Missouri doesn’t have one insulation number. The state stretches across three IECC climate zones, and the county a home sits in decides which row of the R-value table applies: most counties (87 of 115) fall in zone 4A, which calls for R60 in an uninsulated attic, R49 if there’s already 3-4 inches up there, and R19 under the floor. Twenty-six counties need more floor insulation, and two need less overall.
Which climate zone Missouri is in
There’s no single answer for this state, and any page that gives you one figure for “Missouri” is skipping a step. The 2021 International Energy Conservation Code splits the state’s 115 counties across three zones: zone 4A covers 87 counties, zone 5A covers 26, and zone 3A covers just 2. That’s a county count, not a population share, so don’t read it as “most of Missouri” without the number attached. A pair of counties in the far southeast corner, where the Bootheel region dips toward the Mississippi Delta, sits in the milder 3A zone, while the northern tier of the state runs into the cooler 5A zone.
A climate zone is a shorthand for how much a building has to fight the outside temperature over a year, and the number does most of the work: higher numbers mean colder winters and a bigger insulation job. The letter that follows the number describes moisture, not temperature. A means moist, B means dry, C means marine, and zones 7 and 8 (found only in the coldest parts of the country) don’t carry a letter at all. That letter matters here because the ENERGY STAR insulation table groups some zones together by moisture regime rather than by number alone, so getting the letter wrong can point you at the wrong row.
None of this tells you which zone your own house sits in, and that’s on purpose. Zone assignment happens at the county level under the 2021 IECC’s Table R301.1, and the only reliable way to check is to look up your specific county rather than assume based on region. A homeowner near St. Louis and one near the Iowa border are working from different assumptions about How Much Insulation “enough” actually means, even though they live in the same state.
The insulation levels that apply here
ENERGY STAR publishes a retrofit table for existing wood-framed homes, and it’s built around the same zone numbers described above. The table’s header is merged in a way that trips people up: two of the three figures for each zone cover the attic (one for a space with no insulation at all, one for a space that already has 3-4 inches), and the third figure is a completely different location, the floor. Reading all three as attic numbers is the single most common misread of this table.
| Climate zone | Attic (uninsulated) | Attic (already has 3-4 in.) | Floor |
|---|---|---|---|
| Zone 1 | R30 | R25 | R13 |
| Zone 2 | R49 | R38 | R13 |
| Zone 3 (includes Missouri’s 3A counties) | R49 | R38 | R19 |
| Zones 4A and 4B (includes Missouri’s 4A counties) | R60 | R49 | R19 |
| Zones 6, 5, and 4C (includes Missouri’s 5A counties) | R60 | R49 | R30 |
| Zones 7 and 8 | R60 | R49 | R38 |
Three of those rows matter in Missouri, and they don’t all say the same thing. The two counties in zone 3A land on the “Zone 3” row: R49 attic, or R38 over existing insulation, with R19 under the floor. The 87 counties in zone 4A use the “Zones 4A and 4B” row: R60 attic, R49 over existing insulation, R19 floor, the same attic target as zone 3 counties see for the floor but a bigger attic job. The 26 counties in zone 5A move to the “Zones 6, 5, and 4C” row, where the attic numbers stay at R60 and R49 but the floor recommendation jumps to R30, a meaningfully thicker layer than what a 4A or 3A county needs underneath.
These are retrofit levels for existing wood-framed homes, based on the 2021 IECC, not new-construction code minimums. A house being built today follows a different rulebook. Don’t average these three rows into one Missouri number and don’t estimate a depth in inches from the R-value; actual thickness depends on which material you use, and that’s printed on the product’s own packaging, not on this table.
Sealing comes before insulating
ENERGY STAR treats air sealing and insulation as two parts of one project, and its own guidance puts attic air sealing before attic insulation, not the other way around. The reasoning is simple: insulation slows heat moving through a material, but it does nothing to stop air moving around it. Lay batts or blown-in fiber over a gap around a can light or a plumbing chase, and you’ve hidden the leak, not closed it. Air keeps moving through that gap all winter, carrying heated air out and pulling cold air in, while the insulation on top does its job everywhere except the one spot that was actually costing you money.
The order ENERGY STAR lays out runs roughly like this:
- Seal the attic first, closing gaps around penetrations, chases, and the attic hatch itself.
- Add or top up attic insulation once the leaks underneath it are closed.
- Address the rim joist, the band around the top of the foundation where framing meets masonry.
- Handle the floor over a crawlspace or unconditioned basement.
- Move to walls last, since they’re typically the hardest and most disruptive part of the job to retrofit.
Don’t attach a specific savings figure to sealing on its own. The percentage ENERGY STAR publishes covers sealing and insulating together as a combined project, and pulling them apart to claim a number for sealing alone isn’t something the source data supports. For the mechanics of each step, from finding the leaks to choosing a method for the attic floor, the national insulation guides on this site cover the detail that a state-level page like this one isn’t the place for.
What the winter here actually asks for
Heating degree days measure demand, not temperature. The figure adds up, for every day of the year, how far the average temperature fell below 65°F, so a bigger number means a longer or colder heating season, not a lower thermometer reading on any single day. At Kansas City International Airport, the 1991-2020 NOAA normal comes to about 5,071 heating degree days a year, alongside roughly 1,363 cooling degree days. That’s a heating-dominant climate with a real cooling season layered on top, which is exactly the kind of profile where both attic insulation and rim joist work pull their weight in different seasons. That figure is for one reference station; a colder pocket of the state can run higher.
The U.S. Energy Information Administration’s Residential Energy Consumption Survey shows what Missouri homes actually burn to get through that demand. Furnaces heat 76% of homes, and central heat pumps cover another 10%. The survey doesn’t publish a reliable figure for steam or hot-water boilers here, which means too few households showed up in the sample to report a number, not that boilers are absent. Natural gas fuels 59% of homes and electricity another 31%, with propane at 7% and no reportable figure for fuel oil or kerosene.
A state running mostly on furnaces pushing air through ductwork has a different weak point than one heated by boilers and radiators. Ducts routed through an unconditioned attic lose heat straight into that attic space, and no amount of insulation laid at the ceiling plane fixes a leaky duct sitting above it. Local priorities follow from that:
- Seal and insulate the attic floor to the levels in the table above, since that’s the layer separating living space from an unconditioned attic in most homes here.
- Check any ductwork routed through that attic or a crawlspace, since duct leakage bypasses insulation entirely.
- Address the rim joist and floor over crawlspaces or basements, given how much of the year sits below 65°F.
The duct guides on this site go into how to find and seal that kind of leakage, which matters more in a furnace-and-ductwork state than in one running mostly on hydronic heat.
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, and so does what each one is measured against, 15% of the heating and cooling portion of a bill, 11% of the whole energy bill.
These are averages built from energy modeling of a typical existing U.S. home, not a guarantee for any specific house in Missouri. A home that’s already well sealed will see less benefit than one with an untouched attic and an open rim joist. The estimate also names exactly where the work was modeled, attics, floors over crawl spaces, and accessible basement rim joists, and doesn’t extend to walls, windows, or doors, so don’t read a wall retrofit into the same figure.
Two other numbers float around that shouldn’t get blended with this one. ENERGY STAR’s own program landing page cites “up to a 10% savings on your annual energy bills” for the same kind of work, phrased as a ceiling rather than an average. The Department of Energy’s separate 10% figure applies to a thermostat setback of 7-10°F held for eight hours a day, which has nothing to do with insulation at all. Three real numbers, three different claims, and only the 15%/11% pairing above describes what sealing and insulating an attic, crawlspace floor, and rim joist actually does.