Calculate RSI and imperial R-value from U-value, or reverse-convert resistance to U-value. An optional steady conductive heat-flow estimate uses both assembly area and temperature difference.
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U Value to RSI Formula
RSI is the reciprocal of U-value in matching SI units. For an ideal uniform stack of layers in series, layer resistances sum as thickness divided by conductivity. This calculator converts a supplied U-value or resistance; it does not build a layer stack.
- 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-value is expressed in W/(m²·K), and RSI in m²·K/W. Multiply imperial R-value (ft²·°F·hr/BTU) by approximately 0.17611 for RSI; multiply imperial U-factor by approximately 5.67826 for SI U-value. The conversion preserves whatever surface films and thermal bridges are already represented by the supplied assembly value. Thermal bridges require appropriate parallel-path or detailed modelling, not simply another layer.
Reference Values
Illustrative U-values and reciprocal RSI ranges follow. Actual performance and code compliance depend on the assembly, project and climate; these ranges are not compliance ratings.
| Building Element | U [W/m²K] | RSI [m²K/W] |
|---|---|---|
| Illustrative highly insulated wall | 0.10 – 0.15 | 6.7 – 10.0 |
| Illustrative insulated 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? It reflects the boundary definition of the U-value or resistance you enter. If that value already includes surface films, the converted result includes them too. This converter adds no film resistance; appropriate film values depend on the applicable method and heat-flow conditions.
Why can an ideal layer sum differ from a published assembly value? Published values may represent surface films, framing and thermal bridges. Surface films add resistance, while parallel heat-flow paths can reduce effective resistance. Compare the same assembly boundaries and calculation method; the difference does not always have the same direction.
