Insulation U-Value Calculator
Calculate the U-value and R-value of a wall, roof or floor from insulation thickness and λ — and the thickness you need.
Simplified calculation for uniform layers. Studs, rafters, fixings and air gaps (thermal bridges) make the real U-value somewhat higher. Use the λ value from the product data sheet, and have a professional check designs that must document TEK17 compliance.
About this tool
Choose the building element, enter the insulation thickness and its thermal conductivity (λ), and optionally the R-value of other layers. The calculator uses U = 1 ÷ (Rsi + d/λ + R other + Rse) with the standard surface resistances from EN ISO 6946, checks the result against the Norwegian TEK17 minimum, and tells you how thick the insulation must be for a target U-value.
Frequently asked questions
The U-value is how much heat passes through 1 m² of a building element per degree of temperature difference, in W/(m²·K). Lower is better. U = 1 ÷ total R, and an insulation layer has R = thickness (m) ÷ λ, so 200 mm with λ 0.035 gives R = 5.71 m²·K/W.
EN ISO 6946 adds a fixed resistance for the air film on each side: Rse = 0.04 outside, and inside Rsi = 0.13 for walls (horizontal heat flow), 0.10 for roofs (upward) and 0.17 for floors (downward). They add 0.14–0.21 m²·K/W.
Use the declared λ from the product data sheet. Typical ranges are 0.032–0.038 W/(m·K) for glass wool, 0.035–0.040 for stone wool, 0.030–0.040 for EPS and 0.022–0.028 for PUR/PIR boards — a lower λ insulates better per mm.
Multiply the metric R-value (RSI, m²·K/W) by 5.678 to get the US R-value in ft²·°F·h/BTU. A total RSI of 5.88 is about R-33.4, and the US R-value per inch of λ 0.035 insulation is about R-4.1.
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