Insulation Calculator
Find the DOE-recommended R-value for your climate zone and assembly, then the insulation thickness, blown-in bags or batt area, and cost — attic, wall, and floor per the 2021 IECC. Free, no sign-up.
What to calculate next
Tools commonly used alongside this calculation
Ceiling Joist Span Calculator
Find the maximum ceiling joist span from the IRC R802.5.1 tables. Pick the attic use, species, grade, size, and spacing to get the max span by size and the minimum joist for your ceiling. Free, no sign-up.
Joist Span Calculator
Find the maximum allowable floor or deck joist span from the IRC span tables. Pick species, grade, size, and spacing to get the code max span by joist size and check it against the span you need. Free, no sign-up.
Rafter Span Calculator
Find the maximum horizontal roof rafter span from the IRC R802.4.1 tables. Pick the roof or ground snow load, species, grade, size, and spacing to get the max span by size and the minimum rafter for your roof. Free, no sign-up.
Explanation
“How much insulation do I need?” has two parts: the R-value your code recommends for where you live, and the thickness of a real product that reaches it. This calculator answers both. Pick your DOE climate zone and the assembly — attic, wall, or floor — to get the recommended R-value, then choose a material to turn that target into an installed thickness, a blown-in bag count or batt area, and a cost.
Recommended R-values by climate zone
The United States is split into eight climate zones, from Zone 1 along the Gulf Coast to Zone 8 in interior Alaska. The colder the zone, the more R-value the building envelope needs. The targets below follow the 2021 International Energy Conservation Code (IECC) Table R402.1.3, the same basis ENERGY STAR uses for its zip-code recommendations:
| Climate zone | Attic | Wall | Floor |
|---|---|---|---|
| 1 — Hot | R-30 | R-13 | R-13 |
| 2 — Hot-Humid | R-49 | R-13 | R-13 |
| 3 — Warm | R-49 | R-20 | R-19 |
| 4 — Mixed | R-60 | R-30 | R-19 |
| 5 — Cold | R-60 | R-30 | R-30 |
| 6 — Cold | R-60 | R-30 | R-30 |
| 7 — Very Cold | R-60 | R-30 | R-38 |
| 8 — Subarctic | R-60 | R-30 | R-38 |
The attic needs the most R-value because heat rises and escapes through the roof. The attic figure sits on top of the framing, so the depth of insulation you can pile on is set by the joists below it — the same ceiling joist span that carries the load. The floor figure is the batt between the framing members, sized the same way a floor joist span is checked for the room above.
Turning R-value into thickness
Every insulation product has an R-value per inch. Divide the R-value you need to add by that figure to get the installed thickness:
The R-value to add is the target minus whatever is already in place, so a Zone 5 attic at R-60 with an existing R-19 only needs R-41 more — about 11.7 in of blown cellulose. The higher the R-per-inch, the thinner the layer, which matters most in a wall cavity where space is fixed:
| Material | R per inch | Form |
|---|---|---|
| Fiberglass batt | 3.1 | Batt / roll |
| Mineral wool batt | 4.2 | Batt / roll |
| Blown cellulose | 3.5 | Loose-fill (bags) |
| Blown fiberglass | 2.5 | Loose-fill (bags) |
| Open-cell spray foam | 3.7 | Spray |
| Closed-cell spray foam | 6.5 | Spray |
| Rigid XPS / EPS / polyiso | 4.0–6.0 | Board |
A 2×4 wall holds about 3.5 in of cavity insulation and a 2×6 about 5.5 in. That is why a high cold-zone wall target often can’t be met with batt alone: R-30 in batt is roughly 9.7 in, far deeper than any stud cavity, so the code allows it to be met with continuous exterior insulation or a denser fill. The calculator flags when the target won’t fit the cavity you picked.
Notes and assumptions
Recommended R-values are the 2021 IECC minimums. Your local code may adopt an older or amended version, and high-performance or rebate programs often ask for more. Confirm the requirement with your building department before ordering.
R-per-inch values are nominal, settled figures; loose-fill should be installed to the bag’s coverage chart to hit its rated R-value after settling, and aged board foam (especially polyiso) drifts below its label value in cold weather. The blown-in bag count is an estimate from a typical bag — always check the count against the coverage chart on the product you buy, since it varies by manufacturer. Treat the result as an ordering estimate and round up at the supplier.