U-value calculator — thermal transmittance of your wall

Each layer gets a thermal resistance R = d/λ, where d is the thickness in meters and λ is the material’s conductivity in W/mK. An air gap counts as a fixed resistance of about 0.18 m²K/W regardless of thickness. To the sum of the layers you add the surface resistances: Rsi = 0.13 on the inside and Rse = 0.04 on the outside. The U-value is then 1 divided by the total resistance: U = 1/R_tot.

Example: 22 mm wood siding + 195 mm mineral wool + 13 mm drywall gives R_tot = 0.13 + 0.16 + 5.27 + 0.05 + 0.04 ≈ 5.65 m²K/W, so U ≈ 0.18 W/m²K — before any correction for studs. In US units the same total corresponds to about R-32, since the imperial R-value is the SI resistance multiplied by 5.678.

The calculation above assumes a homogeneous section straight through the insulation. In a real framed wall the insulation is interrupted by studs with almost four times the conductivity, and the practical lambda value in the built construction is often slightly worse than the datasheet. As a rough rule, a normal framing fraction raises the U-value by 10–25%. Code-compliant energy calculations therefore use corrected methods — ISO 6946 in Europe, or whole-assembly U-factors under the IECC/IRC in the US — that account for framing, thermal bridges and ventilated air gaps.

To reduce the framing penalty, build with crossed framing layers (for example a horizontal service batten inside the main studs) or add a continuous layer of insulation outside the frame. As a benchmark: new-build targets in colder climates sit around 0.18 W/m²K for walls, 0.13 for roofs and 0.15 for floors, and in a renovation every reduction in U-value shows up directly in the heating demand.