How to Use the Insulation R-Value Calculator
R-value (thermal resistance) tells you how effectively a layer of insulation resists heat transfer. It is calculated as thickness (in meters) divided by thermal conductivity (λ, W/mK). The U-factor (thermal transmittance) is the reciprocal of R-value — energy codes specify maximum U-factors.
This calculator also converts SI R-value to imperial (ft²·°F·h/BTU) and checks whether the single insulation layer meets the IECC 2021 climate zone 5 exterior wall assembly requirement of U≤0.060 BTU/(h·ft²·°F), equivalent to U≤0.341 W/m²K in SI.
Insulation Thermal Conductivity (λ) Reference
- EPS: λ = 0.036 W/mK
- XPS: λ = 0.029 W/mK
- Closed-cell spray foam: λ = 0.024 W/mK
- Phenolic foam: λ = 0.022 W/mK (best performance)
Frequently Asked Questions
Divide SI R-value (m²K/W) by 0.176 to get imperial R-value (ft²·°F·h/BTU). For example, R-2.0 (SI) ≈ R-11.4 (imperial).
Yes — wood studs have lower R-value than cavity insulation, reducing effective (clear-wall) R-value by 10–20%. Assembly U-factors used in energy codes account for framing fraction.