Insulation R-value calculator
Enter the R-value you want, what is already there and the material. The calculator tells you the thickness to add and the volume of insulation for the area, in R (imperial) or RSI (metric).
See your design as an AI render — freeValues come from an official table, but the product label wins: swap in the manufacturer's figure.
- R to add
- 30
- Insulation volume
- 694 cu ft
- As RSI (metric)
- RSI 5.28
Material values: Canada, National Building Code 2020, Table A-9.36.2.4.(1)-D. RSI = R ÷ 5.678.
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How to calculate insulation thickness
Thermal resistance (R in imperial, RSI in metric) grows with the thickness of the insulation.
- 1
Find what is missing
R to add = target − existing. To reach R-49 with R-19 already in place, you need R-30 more.
- 2
Divide by R per inch
Thickness = R ÷ R per inch. With loose-fill cellulose at R-3.6 per inch: 30 ÷ 3.6 = 8.33 in.
- 3
Work out the volume
Volume = area × thickness: 1,000 sq ft × 8.33 in ÷ 12 = 694 cu ft of insulation.
- 4
In metric
Metric uses RSI and λ: thickness = RSI × λ. From RSI 2 to RSI 7 with λ 0.040 is 5 × 0.040 = 0.20 m, or 200 mm. RSI = R ÷ 5.678.
Common insulation materials
Values from the Canadian code table (NBC 2020). Always use the figure for the product you buy.
| Material | R per inch | Thickness for R-30 |
|---|---|---|
| Cellulose, loose fill | 3.6 | 8.3 in |
| Cellulose, sprayed | 3.5 | 8.6 in |
| Fiberglass, loose fill | 2.7 | 11.1 in |
| Fiberglass, blown/sprayed | 3.6 | 8.3 in |
| EPS, low density | 3.75 | 8 in |
| EPS, high density | 4.1 | 7.3 in |
| XPS (extruded polystyrene) | 5.08 | 5.9 in |
| Polyisocyanurate (polyiso) | 5.7 | 5.3 in |
| Spray polyurethane foam | 5.19 | 5.8 in |
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