South Dakota’s 66 counties split across two IECC climate zones, but that split doesn’t actually change the insulation numbers much: both zones land on the same ENERGY STAR retrofit row. Attics should reach R60 if uninsulated, R49 if partially insulated, with R30 for floors over unheated spaces. The details below explain why, and what to check for your own county.
Which climate zone South Dakota is in
South Dakota does not have a single climate zone answer. According to the 2021 International Energy Conservation Code (IECC), Table R301.1, the state’s 66 counties are divided between two zones: 48 counties sit in zone 6A, and 18 counties sit in zone 5A.
| Climate zone | Number of counties |
|---|---|
| 6A | 48 of 66 |
| 5A | 18 of 66 |
That’s a county count, not a population share. A state can have most of its counties in one zone and most of its people in another, so don’t read “48 counties” as “most South Dakotans.” The right way to use this table is to find your own county, not to assume the majority rule applies to your address.
A climate zone number reflects how cold (or hot) a location runs on average, with zone 1 being hottest and zone 8 being coldest. The letter that follows the number is the moisture regime: A means moist, B means dry, C means marine. Zones 7 and 8 don’t carry a letter at all, since there’s no meaningful moisture distinction that far north. This letter matters because the insulation table in the next section groups zones by both number and letter, and a state that straddles two zones can still end up on the same row, or on two different rows, depending on how those letters fall.
If you want to confirm which zone applies to your specific county, the IECC table and the ENERGY STAR climate zone map are the sources to check, not a state-wide guess. Nobody reading this article should walk away thinking “South Dakota is zone 6” or “South Dakota is zone 5” as a blanket statement. It’s both, depending on where you live, and the county is what decides it.
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 has a header that’s easy to misread: “attic” is split into two columns (one for an attic that currently has no insulation, one for an attic that already has 3 to 4 inches), and floor insulation is a separate, third column. So the three numbers listed per zone aren’t three attic figures. Two describe the attic under different starting conditions, and the third describes the floor.
| Zone | Attic (uninsulated) | Attic (already has 3-4 in.) | Floor |
|---|---|---|---|
| 1 | R30 | R25 | R13 |
| 2 | R49 | R38 | R13 |
| 3 | R49 | R38 | R19 |
| 4A and 4B | R60 | R49 | R19 |
| 6, 5 and 4C | R60 | R49 | R30 |
| 7 and 8 | R60 | R49 | R38 |
Here’s the part specific to South Dakota: both of its zones, 6A and 5A, fall under the same combined row in this table, “Zones 6, 5 and 4C.” That means the retrofit numbers don’t actually change depending on which of the state’s 66 counties you’re in. Whether a home sits among the 48 counties in zone 6A or the 18 counties in zone 5A, the same targets apply: R60 for an uninsulated attic, R49 for an attic that already has 3 to 4 inches, and R30 for the floor.
That’s a real convenience for a state split across zones, and it’s worth stating plainly rather than assuming a two-zone state always means two different sets of numbers. It doesn’t here.
None of these figures should be converted into a thickness in inches. The actual depth of material needed to hit R60 or R30 depends on what you’re using, fiberglass batts, blown cellulose, or spray foam all cover different R-value per inch, and that number is printed on the product packaging, not derived from the R-value alone. And to be clear, these are retrofit levels for existing wood-framed buildings. New construction in South Dakota follows separate code requirements tied to the same zone data.
Sealing comes before insulating
ENERGY STAR treats air sealing and insulation as two parts of one project, not two separate options, and its own guidance puts attic air sealing ahead of attic insulation in the order of work. That sequencing isn’t arbitrary.
Insulation slows heat moving through a material, but it does nothing to stop air moving around it. A gap around a can light, a plumbing stack, or an attic hatch keeps leaking warm air whether or not you’ve piled R60 worth of insulation on top of it. Insulate first, and you’ve just buried the leak instead of fixing it, which makes it harder to find later.
The general order of work looks like this:
- Seal air leaks in the attic
- Add or upgrade attic insulation
- Seal and insulate the rim joist
- Address the floor or crawlspace
- Move to wall insulation last
Notice that walls come last, not first. Attics and rim joists tend to be the biggest sources of both air leakage and heat loss in a typical wood-framed home, which is why they’re prioritized ahead of wall cavities that are harder and more expensive to access anyway.
One thing worth being precise about: the published savings estimate for this kind of work covers sealing and insulating together, as a combined project. There isn’t a separate, standalone savings figure for air sealing by itself. For the specific mechanics of sealing an attic, insulating a rim joist, or handling a crawlspace, the national insulation guides on this site walk through each step in more detail than a state-level overview can.
What the winter here actually asks for
Heating degree days are a way of measuring demand, not a temperature reading. The calculation adds up, for every day of the year, how far the average temperature fell below 65°F. At Sioux Falls, that comes to about 7,248 heating degree days a year, against roughly 847 cooling degree days. That lopsided ratio, nearly nine heating days for every cooling day, tells you this is a state where the heating season, not the cooling season, drives most of the annual energy load.
Sioux Falls is one reference station, not a statewide average. Counties farther north or at higher elevation can run colder still, so treat this number as a benchmark for comparison rather than a promise for every address in the state.
What are homes actually heating with? According to the Energy Information Administration’s Residential Energy Consumption Survey, 75% of South Dakota homes use a furnace as their main heating equipment, and 7% use a steam or hot-water boiler. By fuel, 58% run on natural gas, 25% on electricity, and 14% on propane. Figures for central heat pumps and for fuel oil or kerosene were not reported or had estimates suppressed by the survey, which means too few households were sampled to publish a reliable number, not that nobody uses that equipment.
A state that heats mostly with furnaces through ductwork has a different set of priorities than one leaning on boilers and radiators. Ductwork routed through an unconditioned attic is its own source of heat loss, one that ceiling insulation alone doesn’t fix, since the loss happens inside the duct run before the air even reaches a room.
Given that, the practical priorities for South Dakota homes line up like this:
- Seal and insulate the attic to the R60/R49 targets, since this is the largest heat-loss surface in most homes
- Check and seal any attic ductwork, given how common furnace-and-duct systems are in this state
- Address the rim joist and floor insulation, particularly over crawlspaces
- Confirm boiler-heated homes have adequate wall and floor insulation, since they don’t carry the same duct-loss risk but still need envelope work
For homes with ductwork running through attics or crawlspaces, the duct sealing and insulation guides on this site go into the specifics that a state overview like this one can’t cover in full.
What the work is worth
ENERGY STAR’s own modeling puts a number on this kind of project. Their stated estimate: “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, and they’re not interchangeable. The 15% applies specifically to heating and cooling costs. The 11% applies to total energy costs, which includes everything else running through the meter, water heating, appliances, lighting. Quoting one figure without its denominator turns an accurate number into a misleading one.
These are averages drawn from energy modeling of a “typical” existing U.S. home, not a guarantee tied to any specific house in South Dakota. Given how much heating demand this state carries, a home with genuinely uninsulated attic space and leaky ductwork could plausibly see results at the higher end of that range, but the source doesn’t make that promise, and neither should this page.
It’s also worth noting exactly where this figure applies: attics, floors over crawl spaces, and accessible basement rim joists. It does not extend to walls, windows, or doors, even though those are common places homeowners assume insulation upgrades pay off. If you’re weighing a wall insulation project, treat it as a separate question with its own return, not part of this 15%/11% figure.
One more distinction worth keeping straight: ENERGY STAR’s general program page separately advertises “up to a 10% savings on your annual energy bills” for similar sealing and insulating work, a different denominator and a different framing than the 15%/11% figure above. And the Department of Energy’s often-cited 10% savings figure has nothing to do with insulation at all, it’s tied to a thermostat setback of 7 to 10 degrees held for eight hours a day. Three real numbers, three different claims, and conflating them is how good information turns into a bad estimate.