U Value Calculator
Work out the U value of a wall or roof build-up from its layers.
U value
0.321W/m²K
- Total thermal resistance
- 3.116 m²K/W
Includes standard surface resistances of 0.13 (inside) and 0.04 (outside) m²K/W. Ignores thermal bridging and air gaps.
v1.0 · Reviewed September 2026
How this is worked out
Each layer's thermal resistance is its thickness in metres ÷ its conductivity. Those are added to the standard surface resistances of 0.13 inside and 0.04 outside, and the U value is 1 ÷ the total resistance.
Worked example
- 100 mm at λ 0.035
- 100 mm at λ 1.13
0.17 + 2.857 + 0.088 = 3.116 m²K/W, then 1 ÷ 3.116
0.321 W/m²K
Assumptions and limits
- Surface resistances of 0.13 m²K/W inside and 0.04 m²K/W outside are the standard values for a wall. Roofs and floors use different figures.
- Ignores thermal bridging through studs, joists and fixings, which can raise the real U value substantially, and ignores unventilated air gaps.
- Only three layers are handled, so thin finishes may need to be left out or combined.
- Not a Building Regulations calculation. Compliance work uses BR 443 conventions and accounts for bridging.