Maine Code · New Construction
Insulation Requirements for New Maine Homes: Foundations, Walls & Attics
Every new stick-built home permitted in Maine since April 7, 2025 is governed by the 2021 IECC (and the matching Chapter 11 of the 2021 IRC) as adopted into MUBEC. Here are the prescriptive requirements for each assembly — foundation, walls, and attic — plus the mandatory air leakage test, with citations to the exact code sections.
Start here
Know your climate zone, then read one row of one table
The IECC assigns every county a climate zone (2021 IECC §R301.1 / Table R301.1). Nearly all of Maine is Climate Zone 6; Aroostook County is Climate Zone 7. The prescriptive insulation requirements for your zone live in a single table:
The prescriptive R-value path. Assemblies meeting these R-values comply; alternatively, a builder may use the equivalent U-factor table (R402.1.2), the total UA alternative (R402.1.5), the simulated performance path (R405), or the Energy Rating Index path (R406). The numbers below are the Zone 6 prescriptive minimums as adopted in Maine.
| Component | 2021 requirement | What it means in practice |
|---|---|---|
| Ceiling / attic | R-60 | About 16–17″ of blown cellulose. R-49 satisfies the requirement when full-height R-49 extends over the wall top plate at the eaves (raised-heel/energy truss) — §R402.2.1. |
| Wood-frame wall | R-30 cavity, or R-20 + R-5ci, or R-13 + R-10ci, or R-0 + R-20ci | Four paths; three use continuous insulation (ci) outside the framing to kill thermal bridging. A 2×6 wall alone no longer complies without either deeper cavities or exterior foam/mineral wool. |
| Floor (over unconditioned space) | R-30 | Insulation must maintain permanent contact with the subfloor and be air sealed — §R402.2.7. |
| Basement wall | R-15ci, or R-19 cavity, or R-13 + R-5ci | Insulated from the top of the wall down 10 ft or to the basement floor, whichever is less — §R402.2.8. |
| Slab-on-grade | R-10ci, 4 ft depth | From the top of the slab, extending down/under/out 4 ft; heated slabs add R-5 under the full slab — §R402.2.9. |
| Crawl space wall | R-15ci, or R-19, or R-13 + R-5ci | For unvented, conditioned crawl spaces insulated at the walls — §R402.2.10. |
| Fenestration (windows/doors) | U-0.30 max | Skylights U-0.55 max; NFRC-labeled ratings — §R402.1.3 / §R303.1.3. |
Aroostook County (Zone 7): the same table's Zone 7 row applies — the headline differences are a higher floor requirement (R-38) with the same R-60 ceiling; verify the full Zone 7 row and footnotes in Table R402.1.3 before design. Stretch-code municipalities exceed these values. R-values in the table are minimums; where cavity insulation is compressed below its labeled thickness, the installed R-value governs (table footnote a).
Assembly by assembly
Foundations: the code finally treats the basement as part of the house
Uninsulated concrete is one of Maine's great heat drains — a full-height foundation can account for a fifth or more of a home's heat loss, and it runs the coldest surfaces in the building. The 2021 IECC closes this out for new construction:
Basement walls forming part of the thermal envelope must be insulated from the top of the basement wall down to 10 feet below grade or to the basement floor, whichever is less, at R-15 continuous, R-19 cavity, or R-13 cavity + R-5 continuous (Zone 6). Walls of unconditioned basements are treated as outside the envelope only if the floor above is insulated to the floor requirement (R-30) — you insulate the walls or the ceiling; either way the boundary must be complete.
Slabs less than 12 inches below grade: R-10 insulation from the top of the slab extending 4 feet down, under, or outward, with the top edge permitted to be cut at 45° where it meets the interior slab edge; heated slabs require an additional R-5 beneath the entire slab. Unvented crawl space walls: R-15ci / R-19 / R-13+5ci, run from the sill down and along the ground per the section, with the crawl space sealed and conditioned or mechanically ventilated as the IRC requires.
In practice on Maine foundations: rigid foam (or closed-cell spray foam) directly against the concrete, seams sealed, protected where exposed, integrated with a fully foamed rim joist — the rim is part of the wall assembly and is one of the leakiest, most condensation-prone details in the house. Fire-protective coverings over foam plastics (IRC R316) apply in habitable basements.
Walls: cavity insulation plus a thermal-bridge strategy
Zone 6's wall row offers four prescriptive paths (table above). The three continuous-insulation options reflect the physics on our envelope page: covering the studs matters as much as filling the bays. Builders can also trade assemblies through the U-factor (§R402.1.2) or total-UA (§R402.1.5) alternatives — a common route for double-stud, deep-cavity, or dense-pack designs. Whatever the path, §R402.4.1 requires the wall's air barrier to be continuous and in contact with the insulation, verified per Table R402.4.1.1.
Attics: R-60, ventilated, sealed at the floor
R-60 minimum in the ceiling/attic. Installing R-49 over 100 percent of the ceiling area satisfies the R-60 requirement wherever uncompressed insulation extends over the wall top plate at the eaves — the raised-heel truss allowance, which rewards full-depth insulation at the building's coldest, leakiest edge. Ceilings without attic space may reduce to not less than R-30 for up to 500 sq ft or 20% of ceiling area (§R402.2.2). Attic access hatches and doors must be insulated to the surrounding assembly's R-value and weatherstripped (§R402.2.5), with baffles/dams retaining loose fill at hatches and eaves.
An R-60 attic is roughly 16–17 inches of blown cellulose or 20+ inches of blown fiberglass — with eave baffles maintaining ventilation from soffit to ridge (IRC §R806) and, critically, a sealed attic plane beneath it. The code's air barrier table requires the ceiling air barrier to be continuous and all penetrations sealed before the insulation goes in — the same seal-then-insulate sequence BPI practice prescribes for existing homes.
The test that makes it real
Every new home gets a blower door: 3.0 ACH50 maximum
The building thermal envelope must be constructed to limit air leakage (§R402.4), the air barrier installed per Table R402.4.1.1, and the dwelling tested for air leakage. The measured leakage rate must not exceed 3.0 air changes per hour at 50 pascals (3.0 ACH50), with a signed written test report; where required by the code official, testing is by an approved third party. Ducts outside the envelope must be insulated (R-8/R-6) and duct systems pressure-tested (§R403.3).
This is the provision that changed Maine homebuilding culture: airtightness is no longer a virtue, it's a pass/fail number. Hitting 3.0 ACH50 reliably requires the air barrier to be planned, not improvised — sealed sheathing or membranes, gasketed plates, sealed penetrations, airtight electrical boxes at the ceiling. For the full story on the test itself — and what each CFM is worth — see our blower door guide. Ask your installer for mid-construction blower door testing so leaks are found while they're still reachable, and final testing with the documentation your code officer and §R401.3 energy certificate require.
Don't forget the paperwork: the energy certificate
§R401.3 requires a permanent certificate posted in the home listing the installed R-values, fenestration U-factors, blower door and duct test results, equipment efficiencies, and the code edition and compliance path used. It's the home's energy birth certificate — and the first thing a future buyer's inspector will look for.
Building to the 2021 code?
Our recommended installers work with Maine builders and owner-builders on code-compliant insulation packages: R-60 attics, continuous-insulation walls, foundation and rim joist systems, and mid-build plus final blower door testing.
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