Wisconsin’s 72 counties split across two federal climate zones, and that split is exactly why a single “recommended R-value” for the state doesn’t exist. The right insulation level depends on which zone a county sits in, whether the attic is bare or already has a few inches down, and whether you’re insulating an attic or a floor. Here’s how those pieces fit together.
Which climate zone Wisconsin is in
Wisconsin has no single climate zone. According to the 2021 International Energy Conservation Code (IECC), the state’s 72 counties are divided between two zones: zone 6A covers 42 counties, and zone 5A covers 30 counties. That’s a real split, not a rounding error, and it means a homeowner in the far north of the state and one near the Illinois border are technically working from different rows of the same insulation table.
It’s worth being precise about what that split actually tells you. Forty-two counties in zone 6A is a county count, not a population share. A handful of southern counties in zone 5A hold a large chunk of the state’s residents, including the Milwaukee and Madison metro areas, while the 6A counties stretch across a wider, less densely populated northern and central swath. So don’t read “42 of 72” as “most of the state’s homes.” Read it as “most of the state’s land area, but not necessarily most of its households.”
What the zone number and letter mean
A climate zone number describes how cold or hot a location’s winters and summers run, with zone 1 being hot and zone 8 being the coldest parts of the country. The letter that follows the number describes moisture, not temperature: A means moist, B means dry, and C means marine. Zones 7 and 8 don’t carry a letter at all, because at that point cold dominates the building code more than humidity does. That letter matters here because the insulation table in the next section groups zones by both number and letter, and a 5A county and a 6A county can end up on the same row or on different ones depending on how the table is built.
Because this page can’t tell an individual reader which of the two zones their own county falls into, the right move is to check your county against the official IECC zone map or the ENERGY STAR climate zone map before buying material. Guessing wrong means buying either too little insulation for the winter you actually get, or paying for R-value you don’t need.
The insulation levels that apply here
ENERGY STAR publishes recommended insulation levels for retrofitting existing wood-framed homes, based on the 2021 IECC. The table below is reproduced exactly as published, and it needs a careful read: each zone row has three numbers, but only two of them describe the attic. The first is what an uninsulated attic needs; the second is what an attic already holding 3 to 4 inches of existing insulation needs on top of that; the third number is a completely separate figure, for the floor, not a third attic value.
| Zone | Attic if uninsulated | Attic if already 3-4 inches | 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 |
Here’s the part specific to Wisconsin: both of the state’s zones, 5A and 6A, fall inside the same row of this table, “Zones 6, 5 and 4C.” That means a bare attic in either zone should be brought up to R60, an attic that already has 3 to 4 inches down should be topped up to R49, and a floor over an unheated space should reach R30. This isn’t an average calculated to make the state’s two zones fit together; it’s simply where the ENERGY STAR table already groups them. A homeowner in a 5A county and one in a 6A county are, in this specific case, shopping for the same numbers.
These figures are for retrofitting an existing wood-framed house, not a spec for new construction, and the depth of material needed to hit each R-value depends on what you buy. Fiberglass batts, blown cellulose, and spray foam all reach the same R-value at different thicknesses, and that number is printed on the product’s own packaging, not derived from the R-value itself.
Sealing comes before insulating
ENERGY STAR treats a home’s insulation project as two steps, not one, and it puts air sealing first, before any insulation goes in. The reasoning is straightforward: insulation slows heat moving through a material, but it does nothing to stop air moving around gaps, cracks, and penetrations. Lay insulation over an unsealed attic and you haven’t closed those gaps, you’ve just hidden them under a blanket.
The recommended order of work looks like this:
- Seal air leaks in the attic first, around chimneys, plumbing stacks, wiring penetrations, and the attic hatch.
- Add or top up attic insulation once the sealing is done.
- Seal and insulate the rim joist, where the foundation meets the framing.
- Address the floor over a crawlspace or unconditioned basement.
- Move to walls only after these lower-cost, higher-return areas are handled.
No savings figure attaches to sealing by itself here, because the published estimate covers sealing and insulation together as one combined project, not sealing on its own. For the mechanics of each step, the attic air-sealing checklist and rim joist insulation guides on this site walk through the specifics room by room.
What the winter here actually asks for
At Milwaukee Mitchell Airport, NOAA’s 1991-2020 climate normals record about 6,450 heating degree days a year, against roughly 780 cooling degree days. Heating degree days measure demand, not temperature: they add up, for every day of the year, how far the average temperature fell below 65°F. A figure north of 6,000 marks a long, cold-dominated heating season, and it means furnace and boiler runtime, not air conditioning, is where most of a Wisconsin home’s energy dollar goes.
Federal survey data on how Wisconsin homes actually heat backs that up. Per the Energy Information Administration’s 2020 Residential Energy Consumption Survey, 78% of Wisconsin households heat with a furnace, and 8% use a steam or hot-water boiler. By fuel, 71% run on natural gas, 14% on electricity, and 10% on propane. Figures for central heat pumps and fuel oil weren’t published, which the survey marks as suppressed for reliability, not as zero households using them.
A state running mostly on furnaces through ductwork has a different set of weak points than one on boilers and radiators. Ducts running through an unconditioned attic lose heat straight through their walls, and no amount of ceiling insulation fixes that, because the leak happens below the insulation layer, not above it. That gives Wisconsin homeowners a short, practical list:
- Seal and insulate attic ductwork separately from the attic floor itself.
- Bring attic insulation up to the R60/R49 levels noted above, after sealing.
- Check rim joists and floors over crawlspaces, given how long the heating season runs.
- For the 8% on boilers, prioritize building envelope work over duct work, since there’s no duct loss to address.
The duct sealing and insulation guides on this site cover the furnace-and-ductwork case in more detail, since it applies to the large majority of Wisconsin households.
What the work is worth
ENERGY STAR’s own methodology page puts a number on the combined project: “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 percentages matter together, not separately: 15% applies specifically to heating and cooling spending, while 11% applies to a home’s entire energy bill, which includes things insulation doesn’t touch, like water heating and appliances.
These are averages drawn from energy modeling of a typical existing U.S. home, not a guarantee for any particular house, and they cover only the areas named: attics, floors over crawl spaces, and accessible basement rim joists. Walls, windows, and doors sit outside this specific estimate. It’s also worth keeping this figure separate from two others that sound similar but aren’t the same claim: ENERGY STAR’s general program page cites “up to a 10% savings on your annual energy bills” for the same category of work, phrased as a ceiling rather than an average, and the Department of Energy’s often-cited 10% figure refers to a thermostat setback of 7 to 10°F held for eight hours a day, which has nothing to do with insulation at all. Three real numbers, three different claims, worth keeping straight before deciding what a Wisconsin retrofit is likely to return.