Montana's Residential Energy Code: What's In Force Now, and What's Actually Proposed for 2026

1. What the Code Is and Who Enforces It

Montana is a single-code state, not a home-rule patchwork like Texas or Colorado. The Department of Labor and Industry's Business Standards Division, Building Codes Program adopts one uniform state building code by administrative rule, and that code - including its energy provisions - applies the same way whether your project is in Billings or on a county road outside Roundup.¹

The statewide code

The U.S. Department of Energy's own Building Energy Codes Program state profile lists Montana's Current State Code (Residential) as "2021 IECC with Amendments," with an Effective Date of 09/21/2024 and an Adoption Date of 06/11/2022, State Amendments = Yes, and "Can use COM/REScheck: Yes."¹ Those two dates aren't a typo - Montana's Building Codes Program adopted the base 2021 IECC on June 11, 2022 (effective the same day, per the department's own "Current Codes" page, which lists "International Energy Conservation Code, 2021 Edition - Effective: June 11, 2022"²), then went back and amended several sections of that same rule, with the amendment taking effect September 21, 2024. The administrative rule itself, ARM 24.301.161, carries the amendment history in its own footer: adopted 2022 MAR p. 911, effective 6/11/2022; amended 2024 MAR p. 2223, effective 9/21/2024.³

The rule is adopted under authority granted by 50-60-203, MCA ("The department may adopt by reference nationally recognized building codes in whole or in part... and may adopt rules more stringent than those contained in national codes") and implements the purpose language of 50-60-201, MCA, which directs the state building code to "encourage efficiencies and criteria directed toward design of building envelopes with high thermal resistance and low air leakage" among its stated objectives.⁴

Two tiers of enforcement, one code

This is the wrinkle that makes Montana different from most states covered on this site: the code text is uniform, but who reviews your plans and inspects your project depends on where you're building.

• Certified local programs. A city, county, or town may run its own building code enforcement program, but only after that program is certified by the state's Building Codes Program as compliant with applicable statutes and rules - current adopted code, fee schedule, and enforcement plan all on file and approved.⁵ Certified jurisdictions include Billings, Bozeman, Missoula (city and county), Great Falls, Helena, Kalispell, and dozens of smaller cities and counties, each certified for some or all of building, plumbing, electrical, and mechanical trades.⁵
• The state program directly. Everywhere a city, county, or town hasn't run its own certified program, the Building Codes Program itself is the authority having jurisdiction - reviewing plans, issuing permits, and performing inspections through field staff positioned around the state.⁵

Whichever tier applies to your project, the underlying code and amendments are identical - Montana's rule does not let a certified jurisdiction adopt a different edition or weaker amendments than the state's.⁶

A genuine Montana wrinkle: some sections differ by who's reviewing

Even though the base code and its numeric requirements are the same statewide, ARM 24.301.161 itself carves out a handful of administrative sections that apply differently depending on whether the state Building Codes Program or a certified local government is doing the reviewing. Two examples pulled directly from the current rule text:

• Subsections C105.2 and R105.2 of the IECC, "Required Inspections," are deleted in their entirety when the code is used by the department - but the same subsections "remain undeleted and available for use for certified local governments using the code."³
• Appendix CB (Solar-Ready Zone - Commercial) and Appendix RB (Solar-Ready Provisions for detached one- and two-family dwellings) may be adopted by a certified city, county, or town - but "the department shall not apply or enforce" either appendix when it is the reviewing authority.³

Neither of those specific carve-outs changes your prescriptive R-values or your blower-door number, but they illustrate a real administrative difference: a plan reviewed by the state Building Codes Program and a plan reviewed by a certified city like Billings or Missoula can, on paper, be checked against a slightly different administrative rule set even though both are working from the same base 2021 IECC. Confirm with your specific reviewing authority which optional sections apply to your project.

Who enforces it, concretely

If you're pulling a permit in Billings, Bozeman, Missoula, Great Falls, Helena, or another certified jurisdiction, that city or county's own building department is your point of contact and your plan reviewer.⁵ Outside those areas, permits and inspections run through the state's own Building Codes Program, which issues permits online through EBIZ.MT.GOV and staffs field inspectors by region.⁵

A narrow but real exemption for small residential buildings

Not every residential building falls under the state code at all. Under 50-60-102, MCA, the state building code generally does not apply to residential buildings containing fewer than five dwelling units (plus farm/ranch buildings and attached private garages used only for the owner's own use) unless the local jurisdiction has by ordinance made the code applicable to those structures.⁷ For energy conservation specifically, that same statute carves the requirement back in a different way: the energy-code provisions apply to residential buildings generally, but they are enforced by the department only for buildings with five or more dwelling units, while smaller residential buildings not otherwise subject to the state code comply through the builder self-certification program under 50-60-802, MCA instead of a plan-review process.⁷ In practice, this mostly matters outside certified jurisdictions; certified cities and counties commonly apply their adopted building code to residential construction by local ordinance regardless of unit count, which is why Billings, Bozeman, and Missoula all show live building-permit and REScheck activity for single-family homes despite the state statute's general small-residential carve-out.

Builder self-certification outside code jurisdictions

Where the state energy code is enforced through self-certification rather than plan review, 50-60-802, MCA requires the person who begins construction on a residential building to certify in writing to the building owner, at the conclusion of construction, that the building was built in compliance with the energy-efficient construction standards adopted under 50-60-203(1), MCA.⁸ Montana's Department of Environmental Quality, which runs public education for the residential energy code, confirms the practical mechanics: outside certified code jurisdictions, "provisions of the Energy Code became effective as of June 10, 2022," and builders document compliance through the self-certification process rather than a third-party plan review, with a blue Energy Code Compliance Label affixed to the electrical panel of the completed home.⁹

Climate zone

Montana sits in IECC Climate Zone 6B statewide. The 2021 IECC's own published climate-zone table (Table R301.1 / Table A101.1, "Climate Zones, Moisture Regimes, and Warm-Humid Designations by State, County and Territory," reproduced without modification across recent code-year sample chapters) lists Montana as a single entry: "MONTANA - 6B (all)" - meaning every county in the state carries the same climate-zone designation, with no county-by-county split the way neighboring Idaho, Wyoming, or Colorado have.¹⁰ Zone 6 is a cold climate (7,200 to 9,000 heating degree days on a 65°F base); the "B" suffix denotes a dry moisture regime. Practically, that means the same prescriptive insulation table and the same air-leakage number in Section 2 below apply whether your project is in Kalispell, Billings, or Ekalaka - there is no zone-boundary question to resolve within Montana the way there is in most other states this site covers.

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2. What Actually Changed / Where Jurisdictions Differ

Two different things are true about Montana's code right now, and keeping them separate matters more here than on almost any other state page on this site:

1. The code in force today is the 2021 IECC with Montana amendments, effective September 21, 2024. That's what every plan reviewer in the state - certified local program or the state Building Codes Program - is checking submittals against right now.
2. A proposed rule (not yet adopted, not yet effective) would move the reference edition to the 2024 IECC. It has a hearing date and a comment deadline, but no adoption date and no effective date.

Confusing "proposed" with "in force" is the single most consequential mistake a builder or designer could make reading about Montana's code right now, so this section covers each precisely, with the exact source for every number.

The 2021 IECC with Montana amendments - in force today

ARM 24.301.161 adopts the 2021 IECC and layers a specific set of Montana amendments on top. Pulled directly from the current rule text (Mont. Admin. r. 24.301.161, current through the 2024 amendment), the residential-relevant amendments include:

• Table R402.1.3 (Table 402.1.2 in the current rule numbering), Zone 6 wood-framed wall R-value: amended to "R-21 or R-20 + R-5ci or R-13 + R-10ci or R15ci" - a Montana-specific set of equivalent wall assemblies, not a single fixed R-value.¹⁰
• Table 402.1.2, maximum assembly U-factors for Climate Zone 6: Fenestration U-factor 0.30; Skylight U-factor 0.55; Ceiling U-factor 0.026; Frame Wall U-factor 0.045; Mass Wall U-factor 0.060; Floor U-factor 0.033; Basement Wall U-factor 0.050; Crawl Space Wall U-factor 0.055.¹⁰
• Section R402.4.1.2/R402.4.1.3, air leakage testing: the building or dwelling unit "shall be tested and verified as having an air leakage rate of not exceeding four (4.0) air changes per hour in Climate Zone 6," tested with a blower door at 0.2 in. w.g. (50 Pascals) - looser than the 2021 IECC's unamended national default of 3.0 ACH50 for Zone 6, and a real, deliberate Montana amendment.¹⁰
• Section R402.2.2, ceilings without attics: where the standard table would require insulation above R-30 and the roof/ceiling assembly can't fit it, the minimum required insulation drops to R-30, limited to 250 sq ft or 10% of the total insulated ceiling area, whichever is less - and this reduction doesn't apply if you're using the U-factor alternative (R402.1.4) or total UA alternative (R402.1.5) instead of the straight prescriptive table.¹⁰
• Section R402.2.10, crawl space walls: as an alternative to insulating floors over crawl spaces, crawl space walls may be insulated instead when the space isn't vented to the outside, subject to a detailed vapor-retarder and sealing specification in the rule text.¹⁰
• Section R403.3.7, building cavities as return ducts: Montana adds an exception allowing framing cavities to be used as return ducts, provided there's no atmospherically vented furnace, boiler, or water heater outside a sealed, isolated room, and the duct system tests at no more than 4 cfm/SF total leakage.¹⁰
• Sections R403.4 and R403.5, mechanical piping and hot-water pipe insulation: both deleted and replaced with a cross-reference to the Uniform Plumbing Code (2021 edition) instead of the IECC's own piping-insulation language - consistent with Montana using the UPC rather than the International Plumbing Code.¹⁰
• Section 103.1/105.1, plans and specifications: the building official may require plans and specifications prepared by an architect or other design professional licensed to practice by the state, except for owner-occupied, single-family dwelling houses - with the code official separately authorized to waive construction-document requirements entirely if not necessary to confirm compliance.¹⁰

Montana's Department of Environmental Quality corroborates the 4.0 ACH50 amendment directly on its own residential energy code page: "The building tightness standard is targeted at 4 air changes per hour (measured at 50 Pascals pressure control), or 4 ACH50. This is a Montana amendment to the 2021 IECC, which recommended 3 ACH50," and separately confirms the building-cavity return-duct amendment and the duct-testing exemption for ducts and air handlers located entirely within the building thermal envelope.⁹

The proposed 2024 IECC rule - MAR Notice 2026-95.1

On August 7, 2026, the Department of Labor and Industry published MAR Notice No. 2026-95.1, "2026 Updates to the State Building Code," in Montana Administrative Register Issue No. 15.¹¹ Directly from the notice's own text:

• A virtual public hearing was held Friday, August 28, 2026, at 9:00 a.m. - remote conferencing only, no in-person hearing.¹¹
• Written comments were due Friday, September 4, 2026, at 5:00 p.m., submitted via dli.mt.gov/rules or by mail to P.O. Box 1728, Helena, Montana 59624.¹¹
• The department's Building Codes Council met June 8, 2026, in Helena and over Zoom to review and provide feedback on the proposed amendments, as required by 50-60-115, MCA.¹¹

The proposed rule text itself would amend ARM 24.301.161(1) to read: "The department adopts and incorporates by reference the International Code Council's International Energy Conservation Code, 2021 2024 Edition" - with the same 2021-to-2024 strikethrough/insert pattern applied to the IBC (24.301.131), IRC (24.301.154), IMC and IFGC (24.301.109), IEBC (24.301.171), and other referenced codes across the same notice.¹¹ The notice's own Reasonable Necessity Statement is direct about the purpose: "The proposed amendments are necessary to update the International Energy Conservation Code to the 2024 version, along with updating the associated subsection numbers and titles."¹¹

What the proposed rule would change for residential energy compliance specifically (verified against the notice's numbered amendments to ARM 24.301.161):

• The single biggest number to watch: air leakage tightens from 4.0 to 3.0 ACH50 in Zone 6. The proposed amendment to the testing and leakage-rate subsections strikes "four" and inserts "3.0" - meaning Montana's current amendment loosening the 2021 IECC's blower-door requirement would be reversed, bringing Montana back in line with the national default air-leakage number.¹¹ This is not yet in force. The number a builder needs to hit today is still 4.0 ACH50; 3.0 ACH50 is proposed only.
• Table R402.1.2, Zone 6 U-factors, are proposed to carry forward unchanged at the same values currently in force: Fenestration 0.30, Skylight 0.55, Ceiling 0.026, Frame Wall 0.045, Mass Wall 0.060, Floor 0.033, Basement Wall 0.050, Crawl Space Wall 0.055 - just renumbered within the 2024 IECC's section structure.¹¹
• The Zone 6 wood-framed wall R-value alternative is proposed to change in one option: the notice strikes the current "R-21 or R-20 + R-5ci or R-13 + R-10ci or R15ci" and inserts "R-21 or R-20 + R-5ci or R-13 + R-10ci or R-20ci" - the R-21 cavity, R-20 + R-5ci and R-13 + R-10ci assemblies carry forward unchanged, but the continuous-insulation-only option would rise from R-15ci to R-20ci. Proposed only; the R15ci figure is what applies today.¹¹
• The ceilings-without-attics reduction and crawl-space-wall alternative are both proposed to carry forward in substantially the same form as the current rule, with the reduction floor for ceilings without attics remaining R-30.¹¹
• Piping insulation cross-references shift from the 2021 to the 2024 edition of the Uniform Plumbing Code, consistent with the UPC itself being proposed for an edition update in the same notice.¹¹
• The plans/specifications requirement, including the owner-occupied single-family dwelling exception, is proposed to carry forward unchanged - the same design-professional stamp requirement, the same exception, and the same code-official waiver authority.¹¹
• The building-cavity return-duct exception is proposed to carry forward with the same 4 cfm/SF total-leakage threshold and the same exemption for ducts and air handlers located entirely within the thermal envelope.¹¹

No adoption date and no effective date have been set for the 2024 IECC edition change. The notice is a Notice of Proposed Rulemaking - a formal step that precedes a Notice of Adoption, which the department has not yet issued as of this page's research. Nothing on this page should be read as predicting when, or whether in its currently proposed form, the 2024 IECC will take effect in Montana.

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3. Your Ways to Comply

Whichever edition ultimately governs your project - the 2021 IECC with Montana amendments today, or the 2024 IECC once and if the proposed rule is adopted - the same three compliance paths carry forward, consistent with how the IECC's residential chapter is structured across recent editions:

Path	Typical code section	What it is	Best fit
Prescriptive	R402.1.1/R402.1.2	Meet every line item in the R-value/U-factor table for Zone 6, exactly as amended by Montana.	Simple plans, standard 2x6 framing, no unusual glazing, tight budget and timeline.
UA / total-building-envelope trade-off (REScheck)	R402.1.5	Total heat loss (UA) of the actual envelope must be at or below the UA of a prescriptive-minimum reference home. Weak components can be offset by strong ones elsewhere.	2x4 walls, large glazing packages, log construction, or an assembly that can't hit every prescriptive number individually but performs well overall.
Simulated performance / Energy Rating Index (ERI/HERS)	R405 / R406	A full energy model or HERS rating compares the proposed design's performance against a standard reference design or scores it on the 0-100 ERI scale.	Custom homes, non-standard HVAC or renewable strategies, or projects already doing a HERS rating for a rebate, tax credit, or green certification.

Which path fits which Montana project:

• Simple, code-minimum spec homes with standard 2x6 framing - the prescriptive path is fastest through plan review and cheapest to document, especially since Montana's climate zone is uniform statewide and there's no zone-boundary ambiguity to resolve.
• 2x4 walls, large window packages, log or timber-frame construction, or one component short of the table - the UA trade-off (REScheck) is usually the cheapest fix. It's also the pathway most Montana plan reviewers, certified local or state, see most often on residential submittals.
• Custom homes, unusual HVAC, or a design the prescriptive/UA path can't cleanly capture - go simulated performance. It costs more and takes longer, but it's the only path flexible enough for genuinely non-standard designs.
• Already paying for a HERS rating for ENERGY STAR, DOE ZERH, a NorthWestern Energy rebate, or the federal 45L tax credit - use the ERI path and let that rating do double duty as code compliance.

Which REScheck edition to select. Montana does not have its own dedicated state-specific REScheck edition the way Florida, Louisiana, Massachusetts, Denver, or a few other jurisdictions do - DOE's own REScheck supported-codes list names those state-specific editions explicitly and Montana isn't among them.¹² That means the correct approach today is selecting the standard 2021 IECC edition inside REScheck and then applying Montana's amendments - most importantly the 4.0 ACH50 Zone 6 air-leakage number and the Zone 6 wall-assembly alternatives - manually, since the tool's UA calculation doesn't encode state-specific amendments automatically. A Bozeman residential permit file we reviewed for this research shows exactly this pattern in practice: a REScheck compliance certificate stating the building "has been designed to meet the 2021 IECC requirements... and to comply with the mandatory requirements," attached to the permit alongside the jurisdiction's other required documents.¹³ If the proposed 2024 IECC rule is adopted, the correct edition selection inside REScheck will change to 2024 once that edition's effective date arrives - and not before.

What about Manual J? The 2021 IRC, adopted statewide alongside the 2021 IECC under the same rulemaking structure, carries the IRC's standard mechanical-code section requiring HVAC equipment to be sized per ACCA Manual S based on loads calculated with ACCA Manual J or an equivalent approved method. That requirement travels with the IRC adoption itself, not with any one city's local amendment - so it applies in certified jurisdictions and under direct state Building Codes Program review alike. Montana's DEQ energy code page reinforces the same point directly to builders, pointing them to an ACCA Manual J brochure and noting plainly that "heating and cooling equipment must be sized according to a code-acceptable procedure" as "an important consideration in residential construction."⁹ Zone 6B's cold winter design conditions - roughly -10°F to -20°F design temperatures depending on elevation and location within the state - make correct Manual J sizing more consequential in Montana than in a milder climate zone: an oversized furnace short-cycles and wastes energy in mild shoulder-season weather, while an undersized one can't keep up during a genuine cold snap.

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4. Montana-Specific Wrinkles

The state code is uniform, but who's reviewing your plans isn't. Unlike Texas or Colorado, you won't find a different R-value table or a different blower-door number by crossing a city line in Montana. What changes is the reviewing authority: a certified local program (Billings, Bozeman, Missoula, Great Falls, Helena, Kalispell, and dozens of smaller jurisdictions, each certified for some or all trades) versus the state Building Codes Program directly.⁵ Confirm which one applies to your parcel before assuming a submittal process, a fee schedule, or a turnaround time from a project in a different city.

"Proposed" and "in force" are not the same thing, and this page will not blur them. As of this research, the 2021 IECC with Montana amendments - including the looser 4.0 ACH50 Zone 6 air-leakage number - is what your plan reviewer is checking against. MAR Notice 2026-95.1 proposes to move to the 2024 IECC and tighten that same air-leakage number to 3.0 ACH50, but the department has not issued a Notice of Adoption or set an effective date. If your project's permit application lands after an eventual adoption and effective date, you'll need to know which code year and which ACH50 number apply - ask your building department directly rather than assuming either the current or proposed figure.

Montana's Zone 6 air-leakage amendment is unusually visible for a state amendment. Most states that amend the IECC do so quietly in a handful of technical sections; Montana's decision to loosen the national default 3.0 ACH50 Zone 6 requirement to 4.0 ACH50 is explained on the state's own public-facing DEQ energy code page, in plain language, specifically so builders self-certifying outside code jurisdictions know the number to test against.⁹ If the proposed 2024 rule is adopted as currently drafted, that 4.0 ACH50 number reverts to 3.0 - a genuinely tighter target that changes air-sealing practice on the job site, not just a paperwork edition bump.

Owner-occupied single-family dwellings get a real, code-level exception on stamped plans. ARM 24.301.161's plans-and-specifications amendment lets the building official require plans stamped by an architect or other design professional licensed to practice by the state - except for owner-occupied, single-family dwelling houses, which are carved out of that requirement by the rule text itself, and the code official can separately waive construction-document requirements entirely where not necessary to confirm compliance.¹⁰ That exception is proposed to carry forward unchanged under MAR Notice 2026-95.1.¹¹

Small residential buildings outside a certified jurisdiction may not see a plan reviewer at all. Under 50-60-102, MCA, residential buildings with fewer than five dwelling units fall outside the state building code's mandatory plan-review structure unless the local jurisdiction has separately made the code applicable - meaning energy-code compliance for many rural single-family homes runs through builder self-certification rather than a REScheck submittal reviewed by a plan checker.⁷ A documented REScheck report is still the clearest way to demonstrate that self-certification is accurate, and it's the same document a certified jurisdiction's reviewer would expect to see if your project is inside one.

ADUs are a live area of Montana law, tangled up in litigation that only recently resolved. Senate Bill 528 (2023), codified at 76-2-345, MCA, requires municipalities to adopt zoning regulations allowing at least one accessory dwelling unit by right on a lot with a single-family dwelling.¹⁵ The related energy-code rulemaking notes that SB 528's implementation was delayed by litigation - Montanans Against Irresponsible Densification (MAID), LLC v. State was enjoined from taking effect January 1, 2024, appealed to the Montana Supreme Court twice, and the injunction was ultimately lifted with the statute held constitutional in a March 17, 2026 decision.¹¹ Appendix BC, Accessory Dwelling Units, is not adopted by the state Building Codes Program itself under the currently proposed rule, but certified cities, counties, and towns have the option of adopting it - another example of the "same base code, different optional layer by jurisdiction" pattern.¹¹ If you're building an ADU in a Montana city, confirm with that specific jurisdiction whether Appendix BC has been locally adopted.

What to ask your local building department (or the state Building Codes Program) before you finalize a compliance strategy:

1. Is my project reviewed by a certified local building code program, or directly by the state Building Codes Program - and does that change which optional sections or appendices apply?
2. Is the 2021 IECC with Montana amendments still the code in force for my permit application date, or has MAR Notice 2026-95.1 (or a successor notice) been adopted with an effective date by then?
3. Which Zone 6 air-leakage number applies to my project - 4.0 ACH50 (current) or 3.0 ACH50 (proposed)?
4. Do you require a Manual J load calculation submitted with the permit application, or only upon request?
5. Does my building fall under the state building code's plan-review structure, or does it qualify for builder self-certification under 50-60-102/50-60-802, MCA?

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5. Frequently Asked Questions

What is Montana's current residential energy code? The 2021 IECC with Montana amendments, adopted statewide by rule (ARM 24.301.161) on June 11, 2022 and last amended effective September 21, 2024.¹ A proposed rule published August 7, 2026 would move the reference edition to the 2024 IECC, but no adoption date or effective date has been set for that change - the 2021 IECC with Montana amendments remains the code in force.

Is Montana about to switch to the 2024 IECC? A change is proposed, not adopted. MAR Notice No. 2026-95.1, published August 7, 2026, proposes moving ARM 24.301.161 from the 2021 to the 2024 IECC.¹¹ A virtual hearing was held August 28, 2026, comments were due September 4, 2026, and the department's Building Codes Council reviewed the proposal June 8, 2026. Until a Notice of Adoption with an effective date is issued, the 2021 IECC stays in force.

Will a REScheck report be accepted for a Montana permit? Yes. DOE's Montana state profile lists "Can use COM/REScheck: Yes" for both commercial and residential.¹ Montana doesn't have its own dedicated REScheck edition, so you run the standard 2021 IECC edition inside the tool and apply Montana's amendments - the 4.0 ACH50 Zone 6 number chief among them - manually.

Is a Manual J load calculation required in Montana? The 2021 IRC, adopted statewide alongside the 2021 IECC, requires HVAC equipment to be sized per ACCA Manual S based on ACCA Manual J loads. That travels with the state code itself, not any one city's amendment, so it applies under certified local programs and direct state review alike.

Who prepares these reports? Travis Smith, a certified Residential Energy Inspector / Plans Examiner - the same certification your building department's reviewer holds.

My city isn't Billings, Bozeman, or Missoula - what code applies to me? Montana runs a single state building code, so the base edition and amendments are the same everywhere - the difference is who enforces it. In a certified jurisdiction, that local building department reviews your plans; everywhere else, the state Building Codes Program does. Tell us your city or county when you order a report and we'll confirm which office is reviewing your permit.

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6. Get It Right the First Time - Before Plan Review Sends It Back

Need a REScheck report matched to the code actually in force? The most consequential Montana-specific mistake right now isn't picking the wrong city - it's picking the wrong code year or the wrong ACH50 target during a rulemaking transition. We run the report against the 2021 IECC with Montana's amendments, currently the code in force everywhere in the state, and we track MAR Notice 2026-95.1's status so your report reflects whatever your AHJ is actually enforcing on your submittal date - not a default the software happens to suggest. A REScheck report is $149, delivered ready to attach to your permit application.

Need a Manual J load calculation? Zone 6B's cold winter design conditions make equipment sizing consequential, and the 2021 IRC's equipment-sizing requirement applies statewide regardless of whether a certified city or the state Building Codes Program is reviewing your permit. A room-by-room Manual J report gives you defensible equipment sizing your HVAC contractor and building department can both stand behind. A Manual J report is $249 for homes up to 3,000 sq ft.

Need both? The combined Manual J + REScheck package is $349 - one submission covering the equipment-sizing requirement under the residential code and the envelope/compliance requirement under whichever IECC edition is in force on your submittal date.

Every report comes with the same guarantee: accepted by your building department, or we revise it free until it clears.

Adding a heat pump or replacing existing equipment? See our heat pump changeout Manual J guide for when a load calculation is expected on an equipment swap, not just new construction.

Building an ADU? With SB 528's litigation resolved and municipalities required to allow ADUs by right, see our Manual J for ADU guide for what that calculation needs to cover, and confirm with your city whether Appendix BC has been locally adopted.

Installing a ductless mini-split? Our mini-split Manual J and permit requirements guide covers sizing documentation for ductless systems specifically.

Contact us with your plans and the city or county where you're pulling the permit, and we'll confirm whether a certified local program or the state Building Codes Program is your reviewing authority, and which code edition and ACH50 number apply on your submittal date.

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Verification Appendix

Every claim on this page was checked directly against Montana's own administrative rules, the state's own agency pages, the primary-source text of the proposed rulemaking notice, or a jurisdiction's own published document during this research pass - not a national aggregator or a secondary blog summary, except where explicitly noted below.

1. Current state code = 2021 IECC with Amendments, effective 09/21/2024, adopted 06/11/2022; "Can use COM/REScheck: Yes." VERIFIED (primary source). energycodes.gov/status/states/montana's own State Profile table lists exactly these values for both Commercial and Residential code type, linking directly to the underlying rule at rules.mt.gov.

2. Montana's Building Codes Program current codes list, including "International Energy Conservation Code, 2021 Edition - Effective: June 11, 2022." VERIFIED (primary source). bsd.dli.mt.gov/building-codes-permits/current-codes, the department's own "Current Codes" page, lists this line item alongside the 2021 editions of the IBC, IRC, IEBC, UPC, IMC, and IFGC, all effective the same date, and separately notes the codes are amended by ARM Title 24, Chapter 301.

3. ARM 24.301.161 text and amendment history (adopted 2022 MAR p. 911, Eff. 6/11/2022; amended 2024 MAR p. 2223, Eff. 9/21/2024), including the Zone 6 wall R-value table, U-factor table, and the 4.0 ACH50 air-leakage amendment. VERIFIED (primary source). Cross-checked against two independent legal-publisher mirrors of the current rule text (law.cornell.edu/regulations/montana/Mont-Admin-r-24.301.161 and Montana's own rules.mt.gov/Administrative Rules gateway), both showing identical rule language and the identical amendment-history footer.

4. 50-60-201 and 50-60-203, MCA (purpose of the state building code; department's authority to adopt by reference). VERIFIED (primary source). mca.legmt.gov's own published Montana Code Annotated 2025 text for both sections, matching the authorizing/implementing statute citations printed at the end of ARM 24.301.161 itself.

5. Certified local government structure; list of certified jurisdictions including Billings, Bozeman, Missoula (city and county), Great Falls, Helena, Kalispell. VERIFIED (primary source). bsd.dli.mt.gov/building-codes-permits/certified-government, the department's own "Certified City, County and Town Programs" page, publishes the full current list with building official contact information for each certified jurisdiction and trade scope (Building/Plumbing/Electrical/Mechanical/Pool/WUI).

6. Certified jurisdictions must use the same code edition and amendments as the department (ARM 24.301.202). VERIFIED (primary source). bsd.dli.mt.gov/building-codes-permits/code-interpretation-request states directly: "ARM 24.301.202 requires the codes adopted by certified cities, counties and towns must be same edition with the same amendments as those adopted by the Department."

7. 50-60-102, MCA (small-residential exemption from the state building code; energy-code enforcement split between department plan review for 5+ units and builder self-certification for smaller residential buildings). VERIFIED (primary source). mca.legmt.gov's published text of 50-60-102, cross-referencing 50-60-203 and 50-60-802 exactly as printed in the statute.

8. 50-60-802, MCA (builder self-certification requirement and mechanism). VERIFIED (primary source). mca.legmt.gov's published text of 50-60-802, Montana Code Annotated 2023/2025.

9. DEQ's public guidance on the 4.0 ACH50 Montana amendment, self-certification outside code jurisdictions, building-cavity return-duct amendment, and duct-testing exemption for fully-conditioned ducts/air handlers. VERIFIED (primary source). deq.mt.gov/energy/Programs/code, the Montana Department of Environmental Quality's own Residential Energy Code and Energy Efficiency program page, states each of these points in its own words and separately links the underlying rule (ARM 24.301.161) and an ACCA Manual J brochure it hosts directly.

10. Montana climate zone = 6B statewide, per the IECC's own published climate-zone table; Zone 6 R402.1.2/R402.1.3 U-factor and wall-R-value amendments; Zone 6 4.0 ACH50 amendment text. VERIFIED (primary source), with a scope note. The official 2021 IECC Table R301.1/Table A101.1, "Climate Zones, Moisture Regimes, and Warm-Humid Designations by State, County and Territory" (reproduced on the state's own up.codes viewer of the adopted Montana Energy Code 2021, which mirrors the official code text and carries no separate editorial content in the table itself), lists a single entry for Montana - "MONTANA 6B (all)" - with no county-level exceptions, matching the same table's treatment of Nebraska ("5A (all)") and Connecticut ("5A (all)") as other single-zone states. This page prints only the zone designation and county-list result actually shown in that official table; it does not rely on secondary HVAC-contractor or lead-generation sites that describe Montana as spanning three zones (5B/6B/7), since the code's own current table for Montana shows no such split. If your project sits at unusual elevation or you have independent reason to believe a specific Montana county carries a different designation, confirm directly with the published code text or your local building department before finalizing a design.

11. MAR Notice No. 2026-95.1: publication date (August 7, 2026, Issue No. 15), hearing date (August 28, 2026, 9:00 a.m., virtual only), comment deadline (September 4, 2026, 5:00 p.m.), Building Codes Council review date (June 8, 2026), proposed 2021-to-2024 IECC edition change to ARM 24.301.161, and the specific proposed amendments to the Zone 6 air-leakage rate (4.0 to 3.0 ACH50), Table 402.1.2 U-factors, wall R-value alternative, ceilings-without-attics reduction, crawl-space-wall alternative, piping-insulation UPC cross-reference, plans/specifications and owner-occupied exception, building-cavity return-duct exception, and the SB 528/MAID v. State ADU litigation history. VERIFIED (primary source, PDF). dli.mt.gov/_docs/rules/2026-95.pdf, the department's own published Notice of Proposed Rulemaking, fetched and extracted in full for this research pass (55 pages). The department's own bsd.dli.mt.gov/building-codes-permits landing page independently confirms the same hearing date, hearing format, and comment deadline in its own words, corroborating the notice text.

12. REScheck supported codes list; no Montana-specific state edition exists (only Florida, Louisiana, Massachusetts, New York City, DC, Denver, Puerto Rico, Utah, and Vermont have dedicated state editions). VERIFIED (primary source). energycodes.gov/rescheck, DOE's own REScheck program page, publishes the exact supported-codes list (version 4.2.0, October 6, 2025 release) naming the base 2009-2024 IECC editions plus each state-specific edition by name; Montana does not appear in the state-specific list.

13. Billings: adopted the 2021 IECC as amended by ARM 24.301.161 via Administrative Order No. 150; Bozeman: live REScheck 2021 IECC compliance certificate attached to an actual residential permit file. VERIFIED (primary source). billingsmt.gov's own published Administrative Order No. 150 ("2021 Edition (IECC), International Energy Conservation Code, 2021 Edition (IECC), as amended by ARM 24.301.161") and its "Adopted Codes" page (billingsmt.gov/323/Adopted-Codes) both confirm the 2021 IECC adoption directly. Separately, a Bozeman residential permit document hosted on the city's own document-management system (weblink.bozeman.net) shows a REScheck compliance certificate stating the building "has been designed to meet the 2021 IECC requirements... and to comply with the mandatory requirements" - direct evidence of REScheck's actual use on a real Montana residential submittal, not just a theoretical acceptance.

14. IRC equipment-sizing requirement (Manual J/Manual S) as a stable, standard IRC citation carried by Montana's 2021 IRC adoption. VERIFIED as a stable, standard IRC provision. This is the same nationally standard mechanical-sizing section discussed on this site's other state pages; it was not independently re-pulled from the published code text specifically for Montana's adopted 2021 IRC edition during this research pass, since the section's substance (equipment sized per ACCA Manual S based on ACCA Manual J loads) has not materially changed across recent IRC editions nationally, and Montana's DEQ energy code page independently corroborates the sizing requirement in its own guidance to builders.

15. SB 528 (2023), codified as 76-2-345, MCA: municipal ADU by-right zoning requirement. VERIFIED (primary source). mca.legmt.gov's published text of 76-2-345, MCA, confirming the by-right ADU zoning requirement, cross-checked against the MAR Notice 2026-95.1 text's own description of SB 528 and the MAID v. State litigation history.

What remains genuinely unconfirmed: the exact energy code enforced by every certified jurisdiction not named on this page (dozens exist beyond Billings, Bozeman, and Missoula); whether MAR Notice 2026-95.1 will be adopted in its currently proposed form, with amendments, or withdrawn, and on what effective date; and the precise permit-application-date cutover rule each certified jurisdiction would apply if and when a 2024 IECC adoption takes effect mid-project. Confirm any of these directly with the relevant building department or the state Building Codes Program before finalizing a compliance strategy.

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Sources

This page summarizes Montana's statewide residential energy code, currently in force, and a pending proposed rule change, for general guidance. Because a rulemaking is in progress, confirm the code edition and air-leakage requirement in force on your specific permit application date with your building department before finalizing design or bidding.