Michigan’s insulation code sets minimum R-value requirements by climate zone, with the state divided into three zones under the International Energy Conservation Code system. Southern Michigan sits in Zone 5A, the northern Lower Peninsula and eastern Upper Peninsula in Zone 6A, and the western Upper Peninsula in Zone 7. R-values rise as you move north, and each building component — ceiling, wall, basement, floor, slab — has its own threshold. The figures below reflect the 2021 IECC standards that Michigan’s updated code is built around.
Finding Your Climate Zone First
Every R-value in the code is keyed to climate zone, so the zone is the first thing to pin down. Zone 5A covers Detroit, Ann Arbor, Grand Rapids, and Kalamazoo along with the rest of southern Michigan. Zone 6A takes in Traverse City, Alpena, Sault Ste. Marie, and much of the northern Lower Peninsula and eastern U.P. Zone 7, the coldest classification in the state, covers the western Upper Peninsula, including Houghton, Gogebic, and Baraga counties.
Get the zone wrong and you can undersize insulation across the entire envelope. If your county sits near a zone boundary, your local building department can confirm the classification before you order materials.
Ceiling and Attic R-Values
Attic ceilings carry the highest R-value requirement in a Michigan home. The 2021 IECC sets a baseline of R-60 for Climate Zones 5 through 8, which covers every part of the state. The code allows R-49 as an alternative when the roof truss is tall enough at the eaves to keep the full insulation depth continuous over the exterior wall top plate.1Building America Solution Center. 2021 IECC Climate Zone 5A: Vented Attic, Interior Double Wall, Interior Insulated Basement
In practice, most builders use raised-heel trusses and blow in R-49, which is substantially deeper than what older Michigan homes typically have in place.
Exterior Wall R-Values
Wood-framed above-grade walls have the same requirement in both Zone 5A and Zone 6A: R-20 cavity insulation on its own, or R-13 cavity insulation plus R-5 continuous insulation on the exterior sheathing. Where structural sheathing covers 40 percent or less of the wall’s exterior, the continuous insulation value can be reduced by up to R-3 at those locations so the sheathing thickness stays consistent.2Department of Licensing and Regulatory Affairs. Part 10 Michigan Uniform Energy Code – Public Hearing Draft
The R-13+5 option is common because it uses standard 2×4 framing with rigid foam board on the outside. That combination is often cheaper than building with 2×6 lumber sized for R-20 batts.
Basement and Crawl Space Wall R-Values
Below-grade walls are where the zones start to diverge on the specifics, though the spread stays narrow.
In Zone 5A, basement walls require R-15 continuous insulation, R-19 cavity insulation, or R-13 cavity insulation plus R-5 continuous insulation. Zone 6A calls for R-15 continuous or R-19 cavity, and also accepts the R-13+5 combination.3State of Michigan – EGLE. Residential Compliance Paths – Michigan Energy Code Compliance Aid Crawl space walls follow the same values.
Basements and crawl spaces have historically been the most under-insulated parts of Michigan homes, so these numbers represent a real jump for new construction compared with older stock.
Floor and Slab R-Values
Floors over unconditioned spaces such as unheated garages or vented crawl spaces must reach R-30 in Zones 5 and 6.4Energy Star. Recommended Home Insulation R-Values
For slab-on-grade construction, the code requires R-10 insulation extending 4 feet down from the top of the slab or along the slab perimeter. Heated slabs need an additional R-5 beneath the full slab area on top of the perimeter insulation.
Vapor Retarder Rules That Go With the R-Values
Insulation performance in a cold climate depends on keeping moisture out of the wall cavity, so the code pairs R-value requirements with vapor retarder rules. Wood-framed walls in Zones 5, 6, and 7, covering the entire state, need a Class I or Class II vapor retarder on the interior side.5Legal Information Institute. Michigan Administrative Code R 408.30522a – Vapor Retarders Class I materials include polyethylene sheeting; Class II includes kraft-faced fiberglass batts.
Below-grade walls are the exception. They can use a Class III vapor retarder or omit one, as long as no air-permeable insulation is installed without a Class I or II retarder in place.
If you are building with continuous exterior insulation under the R-13+5 approach, the exterior foam shifts where the dew point falls inside the wall. Verify that your assembly keeps the condensation plane outside the cavity insulation before you close the walls up.
Air Leakage: The Number That Goes With the R-Values
R-values do not perform if air moves freely through the envelope, and Michigan’s code treats air sealing as a paired requirement. The current residential code requires new homes to test at or below 4 air changes per hour at 50 pascals (4 ACH50) with a blower door. The 2021 IECC tightens that to 3 ACH50, a 25 percent reduction that requires substantially more attention to sealing details during construction.3State of Michigan – EGLE. Residential Compliance Paths – Michigan Energy Code Compliance Aid
Hitting 3 ACH50 means sealing every penetration through the envelope: electrical boxes, plumbing runs, duct shafts, dropped soffits, and the joint between foundation and framing. Access openings to unconditioned attics, including pull-down stairs and knee wall doors, must be sealed and insulated. An air barrier is required between any attached garage and the conditioned portion of the home.2Department of Licensing and Regulatory Affairs. Part 10 Michigan Uniform Energy Code – Public Hearing Draft The test must be completed before the local department will issue a final certificate of occupancy, and no compliance path lets you skip it.
Which Code Edition Applies to Your Project
The R-values above reflect the 2021 IECC standards Michigan’s updated code is built on, but adoption is not uniform across building types. The 2021 Michigan Building Code was adopted at the state level in late 2024, and local jurisdictions began enforcing it for commercial buildings in April and May 2025.6City of Ann Arbor. Enforcement of 2021 Michigan Building Code Begins May 1, 2025 Michigan’s commercial energy code now follows the 2021 IECC and ASHRAE 90.1-2019.
The residential picture is less settled. The residential energy code listed with the U.S. Department of Energy still references the 2015 IECC with Michigan-specific amendments, and a court order issued in mid-2025 delayed full adoption of the 2021 Michigan Residential Code.7Building Energy Codes Program. Michigan Enforcement timelines can differ by municipality even after a statewide adoption date, so before you finalize an insulation package for a residential project, confirm with the local building department which edition currently governs permits in that jurisdiction. Many departments are already applying the 2021 values regardless of the statewide status.
Prescriptive R-Values or a Trade-Off
The R-values in the tables above come from the prescriptive path, which is the simplest route to compliance: meet or exceed every component number and you are done. That path leaves no room for trade-offs, though, so if one component is hard to hit for your design, two other paths are available.
- The trade-off path uses software — REScheck for residential, COMcheck for commercial — to show the overall envelope meets or exceeds the prescriptive standard even when individual components fall short. Slightly lower wall insulation can be offset by windows that beat the minimum U-factor, for example. Michigan is on the list of states where COMcheck can be used to show compliance. For residential trade-offs, use REScheck-Web version 4.2.0 or later, which supports the 2021 IECC; the older desktop version only goes through 2015.8Building Energy Codes Program. States That Can Use COMcheck to Show Compliance9Building Energy Codes Program. REScheck
- The performance path uses whole-building energy simulation to show the proposed design uses no more energy than a code-compliant reference building. Under the 2021 IECC, the proposed design must show an additional 5 percent improvement over that reference. This path gives the most design freedom but requires energy modeling expertise.
All three paths still require the mandatory air leakage test at the applicable ACH50 threshold. That number is not tradeable.