compliance option among the code's multiple allowed paths

 

Major Cities and Jurisdictions in Colorado

Denver

Zone 5. Denver administers its own energy code with regular local updates and references REScheck in its own posted submittal documentation. Confirm the current Denver Energy Code edition and compliance pathway with Community Planning & Development before starting a project.

 

Colorado Springs / Pikes Peak Region

Zone 5. The Pikes Peak Regional Building Department explicitly accepts RESCheck as a Total UA Alternative compliance method under its adopted IECC edition (2021 IECC minimum as of mid-2026, with 2024 IECC and MLECC available as alternate options).

 

Boulder

Zone 5. Runs its own 2024 City of Boulder Energy Conservation Code — an ERI-based, all-electric, net-zero-oriented standard well ahead of the state minimum. REScheck is not the applicable compliance tool here; confirm the correct performance-path documentation with the city.

 

Fort Collins

Zone 5. Confirm the current adopted IECC edition and REScheck acceptance directly with the city's building department.

 

Aurora

Zone 5, part of the Denver metro area. Confirm current code edition with Aurora's building division.

 

Mountain and Resort Communities (Aspen, Breckenridge, Steamboat Springs, Summit County)

Zone 7 — the state's coldest and most stringent zone, with prescriptive requirements at the top end of Colorado's range. Confirm locally; several mountain resort communities have adopted energy standards well beyond the state floor.

 

Frequently Asked Questions About REScheck in Colorado

Does Colorado have a statewide energy code?

As of July 1, 2026, yes — in the form of a mandatory floor. The Colorado Model Low Energy and Carbon Code (MLECC), based on the 2024 IECC, is now the minimum every jurisdiction must meet or exceed the next time it updates its building code. Before that date, Colorado was a purely home-rule state with no statewide requirement.

 

Is REScheck accepted in Colorado?

In at least some major jurisdictions, yes. The Pikes Peak Regional Building Department (Colorado Springs area) explicitly names RESCheck as an accepted Total UA Alternative compliance method. Denver's building department references REScheck in its own posted documentation. Boulder is a clear exception — its ERI-based, all-electric code doesn't use REScheck as the compliance tool. Always confirm with the specific local building department.

 

What climate zones does Colorado have?

Four: Zone 4 (Baca, Las Animas, and Otero counties), Zone 5 (most of the state, including Denver, Colorado Springs, Boulder, and Fort Collins), Zone 6 (San Luis Valley and Western Slope counties), and Zone 7 (high mountain counties, including resort areas like Aspen, Breckenridge, and Steamboat Springs).

 

What is Colorado's blower door requirement?

3 ACH50, tested at 50 Pascals — the standard IECC threshold for Climate Zones 3 through 8, which covers all of Colorado.

 

What's different about the new Colorado MLECC?

It scales requirements by home size — homes under 5,000 square feet meet a baseline standard, 5,000-7,499 square feet face stricter requirements, and homes 7,500 square feet or larger must fully offset their energy use through on-site or community solar. It also puts all-electric heat pump systems and mixed-fuel systems on equal energy-performance footing.

 

Does REScheck work for Boulder projects?

No — Boulder's 2024 Energy Conservation Code is an ERI-based, outcome-verified performance code requiring all-electric new construction and, for larger homes, a net-zero ERI score. That's a different compliance framework than REScheck's prescriptive and UA-tradeoff paths. Confirm the correct documentation with the City of Boulder directly.

 

Is Colorado's energy code the same in every city?

No. Colorado has historically been a home-rule state, so individual cities and counties adopt and amend their own IECC editions. The new MLECC sets a statewide floor as of July 1, 2026, but it only takes effect in a given jurisdiction once that jurisdiction next updates its building code — so expect variation to continue for a while as different cities transition on their own timelines.

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