About this calculator
Bags of batts, or blown cellulose or fiberglass, to reach a target R-value in an attic, wall or floor.
How it’s calculated
Batts: bags = area ÷ coverage per bag. Blown: settled depth = target R ÷ R per inch (cellulose about 3.6, fiberglass about 2.5); weight = area × depth × density.
Worked example
With these inputs:
- Area length
- 40 ft
- Area width
- 28 ft
- Product
- Batts / rolls
- Batt R-value
- R-30 (10")
- Coverage per bag (sq ft)
- 58
- Waste allowance
- 5%
the result is:
- Bags of R-30
- 21 bags
- Coverage per bag
- 58 sq ft
Tips from the trade
- DOE recommends R-49 to R-60 for attics in most of the US. Check your local energy code.
- Keep insulation clear of eaves vents and recessed light fittings unless they are rated for contact.
- Board out over the top layer only if you raise the boards on legs. Compressed insulation loses performance.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and local building codes before ordering.