Insulation calculator
How many inches of insulation you need to reach a target R-value, counting what's already in your attic or walls.
How to use it
Enter the R-value you want to reach, or tap one of the common targets, and pick the insulation you'll add. If there's insulation there already, measure its depth with a ruler and pick its type. The calculator works out how many inches of new insulation you need on top.
R-value measures how well insulation resists heat flow. Higher is better. Each material gives a different R-value per inch, so the same target needs very different depths.
How it's calculated
- Existing R-value = existing depth × its R-value per inch.
- R-value still needed = target − existing.
- Depth to add = R-value needed ÷ R-value per inch of the new material.
| Material | R-value per inch | Depth for R-38 | Depth for R-49 |
|---|---|---|---|
| Blown fiberglass | 2.2 | 17.3 in | 22.3 in |
| Fiberglass batts or mineral wool | 3.1 | 12.3 in | 15.8 in |
| Open-cell spray foam | 3.5 | 10.9 in | 14 in |
| Blown cellulose | 3.7 | 10.3 in | 13.2 in |
| Polystyrene board | 4.0 | 9.5 in | 12.3 in |
| Polyurethane or polyiso board | 6.0 | 6.3 in | 8.2 in |
| Closed-cell spray foam | 6.2 | 6.1 in | 7.9 in |
R-values per inch are typical figures from a DOE-based chart. Products vary: check the R-value printed on the bag or batt, which is the rated figure for that product.
Which target R-value?
The right target depends on your climate zone and where the insulation goes. The US Department of Energy publishes recommended R-values by zone; attics in most of the US call for roughly R-38 to R-60, and 2 × 4 walls hold about R-13 to R-15. See the DOE link under Sources.
Tips
- Don't compress batts. Squashing a batt into a smaller space lowers its R-value.
- Seal air leaks first. Gaps around pipes, wires, lights and the attic hatch let heat escape no matter how much insulation is on top.
- Keep soffit vents clear with baffles when adding attic insulation, so the attic still ventilates.
- Blown insulation settles. Bags list both the installed and the settled thickness; aim for the settled figure to reach your target.