Costs & R-Value
How Much Insulation Do I Need?
Every insulation product is sold in a different unit, which is why estimating is so annoying. Here is the conversion for each one.
Updated January 2026 · about 6 min read
Start with net square footage
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.
Bags of blown insulation
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 packs
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.
Gear for this job
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Foam board sheets
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.
Spray foam board feet
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.
Insulating a vehicle rather than a house? Use the van insulation calculator.
Run the numbers
Frequently asked questions
- How many bags of insulation for a 1,500 sq ft attic?
- At R-49, roughly 125–135 bags of cellulose in 19 lb bags (GreenFiber's chart lists 12 sq ft per bag at R-49), or about 40–45 bags of a common big-box blown fiberglass such as AttiCat (its chart lists about 40 sq ft per bag at R-49). Bag weights and ratings vary by brand, so the coverage chart on your bag decides.
- How much waste should I add?
- 5–10% for batts and rigid board, 10% for loose fill, and 15–20% for DIY spray foam kits.
- How many square feet in a batt bag?
- Usually 40–88 sq ft depending on R-value and width; the number is printed on the bag.
Insulation gear worth owning
The tools and materials that show up on almost every insulation job.
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