Insulation R-Value Thickness Calculator
Thickness needed to reach a target R-value for the R-per-inch printed on your insulation product.
Formula
Thickness(in) = target R ÷ R per inch. Volume in board feet = area(sq ft) × thickness(in).
Worked example
Inputs: Target R-value = 38; R-value per inch (product label) = 3.2; Area (optional) = 1000 sq ft.
| Thickness in | 11.88 |
|---|---|
| Board feet | 11,875 |
Recommended attic R-values (ENERGY STAR, existing wood-framed homes)
| Zone | Uninsulated attic | Attic with 3–4 in | Floor |
|---|---|---|---|
| 1 | R30 | R25 | R13 |
| 2 | R49 | R38 | R13 |
| 3 | R49 | R38 | R19 |
| 4A and 4B | R60 | R49 | R19 |
| 6, 5, and 4C | R60 | R49 | R30 |
| 7 and 8 | R60 | R49 | R38 |
Source: ENERGY STAR (linked below), based on 2021 IECC. Retrieved 2026-10-11.
FAQ
Where do I get R per inch?
The FTC R-Value Rule requires R-value information on insulation labels and fact sheets; use your product's number.
What R-value do I need?
ENERGY STAR publishes recommended levels by climate zone (table below, retrofit of existing wood-framed homes); your local energy code sets legal minimums.
Sources
- ENERGY STAR: Recommended Home Insulation R-Values by climate zone
- FTC R-Value Rule (16 CFR Part 460): R-value must be on product labels
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Last reviewed 2026-10-11.