R-value basics
R-value measures resistance to heat flow; higher is better. Required thickness = target R ÷ R per inch of the material. Typical R per inch: fiberglass batts about 2.9–3.8, blown fiberglass about 2.2–2.9, blown cellulose about 3.2–3.8, closed-cell spray foam about 6–7.
Example: R-38 attic with blown cellulose at R-3.2/in: 38 ÷ 3.2 = 11.9 in deep.
What R-value do I need?
The U.S. Department of Energy/ENERGY STAR recommends about R-30 to R-60 for attics depending on climate zone (colder zones need more). Check your local energy code.
Reading the bag chart
Blown-in bags list coverage per bag for each R-value—use that number in the coverage field, because settled density varies by product. Seal attic air leaks before adding insulation; it often matters as much as the R-value.
✓ Formula reviewed and tested · Updated Oct 2026