Work out u value instantly with clear inputs, formula shown and shareable results.
U-value is the reciprocal of the total thermal resistance, counting every layer plus the internal and external surface films. Adding 2.6, 0.35 and 0.17 gives 3.12 m2 K/W and a U-value of 0.32 W/(m2 K), so each square metre loses 6.4 W under a 20 K difference.
U-value
U = 1 / (R1 + R2 + R(surfaces))
Heat loss
Q = U x A x dT
Because air films on both faces genuinely resist heat flow - typically 0.13 inside and 0.04 outside, which matters for thin constructions.
Around 0.18 W/(m2 K) for new-build walls under current UK standards, and 0.11-0.13 for roofs.