By Zencalcy Editorial TeamLast reviewed About us
R-value measures resistance to heat flow, and each material delivers a fixed amount per inch of thickness. Multiply to go from thickness to R-value, divide to find the thickness that reaches a target, then compare that target against the recommended level for your climate zone.
R-value, thickness and the recommended level for your climate.
R-value = thickness × R per inch thickness = target R ÷ R per inch
Per-inch values: fiberglass batt R-3.1, cellulose R-3.7, open-cell spray foam R-3.6, closed-cell spray foam R-6.0, XPS rigid R-5.0. Attics want R-49 to R-60 in cold zones and R-30 to R-38 in warm ones.
| Symbol | Meaning |
|---|---|
| t | Material thickness in inches. |
| R | Thermal resistance, R-value. |
| R/in | R-value delivered per inch. |
49 ÷ 3.1 = 15.8 inches of fiberglass batt — two 8 inch courses offset, or one blown layer.
Continue with the Paint Calculator, Wallpaper Calculator, Tile Calculator, or the Flooring Calculator — or browse more home & garden projects and all construction calculators.
About 12.3 inches of fiberglass batt (R-3.1 per inch) or 10.3 inches of cellulose (R-3.7 per inch). Materials differ, so check the per-inch R-value of your product.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026