Insulation Calculator
Estimate insulation bags for walls, attics and floors. For blown-in attics, enter what you have now and the R-value you want.
How it's calculated
Batt bags are rated by coverage in square feet. The calculator divides your net area by the coverage per bag and adds 5%. Typical coverages are built in, but the number on your bag is better.
For blown-in insulation, the calculator finds the R-value you need to add, converts it to a settled depth using about R-2.5 per inch for fiberglass and R-3.6 for cellulose, then converts that volume to weight and bags.
Worked example
An 1,800 sq ft attic with R-11 now, topped up to R-38 with fiberglass, needs 10.8" more: about 890 lb, or 32 bags.
Field notes
- Air-seal top plates, wire holes and the attic hatch before adding insulation. It makes more difference than the extra inches.
- Keep insulation off soffit vents with baffles.
- Mark depth rulers on trusses so you blow an even layer.
- Kraft-faced batts go with the paper toward the heated side in most climates.
Common questions
- How much attic insulation do I need?
- The U.S. Department of Energy recommends R-30 to R-60 for attics in most of the country, depending on climate zone.
- How deep is R-38 blown insulation?
- About 15" for fiberglass loose-fill and about 10–11" for cellulose, after settling.
- What R-value goes in a 2×4 wall?
- R-13 or high-density R-15 batts. A 2×6 wall takes R-19 or R-21.