How Much Insulation Do I Need?
Two separate questions decide this: what R-value your climate needs (look up your zone below), and how many bags of your chosen product cover your area (divide your square footage by the coverage printed on the bag). The insulation calculator handles the second part — including door and window subtraction for walls — once you know your target R-value and product. This page is the climate-zone table itself, plus a worked comparison of how much bag count changes between a low- and high-R-value product on the same area.
Recommended R-value chart by climate zone
| IECC climate zone | Attic, uninsulated | Attic, 3–4 in already there | Floor over unheated space | Exterior wall sheathing (when re-siding) | Basement or crawlspace wall |
|---|---|---|---|---|---|
| 1 | R-30 | R-25 | R-13 | — | — |
| 2 | R-49 | R-38 | R-13 | — | — |
| 3 | R-49 | R-38 | R-19 | R-5 | R-5 sheathing or R-13 batt |
| 4A, 4B | R-60 | R-49 | R-19 | R-5 to R-10 | R-10 sheathing or R-13 batt |
| 4C, 5, 6 | R-60 | R-49 | R-30 | R-5 to R-10 | R-15 sheathing or R-19 batt |
| 7, 8 | R-60 | R-49 | R-38 | R-5 to R-10 | R-15 sheathing or R-19 batt |
ENERGY STAR, Recommended Home Insulation R-Values publishes these retrofit levels for existing wood-framed homes. Read the attic columns by what you have now: an empty attic gets the full figure, while an attic that already holds 3 to 4 inches gets the smaller top-up amount added on top. The wall column is the insulating sheathing ENERGY STAR suggests adding under new siding, on top of blowing insulation into any empty cavities; in zones 4 to 8, walls that already have 2x4 cavity insulation get R-10 sheathing. A dash means the table gives no figure for that zone. New construction follows the stricter code minimums in 2021 IECC Table R402.1.3, which your building department enforces. Once you know your target R-value, the number of bags depends on your product's coverage per bag, not on R-value directly, which the worked example below shows.
A full example: why R-value changes the bag count so much
Take the same 800 sq ft attic floor, insulated two different ways. A thin product whose bag label says 100 sq ft needs 800 ÷ 100 = 8 bags; a thick attic batt labeled 35 sq ft per bag needs 800 ÷ 35 ≈ 22.9, rounded up to 23 bags. The area didn't change; the product did. Now suppose you ran the calculator for the thick batt but left its default coverage of 64 sq ft per bag in place: 800 ÷ 64 = 12.5, rounded up to 13 bags — against 23 actually needed, an order short by nearly half (23 ÷ 13 ≈ 1.8, so the real need is nearly 2x the estimate). Always enter the figure from your own bag label; why that figure swings so widely between products is covered in the insulation calculator's FAQ.
Finding your climate zone
The zone numbers in the table come from the International Energy Conservation Code (IECC) climate map, which is drawn county by county rather than state by state. A state can span two or three zones: mountain counties often sit one or two zones colder than the lowlands around them. Look up your county on the DOE or IECC zone map instead of guessing from your state's general climate. Some zones also carry a letter (A for moist, B for dry, C for marine). The letter matters for moisture and vapor-retarder choices, and it matters once in the chart above: marine zone 4C is grouped with zones 5 and 6 for floors and basement walls, while 4A and 4B get the lower figures. The attic columns depend only on the number.
Topping up an attic that already has insulation
Most attics aren't empty, and stacked R-values add, so you buy the gap between the table's target and what is already there — the insulation calculator's common-mistakes section covers choosing the top-up product. The hard part is judging the existing layer. If the old insulation has no visible label, measure its depth in a few places and ask the manufacturer or a local insulation supplier what R-value per inch that material delivers. Old insulation that has been compressed, soaked by a roof leak, or disturbed by pests or wiring work delivers less than its original rating, so judge the thinnest typical area, not the best-looking corner. Where depth varies a lot across the attic, split the floor into zones and work out the top-up for each one; the thin areas near the hatch and along walkways often need more than the rest.
Why walls don't follow the attic table
An attic floor can take almost any depth of insulation, so its target is set by climate. A wall is limited by its framing: the insulation has to fit inside the stud cavity without being crushed. A 2×4 wall has a cavity about 3.5 in deep, which is why it is usually filled with R-13 or R-15 batts. A 2×6 wall, about 5.5 in deep, takes R-19 or R-21. In an existing wall, then, the practical question is which product fills your cavity properly, and that R-13 to R-21 cavity range reflects the framing limit rather than a climate recommendation. Adding more wall R-value in a cold zone normally means continuous rigid insulation outside the studs, which is a re-siding decision rather than a bag count.
When the quick estimate goes wrong
The formula of area divided by coverage assumes you measured the area you are actually covering. For an attic floor, that is the space between the outside walls, not the house footprint including the garage or a porch roof. For walls, it is exterior walls only; interior partitions are normally left empty unless you want sound control. And climate-zone guidance is written for heated homes. A detached garage, a shed, or an unheated storage attic doesn't need to hit the table's numbers, and buying R-49 for one spends money on heat that is never there to keep in.