The formulas
The converter uses three relationships.
US R-value ↔ SI R-value
- R (SI, m²·K/W) = R (US) × 0.1761
- R (US) = R (SI) × 5.678
The US unit is ft²·°F·h/Btu. The SI unit is m²·K/W. One US R is 0.1761 SI R, so the same insulation always has a much bigger number on a US label than on a European data sheet.
R-value ↔ U-value
- U (W/m²·K) = 1 ÷ R (SI)
- R (SI) = 1 ÷ U
U-value is just the reciprocal of R. A US-style U-factor in Btu/h·ft²·°F is 1 ÷ R (US). To turn a metric U into a US U-factor, multiply it by 0.1761.
Thickness ↔ R-value
- R (SI) = thickness (m) ÷ λ (W/m·K)
- R per inch (US) = 0.0254 ÷ λ ÷ 0.1761, or about 0.1442 ÷ λ
λ (lambda) is thermal conductivity, the number European data sheets print instead of R per inch. The lower it is, the better the material insulates.
Worked examples
R-38 to SI and U. This is the converter's starting value. R-38 × 0.1761 = 6.69 m²K/W, and 1 ÷ 6.69 = 0.149 W/m²K.
A European U-value to US R. A data sheet quotes U = 0.25 W/m²K. 1 ÷ 0.25 = R 4.00 m²K/W, which is about R-22.7 in US units.
Thickness and λ to R. Ten inches of a material with λ = 0.040 W/m·K: 0.254 m ÷ 0.040 = R 6.35 m²K/W, or R-36.1 in US units. That's 3.61 per inch, close to blown cellulose.
Common US R-values converted:
| US R-value | SI R (m²K/W) | U (W/m²K) | US U-factor (Btu/h·ft²·°F) |
|---|---|---|---|
| R-13 | 2.29 | 0.437 | 0.077 |
| R-19 | 3.35 | 0.299 | 0.053 |
| R-21 | 3.70 | 0.270 | 0.048 |
| R-30 | 5.28 | 0.189 | 0.033 |
| R-38 | 6.69 | 0.149 | 0.026 |
| R-49 | 8.63 | 0.116 | 0.020 |
| R-60 | 10.57 | 0.095 | 0.017 |
Insulation R vs. whole-assembly U
The converter turns one number into another. It doesn't know what else is in the wall, so be careful what you compare.
- US targets are insulation R-values. When ENERGY STAR recommends R-49 for an attic, it means the R of the insulation itself, printed on the bag or batt. Studs, joists, drywall and sheathing aren't counted.
- European targets are usually whole-element U-values. A UK or German limit for a roof or wall covers everything in it: the insulation, the timber or masonry, the boards and the thin layers of still air on each surface (called surface resistances).
So converting R-21 batts to U = 0.270 tells you about the batt, not the wall. The real wall does worse, because heat also flows through the studs. Our UK and German calculators work this way: they subtract surface resistances and allow for heat flow through the timber when they size insulation.
Common conversion mistakes
- Mixing up US and metric R. A European spec that says "R 5" almost always means 5 m²K/W. That's about R-28 in US units, not R-5.
- Taking 1 ÷ R with the US number. 1 ÷ 38 gives 0.026, which is a US U-factor in Btu/h·ft²·°F. The metric U-value for R-38 is 0.149 W/m²K. Check which unit you need.
- Adding U-values. R-values of layers add up; U-values don't. To combine layers, convert each to R, add them, then take 1 ÷ the total. R-21 batts plus R-10 board make R-31, which is 5.46 m²K/W, or U = 0.183 W/m²K.
- Forgetting the thickness. R per inch describes the material. Multiply by the thickness to get the R-value of the layer.
When you'd use this
- Reading a European data sheet. These often list only λ. Enter it with a thickness to get the US R-value and R per inch.
- Comparing foam boards. Check a product's R per inch against the values on the R-value chart.
- Following a European guide written in U-values while shopping with US labels.
To size insulation for your home, use the insulation calculator, or see the levels for your area on insulation R-values by climate zone.