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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Thickness is the target R-value divided by the material's R-value per inch. The same R-49 attic is about 14 inches of blown cellulose but nearly 20 inches of blown fiberglass, and R-21 fits a 2×6 wall only with a denser batt. Pick a material, enter the R-value you want, and see the inches you need and whether it fits the cavity.
With these inputs:
| Result | Value |
|---|---|
| Thickness needed | 14 in |
| Smallest framing cavity it fills | Deeper than a 2×12: an open attic floor or added depth |
| In feet | 1.17 ft |
Where the R-per-inch figures come from. Batts are rated as a product, so R per inch comes from the rating and the thickness on the data sheet: an Owens Corning EcoTouch R-13 batt is 3-1/2 inches (R-3.71 per inch) and a ROCKWOOL Comfortbatt R-15 is also 3-1/2 inches (R-4.29 per inch). Loose-fill varies with how densely it is blown, so the calculator uses the middle of the ranges the US Department of Energy publishes: R-3.2 to 3.8 per inch for cellulose and R-2.2 to 2.7 for fiberglass. XPS board is made to ASTM C578, which sets R-5.0 per inch for the common types. If your product's label gives a different figure, type it into the R-value per inch box and the result follows it. The label or coverage chart is always the final word for the product you buy. When the cavity is too shallow. A batt squeezed into a shallower space loses R-value, so do not compress a thick batt to fit. If the thickness you need is more than the cavity depth, choose a denser material (mineral wool instead of standard fiberglass), add continuous rigid foam over the framing, or deepen the cavity. In an open attic floor, loose-fill can be blown deeper than the joists.
Thickness (in) = target R-value ÷ R-value per inch · Centimetres = inches × 2.54 · Cavity depths are actual lumber sizes: 2×4 = 3.5 in, 2×6 = 5.5 in, 2×8 = 7.25 in, 2×10 = 9.25 in, 2×12 = 11.25 in
| Material | R-13 | R-21 | R-30 | R-49 | R-60 |
|---|---|---|---|---|---|
| Fiberglass batt | 3.5 in | 5.7 in | 8.1 in | 13.2 in | 16.2 in |
| Mineral wool batt | 3 in | 4.9 in | 7 in | 11.4 in | 14 in |
| Blown cellulose (loose-fill) | 3.7 in | 6 in | 8.6 in | 14 in | 17.1 in |
| Blown fiberglass (loose-fill) | 5.2 in | 8.4 in | 12 in | 19.6 in | 24 in |
| XPS rigid foam board | 2.6 in | 4.2 in | 6 in | 9.8 in | 12 in |
Computed with this calculator. Loose-fill settles over time; the bag's coverage chart lists both installed and settled thickness, so blow to the installed figure.
About 14 inches of blown cellulose at R-3.5 per inch, or about 19.6 inches of blown fiberglass at R-2.5 per inch. Check the bag's coverage chart for the exact installed depth of your product.
About 8.1 inches of fiberglass batt at R-3.71 per inch, 8.6 inches of blown cellulose, or 12 inches of blown fiberglass.
Divide the R-value you want by the material's R-value per inch. R-21 of mineral wool at R-4.29 per inch is 21 ÷ 4.29 = 4.9 inches, which fits a 2×6 wall's 5.5 inch cavity.
The 2021 International Energy Conservation Code asks for R-30 in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8 for new ceilings. Local codes may adopt a different edition.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.