Insulation R-Value Converter
Convert between R-value (imperial) and RSI (metric) insulation ratings.
Includes common material thicknesses and their R-values.
Type in any field — the others update instantly.
The R-value measures thermal resistance — how well a material resists heat flow through it. A higher R-value means better insulation and less heat loss.
Conversion between imperial and metric:
- 1 R-value (ft2 F h/BTU) = 0.176110 RSI (m2 K/W)
- 1 RSI (m2 K/W) = 5.678263 R-value
The formulas are simply:
- RSI = R-value x 0.176110
- R-value = RSI x 5.678263
Common insulation materials and their R-values per inch (per 25.4 mm):
| Material | R per inch | RSI per 25.4mm |
|---|---|---|
| Fiberglass batts | 3.1-3.4 | 0.55-0.60 |
| Cellulose (blown) | 3.2-3.8 | 0.56-0.67 |
| Mineral wool batts | 3.0-3.3 | 0.53-0.58 |
| Spray foam (open cell) | 3.6-3.8 | 0.63-0.67 |
| Spray foam (closed cell) | 6.0-7.0 | 1.06-1.23 |
| Rigid XPS foam board | 5.0 | 0.88 |
| Rigid polyiso foam board | 5.6-6.5 | 0.99-1.14 |
| EPS (expanded polystyrene) | 3.6-4.2 | 0.63-0.74 |
Recommended total R-values by climate zone (USA):
| Climate Zone | Attic | Walls | Floor |
|---|---|---|---|
| Zone 1 (hot/tropical) | R-30 | R-13 | R-13 |
| Zone 3 (warm) | R-38 | R-13 | R-19 |
| Zone 5 (moderate) | R-49 | R-20 | R-25 |
| Zone 7 (cold) | R-60 | R-21 | R-30 |
Practical tips:
- R-values are additive: two layers of R-13 insulation stacked together give R-26 total.
- The R-value per inch varies by material, so thickness alone does not determine insulation performance.
- In metric countries like Canada, Europe, and Australia, RSI is the standard unit used in building codes.
- Gaps, compression, and moisture all reduce the effective R-value significantly, so proper installation is as important as the material rating itself.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.