How the calculation works
- Thickness = target R-value ÷ the R-value per inch printed on your product.
- Packages = area ÷ coverage per package (printed on the package for that thickness), rounded up.
- The reference table below shows approximate R-values per inch from U.S. Department of Energy data. Values vary by product, so always use the product label.
Example
Area to insulate: 500 sq ft · Target R-value: 15 · R-value per inch of your product: 5 · Coverage per package: 40 sq ft
Result: 3 in or 13 packages.
Approximate R-value per inch
| Material | R-value per inch |
|---|---|
| Fiberglass batts | about 2.5–3.7 |
| Loose-fill (fiberglass, cellulose) | about 2.2–3.75 |
| Open-cell spray foam | about 3.6 |
| Closed-cell spray foam | about 6.5 |
| Expanded polystyrene (EPS) board | about 3.8–4.4 |
| Extruded polystyrene (XPS) board | about 5 |
| Polyisocyanurate (polyiso) board | about 6.5 |
Derived from the R-values and thicknesses in the U.S. Department of Energy guide. Use your product's label.
Tips before you buy
- Recommended R-values depend on your climate zone and where the insulation goes.
- Compressed insulation loses R-value, so do not squeeze batts into a thinner space.
- Always confirm coverage and yield with the manufacturer's label — products differ.
FAQ
Why does R-value per inch vary so much?
Density, material and installation all change it. Loose-fill settles over time and batts lose R-value when compressed.