Insulation Calculator
Plan your insulation job. For a 1,200 sq ft attic at R-38 you need about 10.9 inches of loose-fill cellulose or 13 batt bundles.
R-value and depth
R-value measures resistance to heat flow. Higher is better. Loose-fill cellulose gives about R-3.5 per inch and fiberglass about R-2.5, so depth = target R-value / R per inch. For R-38 with cellulose you need about 10.9 inches.
For batts, divide the area plus 5% waste by the coverage per bundle. That is 1,086 cubic feet of blown-in or 13 batt bundles (two options, not both). Batt bundles cost about $780.00.
Recommended R-values
- Attics: R-38 to R-60 depending on climate zone
- Walls: R-13 to R-21
- Floors over unheated space: R-25 to R-30
Check the DOE zone map or local code for the minimum. Always air seal before insulating. For wall coverings see the drywall calculator.
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Frequently asked questions
How thick should attic insulation be?
About 10 to 14 inches of loose-fill for R-38 to R-49.
How do I calculate how many batts I need?
Divide the area by the coverage of one bundle and add 5% waste.
What is R-value per inch of cellulose?
About R-3.5 per inch. Fiberglass loose-fill is about R-2.5 per inch.
Is it worth adding more attic insulation?
Often yes. Air sealing plus more attic insulation is among the best returns for energy upgrades.
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