Calculate U-value and RSI for building assemblies, convert between metric and imperial units, or estimate performance from layered materials.
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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 = 1 / U
RSI_total = Σ (t / k)
- 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.
