Wall U-Value & R-Value (Layers)
Calculate the total thermal resistance, U-value and R-value of a wall, roof or floor built from several layers, free online.
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Each layer adds thermal resistance R = thickness ÷ conductivity. The total includes surface resistances (about 0.13 m²K/W inside, 0.04 outside). U = 1 ÷ R_total, and the imperial R-value is R(m²K/W) × 5.678. List layers as "name: thickness in mm, k in W/m·K".