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Insulation Calculator

Batts or rolls needed and the R-value your climate zone calls for

What are you insulating?

Overrides the dimensions

Feet, or feet-inches like 12' 6" — metric works too (3.5m)

R-60
Attic with no insulation (ENERGY STAR)
R-49
Attic that has 3–4 in (ENERGY STAR)
R-60
New-home ceiling (2021 IECC)

Measure the depth: roughly R-2.5 per inch for loose fiberglass, R-3.5 for cellulose, R-3.2 for batts

Batts or rolls
Joist spacing→ buy 15 in wide batts

Example packages (Owens Corning EcoTouch labels). Brands and bag counts vary — type the sq ft printed on yours.

Printed on the package; sets how many layers reach your target

5 % covers trimming around blocking, wires, and boxes

93 in for 8 ft walls, 105 in for 9 ft, 48 in attic batts

Blown-in (loose fill)

Example chart: Owens Corning AttiCat blown-in fiberglass (27.5 lb bags) — R-30 takes 14.6 bags per 1,000 sq ft, R-38 19, R-49 25, R-60 31.5. Cellulose and other brands differ; enter the figure from your bag's chart to override.

33 packages
of batts/rolls at 64 sq ft each for 1000 sq ft (+5 %) — adding R-60 with 2 layers of R-38 (reaches about R-76)
1000 sq ft
Area
R-60
R-value to add
32 bags
Blown-in, ≥ 20.5 in deep (AttiCat example)

Stacking batts: batt R-values add up, so R-19 over R-30 gives about R-49. Lay the second layer unfaced and perpendicular to the joists.

Estimates only — verify quantities with your supplier and check local building codes.

Sources: Attic and floor recommendations follow ENERGY STAR's retrofit table (based on the 2021 IECC); new-home wall and ceiling minimums are 2021 IECC Table R402.1.3. Your local code may be older or stricter — check with your building department. The state list shows the zones each state spans; zones are assigned by county.
Air-seal first. Insulation slows heat conduction but does not stop air leaks. Seal top plates, plumbing and wiring holes, and the attic hatch before adding insulation, and keep soffit vents clear with baffles.

Find the R-value your climate calls for and how much insulation to buy to get there. Choose attic, walls, or floor, pick your state and climate zone, and the calculator shows ENERGY STAR's retrofit recommendation and the 2021 IECC new-home minimum. Enter what you already have, and it works out the R-value to add, the packages of batts or rolls from the coverage on the label, and — for attics — the bags of blown-in insulation from a manufacturer coverage chart you can override.

Sizing a heater or AC after the upgrade? Better insulation lowers the load, so rerun the BTU calculator. Finishing a basement or addition? The stud wall calculator and the drywall calculator cover the framing and board.

Frequently asked questions

What R-value do I need in my attic?

It depends on your climate zone. ENERGY STAR's retrofit recommendations, based on the 2021 IECC, call for R-30 in an uninsulated attic in zone 1 (south Florida, Hawaii), R-49 in zones 2 and 3, and R-60 in zones 4 through 8. If the attic already has 3–4 inches of insulation, the targets are R-25, R-38, and R-49 respectively. Pick your state to see which zones it spans, then choose your zone — zones follow county lines, so a single state can span three or more.

How many bags of blown-in insulation do I need?

Divide your attic area by 1,000 and multiply by the bags-per-1,000-sq-ft figure for the R-value you are adding, from the chart printed on the bag. As an example, Owens Corning's AttiCat blown-in fiberglass (27.5 lb bags) lists 14.6 bags per 1,000 sq ft for R-30, 19 for R-38, 25 for R-49, and 31.5 for R-60, with minimum depths of 10.75, 13.5, 17, and 20.5 inches. Cellulose is heavier and uses different numbers, so enter the figure from your product's chart to override ours.

How many batts or rolls of insulation do I need?

Divide the area by the coverage printed on the package and round up, adding about 5 % for trimming around blocking, wiring, and boxes. For example, a common R-19 package of eight 15×93 in batts covers 77.5 sq ft, so 1,000 sq ft of wall needs 14 packages with waste. Buy batts that match your framing: 15 in wide for studs or joists 16 in on center, 23 in wide for 24 in on center. Batts should fill the cavity without being compressed — squashing a batt lowers its R-value. If one layer of your batt falls short of the R-value you need to add, the calculator counts enough layers to reach it.

Can I add new insulation on top of old insulation?

Usually yes, as long as the old insulation is dry, free of mold or pests, and not vermiculite (which may contain asbestos — have it tested before disturbing it). R-values add together, so an attic with R-19 that needs R-49 needs about R-30 more. Lay new batts unfaced and perpendicular to the joists over the old layer, or blow loose fill over the top. Never add a second vapor retarder facing on top of an existing one, which can trap moisture.

What R-value do exterior walls need?

For new wood-frame homes, the 2021 IECC calls for R-13 in zones 1 and 2; R-20, or R-13 cavity plus R-5 of rigid foam sheathing, in zone 3; and R-30, or R-20 plus R-5 continuous insulation, or R-13 plus R-10, in zones 4 through 8. Existing 2×4 walls can hold R-13 to R-15 in the cavity and 2×6 walls R-19 to R-21, so reaching the new-home numbers in a retrofit usually means adding foam board when the siding is replaced.

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