Michigan doesn’t have one insulation number. The state sits across three separate IECC climate zones, so the right R-value for an attic, floor, or rim joist depends on which of the state’s 83 counties a home sits in, not on a single statewide rule. Broadly, attics need R49 to R60 and floors R19 to R38, but which exact figure applies changes with the county line.
Which climate zone Michigan is in
An IECC climate zone is a number from 1 to 8, colder as the number climbs, sometimes followed by a letter that marks the moisture regime: A for moist, B for dry, C for marine. Zones 7 and 8 carry no letter at all. That letter matters because the ENERGY STAR insulation table groups zones by both number and letter, and picking the wrong group means buying the wrong amount of material.
Michigan spans three of these zones, according to the 2021 International Energy Conservation Code’s county-by-county table. There is no single zone for the state, and no shortcut that skips the county lookup.
| Climate zone | Number of Michigan counties |
|---|---|
| 6A | 44 of 83 |
| 5A | 37 of 83 |
| 7 | 2 of 83 |
Read that as a county count, not a population share. Forty-four counties in zone 6A doesn’t mean 44 percent of Michigan residents live under that classification; a couple of densely populated counties can outweigh a dozen rural ones on the map above. The right way to use this table is to find your own county in the IECC’s official listing or on the ENERGY STAR climate zone map, then carry that letter and number into the next section. This page won’t guess it for you, and neither should you.
The insulation levels that apply here
ENERGY STAR publishes retrofit insulation levels for existing wood-framed homes, based on the 2021 IECC’s residential provisions. These are not new-construction code minimums; they’re recommended targets for upgrading a house that’s already standing. The table below has a header that’s easy to misread: two of the three numbers cover the attic, split by whether it’s currently uninsulated or already has 3 to 4 inches in place, and the third number is for the floor, a different surface entirely.
| Zone | Attic, uninsulated | Attic, already has 3-4 in. | Floor |
|---|---|---|---|
| Zone 1 | R30 | R25 | R13 |
| Zone 2 | R49 | R38 | R13 |
| Zone 3 | R49 | R38 | R19 |
| Zones 4A and 4B | R60 | R49 | R19 |
| Zones 6, 5, and 4C | R60 | R49 | R30 |
| Zones 7 and 8 | R60 | R49 | R38 |
For Michigan, two rows do the actual work. The 37 counties in zone 5A and the 44 counties in zone 6A both fall under “Zones 6, 5, and 4C,” meaning an uninsulated attic targets R60, a partially insulated one targets R49, and the floor targets R30. The two counties in zone 7 move up one row, to a floor target of R38, everything else in that row staying the same. That’s a meaningful difference for the handful of households sitting in that colder pocket of the state, and it’s exactly why averaging zones into one figure would shortchange them. None of these figures should be converted into a depth in inches; that depends on the material, and the manufacturer prints it on the bag.
Sealing comes before insulating
ENERGY STAR treats air sealing and insulation as two halves of one job, and its own project guidance puts attic air sealing ahead of attic insulation, not alongside it or after. The reasoning is straightforward: insulation slows heat moving through a material, but it does nothing to stop air moving around it. Lay batts or blown-in fiber over an unsealed gap and you’ve hidden the leak, not closed it.
The recommended sequence for a retrofit project runs in a specific order:
- Seal air leaks in the attic (top plates, wiring and plumbing penetrations, the attic hatch)
- Add or upgrade attic insulation to the target R-value
- Seal and insulate the rim joist
- Address the floor over unconditioned space or the crawlspace
- Move to wall insulation last
The published savings estimate covers sealing and insulating together as one project, not sealing by itself, so there’s no separate number to attach to step one alone. For the mechanics of doing each of these steps, the national insulation guides elsewhere on this site walk through the attic, rim joist, and crawlspace work in more detail than a state-level page needs to repeat.
What the winter here actually asks for
At Detroit Metro Airport, the 1991-2020 NOAA climate normal runs about 6,067 heating degree days a year against roughly 847 cooling degree days. Heating degree days aren’t a temperature reading; they’re a running tally of how far below 65°F the average daily temperature sits, added up across the whole year. A higher number means more fuel burned to hold the same indoor temperature, and Michigan’s ratio, seven times more heating demand than cooling demand, tells you where the insulation dollar does the most good: the attic and the shell facing winter, not the AC load.
What Michigan homes actually burn to meet that demand matters just as much as the zone map. Federal survey data puts furnaces at 83 percent of the state’s main heating equipment, with natural gas as the fuel in 77 percent of homes. Steam or hot-water boilers account for 9 percent, electricity 10 percent, and propane 9 percent. Central heat pumps and fuel oil or kerosene systems are listed as “not reported” or suppressed in that survey, which means the sample was too thin to publish a reliable figure, not that nobody uses them.
A furnace-and-ductwork state carries a specific vulnerability that a boiler-and-radiator state doesn’t: when the ducts run through an unconditioned attic, as they often do, that duct run is losing heat regardless of how much insulation sits on the attic floor above it. Ceiling insulation slows heat loss through the ceiling; it does nothing for a leaky duct sitting a few feet away. Given that split, the practical order for most Michigan homes looks like this:
- Seal and insulate the attic to the target for your county’s zone
- Check and seal accessible ductwork running through unconditioned space
- Seal and insulate the rim joist and any floor over unheated space
- Move to walls only after those are addressed
The duct-specific guides on this site cover sealing and insulating ductwork in more depth for anyone whose furnace feeds an attic or crawlspace duct run.
What the work is worth
ENERGY STAR’s own methodology page states it plainly: “EPA estimates that homeowners can save an average of 15% on heating and cooling costs (or an average of 11% on total energy costs) by air sealing their homes and adding insulation in attics, floors over crawl spaces, and accessible basement rim joists.” Both numbers matter together. Fifteen percent applies to the heating and cooling portion of a bill; eleven percent applies to the total energy bill, a broader and necessarily smaller share.
These are averages pulled from energy modeling of a “typical” existing U.S. home, not a guarantee for any single house in Grand Rapids or Marquette. The estimate also names exactly where the modeled work happened: attics, floors over crawl spaces, and accessible basement rim joists. It does not cover walls, windows, or doors, and stretching the figure to include those upgrades misrepresents what was actually modeled.
Worth noting, because it’s easy to mix up: ENERGY STAR’s program landing page separately advertises “up to a 10% savings on your annual energy bills” for the same category of work, a different phrasing with a different ceiling. The Department of Energy’s own 10 percent figure, meanwhile, refers to a thermostat setback of 7 to 10 degrees held for eight hours a day, and has nothing to do with insulation at all. Three real numbers, three different claims. The 15 percent and 11 percent pairing above is the one that actually describes attic and floor insulation work, and it’s the one worth carrying into any comparison of what a project might return.