Insulation Calculator
Work out how thick your insulation needs to be to reach a target R-value, and how many bags or rolls to buy.
About the Insulation Calculator
R-value measures resistance to heat flow; higher is better, and layers add up. Each material gives a typical R per inch, so thickness = R needed ÷ R per inch. Blown products settle, so follow the bag's chart for installed depth and coverage.
The US Department of Energy recommends attic insulation of R-30 to R-49 in the warmest zone, R-30 to R-60 in zones 2–3, R-38 to R-60 in zone 4 and R-49 to R-60 in zones 5–8.
The formula
Thickness (in) = (target R − existing R) ÷ R per inch
Typical R-value per inch
| Material | R / inch |
|---|---|
| Fibreglass batts | 3.2 |
| Blown fibreglass | 2.5 |
| Blown cellulose | 3.5 |
| Mineral wool | 3.3 |
| Open-cell spray foam | 3.6 |
| Closed-cell spray foam | 6.5 |
| XPS foam board | 5 |
| EPS foam board | 4 |
| Polyiso board | 6 |
Typical values; check the product's label.
Worked example
Example: Add = 15.4 in (39.2 cm) of blown cellulose
- Inputs: Area to insulate: 1,100; Area in: ft²; Material: Blown cellulose (≈R-3.5/in); Target R-value (US): 54 R; Existing R-value: 0 R.
- Formula: Thickness (in) = (target R − existing R) ÷ R per inch
- Result: Add = 15.4 in (39.2 cm) of blown cellulose
Questions people ask
What's RSI?
The metric R-value: RSI = R × 0.1761.
Can I put new insulation on top of old?
Yes, unfaced; the R-values add up, if the old insulation is dry.