How Much Insulation Do I Need?

Measure length × width of each rectangle of the surface and add them together. For walls, multiply perimeter by height and subtract windows and doors. Add 5–10% waste for batts and board, 10% for loose fill settling.

Every bag label lists 'maximum coverage at R-XX'. Divide your square footage by that coverage figure. Example: a 1,400 sq ft attic and a bag covering 32.4 sq ft at R-49 needs 44 bags, or 48 with waste.

Batt bags list total square feet, typically 40–88 sq ft. Divide your net area by the bag coverage and round up. Buying by bay count is more accurate for walls: bays = wall length ÷ spacing, then batts per bay = wall height ÷ batt length.

A 4x8 sheet covers 32 sq ft. Divide net area by 32 and add 10% for cuts. For multi-layer applications multiply by the number of layers.

One board foot equals 1 sq ft at 1 inch thick. Board feet = area × thickness in inches. A 1,000 sq ft crawl wall area at 2 inches is 2,000 board feet. Kits list yields at a stated temperature — real yield is usually 10–20% lower.