Insulation calculator
Enter the wall or attic dimensions, pick the R-value, and enter the coverage printed on the package. The cavity adjustment removes the area taken up by studs or joists.
Materials takeoff
Insulation Calculator · https://estimahome.com/insulation-calculator/
Wall length (or attic length): 40 ftWall height (or attic width): 8 ftWindows and doors to subtract: 30 sq ftR-value: R-13 (2×4 wall)
- Net area
- 290 sq ft
- Cavity share
- 90.6%
- Insulated area
- 262.8 sq ft
- Packages
- 7
| No. | Item | Qty | Unit | Unit cost | Amount |
|---|---|---|---|---|---|
| 1 | Fiberglass R-13, 15 in wide | 7 | pkg | $30.00 | $210.00 |
- Subtotal
- $210.00
- Likely range
- $178.50 – $241.50
- Use the R-value your local energy code requires for the wall, floor or attic you are insulating.
- Not included: vapor retarder, baffles, staples, gloves and a respirator.
Buy these materials
- 7 packages of R-13 insulationAmazon search for packages of R-13 insulation (opens in new tab)
Links open a store search for each item. Check sizes and grades against your plan.
Quick reference
- At 16 in on center with 1.5 in studs, cavities make up about 90.6% of the wall area; at 24 in, about 93.8%.
- Packages = net area × cavity fraction × (1 + waste) ÷ package coverage, rounded up.
- Batts for 16 in framing are 15 in wide; for 24 in framing, 23 in wide.
How it's calculated
⌈x⌉ means round up to the next whole unit; ⌊x⌋ means round down. Every quantity you buy is rounded up.
Net area = length × height − openingsCavity fraction = (spacing − framing thickness) ÷ spacingInsulated area = net area × cavity fractionPackages = ⌈ insulated area × (1 + waste ÷ 100) ÷ coverage per package ⌉Cost = Σ (quantity × unit price); range = cost × (1 ± range % ÷ 100)
Worked example
A 40 ft wall 8 ft tall is 320 sq ft; less 30 sq ft of openings that is 290 sq ft. With 1.5 in studs at 16 in on center, cavities are (16 − 1.5) ÷ 16 = 90.63% of the wall, or 262.81 sq ft. With 5% waste and 40 sq ft per package, that is 7 packages. At the default price: $210.00.
Questions
What R-value do I need?
It depends on your climate zone and where the insulation goes. 2×4 walls take R-13 or R-15, 2×6 walls R-19 or R-21, and attics commonly R-38 to R-60. Your local energy code sets the minimum.
Faced or unfaced batts?
Kraft-faced batts include a vapor retarder and are common in exterior walls in cold climates, facing the heated side. Unfaced batts are used for interior walls, sound control and adding a second layer in attics.
Should I compress batts to fit?
No. Compressing fiberglass lowers its R-value. Cut batts to fit around wiring and boxes instead.
Cite: “Insulation Calculator.” EstimaHome, https://estimahome.com/insulation-calculator/. Updated 2026-10-20. Formulas in machine-readable form: /api/calculators.json.