Calculate U-value and RSI for building assemblies, convert between metric and imperial units, or estimate performance from layered materials.
Customize This Calculator
Build your own version. Describe what you want changed, added, or compared.
- All Construction Calculators
- Thermodynamics and Fluid Unit Converts
- Floor to Area Ratio Calculator
- Gross Floor Area Calculator
- U Value of Ground Floor Calculator
U Value to RSI Formula
RSI is the reciprocal of U value. For an assembly built from layers, RSI is the sum of each layer's thickness divided by its thermal conductivity.
- RSI = thermal resistance, m²·K/W
- U = thermal transmittance, W/(m²·K)
- t = layer thickness, m
- k = thermal conductivity of the material, W/(m·K)
U and RSI are SI units. To convert from imperial R-value (ft²·°F·hr/BTU), multiply by 0.17611 to get RSI. To convert U in BTU/(hr·ft²·°F) to W/(m²·K), multiply by 5.67826. Surface film resistances and thermal bridging are not included unless you add them as layers.
Reference Values
Typical U values by element and the matching RSI:
| Building Element | U [W/m²K] | RSI [m²K/W] |
|---|---|---|
| Passive House wall | 0.10 – 0.15 | 6.7 – 10.0 |
| Code-compliant wall | 0.20 – 0.30 | 3.3 – 5.0 |
| Modern roof | 0.13 – 0.18 | 5.6 – 7.7 |
| Triple-glazed window | 0.7 – 1.0 | 1.0 – 1.4 |
| Double-glazed window | 1.4 – 2.0 | 0.5 – 0.7 |
| Single glazing | 5.0 – 6.0 | 0.17 – 0.20 |
Common conversions between systems:
| From | To | Multiply by |
|---|---|---|
| U BTU/(hr·ft²·°F) | U W/(m²·K) | 5.67826 |
| R-value ft²·°F·hr/BTU | RSI m²·K/W | 0.17611 |
| RSI m²·K/W | R-value | 5.67826 |
Worked Example
A wall has a U value of 0.25 W/(m²·K). The RSI is:
RSI = 1 / 0.25 = 4.0 m²·K/W
That equals an imperial R-value of 4.0 × 5.67826 ≈ R-22.7.
FAQ
Is RSI the same as R-value? No. RSI is the metric version in m²·K/W. The imperial R-value uses ft²·°F·hr/BTU. RSI 1.0 ≈ R-5.68.
Does the result include air films? No. The layers calculation gives the construction RSI only. Add interior (≈0.13) and exterior (≈0.04) surface resistances if you want the total assembly RSI.
Why does my layered result differ from a published U value? Published U values usually include surface films, framing factors, and thermal bridging. A pure layer sum will be slightly higher RSI (lower U) than the real assembly.
