How to use the R-value calculator
- Choose Imperial or Metric for layer thicknesses; conductivity is always in W/m·K.
- Add each layer from inside to outside with its thickness and thermal conductivity λ.
- Enter the surface resistances for your element.
- Read the RSI, U-value and US R-value.
How the calculation works
Each layer’s resistance is R = thickness ÷ λ. The total RSI is the sum of the layer resistances plus the surface resistances. U-value = 1 ÷ RSI. US R-value = RSI ÷ 0.1761. Open Show the working to see each layer’s contribution.
Worked examples
Insulated stud bay (clear field). 12.5 mm board (λ 0.25): 0.05. 100 mm insulation (λ 0.035): 2.857. Surfaces: 0.17. Total RSI 3.08, U-value 0.325 W/m²·K, R-17.5.
Single layer. 100 mm of insulation at λ 0.04 with no surface resistances: RSI 2.5, U 0.4, about R-14.2.
How to interpret the results
Compare the U-value or R-value with the requirement in your energy code, which may apply to the whole assembly including thermal bridging. A lower λ or a thicker layer both raise RSI; doubling the thickness doubles that layer’s resistance.
Tips and common mistakes
- Use declared λ values, not generic ones, for compliance work.
- Don’t add air gaps as insulation unless they are sealed and you know their resistance.
- Compressed insulation performs worse than its rated thickness suggests.
- Thermal bridges through studs, joists and fixings lower the real-world value.
Improving an existing wall or roof
Adding a layer changes the total in a predictable way: each new layer adds its own thickness ÷ λ to the RSI. Try adding a row to see how much an extra layer of insulation board or a thicker batt improves the U-value before you buy. Diminishing returns are real: the first layers make the biggest difference to heat loss.
Assumptions and limitations
This is a simplified layer-by-layer calculation. Official calculations for building regulations follow standards such as EN ISO 6946 and account for bridging, fixings and moisture.