Home › Batt Insulation Calculator
Batt Insulation Calculator
Quick answer: Take the gross wall, ceiling or floor area, subtract doors and windows, reduce for framing, add a little waste, and divide by the square-foot coverage printed on the batt package. The result is how many packages to buy. Use batts sized to your stud spacing and cavity depth so they fit without compression.
How it works
Net area = (gross − openings) × (1 − framing %). Packages = net area × (1 + waste %) ÷ coverage per package. Framing allowance accounts for studs, joists and plates that are not insulated; 10% is a common rough figure for typical framing, but use your own. Package coverage depends on width, length and thickness, so read the label. Compressing a batt into a shallower cavity lowers its R-value; see the R-value chart.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 800 sq ft gross wall area, 60 sq ft of openings, 10% framing allowance, an assumed 80 sq ft per package, 5% waste. Net = 740 × 0.9 = 666 sq ft. Packages = 666 × 1.05 ÷ 80 = 8.7, round up to 9.
FAQ
Faced or unfaced?
It depends on where the vapour retarder belongs in your climate and code. Check local rules.
Can I add batts over old attic insulation?
Often yes, unfaced and laid perpendicular; see blown-in vs batt.
What PPE do I need?
See tools and PPE.
Back to the main calculator · Guides & FAQ