How to use this calculator
- Enter or choose units.
- Enter or choose layer 1 thickness.
- Enter or choose layer 1 conductivity or R per inch.
- Enter or choose layer 2 thickness.
- Enter or choose layer 2 conductivity or R per inch.
- The total R-value (SI) updates as you type. Use Copy result or Share to keep it.
Method
R_layer = t ÷ λ (or t × R per inch); R_total = Σ R_layer + R_si + R_se; U = 1 ÷ R_total; R_US = R_SI × 5.678263
- t
- layer thickness
- λ
- thermal conductivity, W/m·K
- R_si, R_se
- inside and outside surface resistances
Worked example
- Insulation: 0.100 ÷ 0.035 = 2.857 m²·K/W.
- Plasterboard: 0.0125 ÷ 0.25 = 0.050; plus surface films 0.17 → total 3.077 m²·K/W.
- U = 1 ÷ 3.077 = 0.325 W/m²·K; US R = 3.077 × 5.678 = R-17.5.
100 mm of 0.035 insulation behind plasterboard gives R 3.08 m²·K/W (about R-17.5 US) and a U-value of 0.325.
What the result means
Higher R (lower U) means less heat flow. Compare the U-value with the target in your energy code, which may count framing and thermal bridges.
Assumptions and limits
- Uniform, continuous layers (one-dimensional heat flow).
- Studs, fixings and gaps form thermal bridges that lower the real R-value; full calculations (e.g. ISO 6946 combined method) account for them.
Questions
Why are US and metric R-values different numbers?
They use different units. One SI unit (m²·K/W) equals 5.678 US units (ft²·°F·h/Btu), so R-19 US is about R 3.35 SI.
Do air films matter?
Slightly: about 0.17 m²·K/W on a wall, which matters more for thin, poorly insulated elements such as single glazing.
Please note: Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.