Insulation calculator
Pick your climate zone and measure what is in the attic now. You get the R-value to aim for, how much to add, the depth of new insulation, and the bags if you enter the bag's coverage.
Inputs
Result
Recommended attic insulation by zone
ENERGY STAR's recommendations for existing homes, based on the 2021 IECC.
| Climate zone | Uninsulated attic, add | 3–4 in already there, add |
|---|
How it is worked out
R per inch is a range for loose fill, because it depends on how densely it settles: about 3.2 to 3.8 for cellulose and 2.2 to 2.7 for fiberglass, by the Department of Energy's figures. The calculator gives the depth at both ends, so blow to the deeper figure.
In zone 4A, ENERGY STAR recommends R-60 in the attic. Four inches of old fiberglass is worth about R-10, so about R-50 more is needed: 13 to 16 in of cellulose, or 19 to 23 in of fiberglass.
Bags
Every bag of blown insulation carries a coverage chart: square feet per bag at each R-value, with the settled thickness and number of bags per 1,000 sq ft. It differs between products, so read it off your bag and enter the square feet per bag at your target. The calculator divides the attic area by it.
Questions
How much attic insulation do I need?
ENERGY STAR recommends R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8, for an uninsulated attic. With 3 to 4 in already there, add R-25, R-38 or R-49.
How deep should blown-in insulation be for R-38?
About 10 to 12 in of cellulose or 14 to 17 in of fiberglass, settled. For R-60, roughly 16 to 19 in of cellulose.
What is the R-value of cellulose per inch?
About 3.2 to 3.8 for loose-fill cellulose and 2.2 to 2.7 for loose-fill fiberglass, according to the Department of Energy.
How many bags of blown-in insulation do I need?
Divide the attic area by the coverage per bag at your target R-value, from the chart on the bag. It varies by product, so the calculator asks for it.
Can I add insulation over old insulation?
Yes, if the old insulation is dry and not moldy or contaminated. Loose fill goes straight over it; new batts should be laid without a facing, across the old ones.
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