Insulation Calculator — Thickness for a Target R-Value
Pick your target R-value and insulation type, enter the area (and any existing R-value), and get how thick the insulation needs to be plus its coverage.
As an Amazon Associate this site may earn from qualifying purchases.
How it works
thickness (in) = (target R − existing R) ÷ R-per-inch; board feet = area × thickness
Insulation thickness for a target R-value (inches)
| Target R-value | Fiberglass batt | Blown cellulose | Closed-cell foam |
|---|---|---|---|
| R-13 | 4.1 in | 3.7 in | 2.0 in |
| R-21 | 6.6 in | 6.0 in | 3.2 in |
| R-30 | 9.4 in | 8.6 in | 4.6 in |
| R-38 | 11.9 in | 10.9 in | 5.8 in |
| R-49 | 15.3 in | 14.0 in | 7.5 in |
| R-60 | 18.8 in | 17.1 in | 9.2 in |
Thickness = target R ÷ R-per-inch (fiberglass batt ≈ 3.2, blown cellulose ≈ 3.5, closed-cell foam ≈ 6.5 per inch). Recommended R-values vary by climate zone — check local code.
Frequently asked questions
How much insulation do I need for an attic?
Most attics target R-38 to R-60 depending on climate. To reach R-49 you need about 15 inches of fiberglass batts, 14 inches of blown cellulose, or 7.5 inches of closed-cell spray foam. Cold zones often require R-49; milder zones R-38.
What is the R-value per inch of insulation?
Roughly: fiberglass batt ≈ 3.2, blown fiberglass ≈ 2.5, blown cellulose ≈ 3.5, closed-cell spray foam ≈ 6.5, and rigid XPS foam ≈ 5.0 per inch. Divide your target R-value by the R-per-inch to get the thickness.
How do I add insulation to an existing attic?
Enter your current R-value as the existing R, and the calculator shows only the extra thickness needed. For example, topping up R-19 to R-49 means 30 more R — about 9.4 inches of fiberglass or 8.6 inches of cellulose.