U-value is thermal transmittance: how much heat actually flows through each square metre of an assembly per kelvin of temperature difference, so smaller is better. It is R-value read upside down — U = 1/R — and it is what European regulations cap, computed layer by layer under ISO 6946: each material contributes R = d/λ (thickness over conductivity), and the assembly adds two resistances that surprise newcomers, the still-air films on its faces. ISO 6946 fixes those by heat-flow direction: Rsi = 0.13 m²K/W inside and Rse = 0.04 outside for walls, Rsi = 0.10 for roofs (heat flowing up) and 0.17 for floors (heat flowing down); the reciprocal is taken only after they join the sum, U = 1/(Rsi + ΣR + Rse).
A wall of 100 mm mineral wool at λ = 0.038 W/m·K shows the whole chain: the insulation contributes R = 0.100/0.038 = 2.63 m²K/W, the films add 0.13 + 0.04 for R_total = 2.80, and U = 1/2.80 = 0.357 W/m²K — an R-15.9 assembly in American units. Every number is auditable: one division per layer, one addition, one reciprocal.
What makes U the honest metric is thermal bridging. Timber studs conduct about three times better than mineral wool (λ = 0.13 vs 0.038), and the ISO 6946 combined method blends the two parallel paths by area fraction: 1/R_combined = f/R_bridge + (1−f)/R_insulation. Give the example wall studs on 15% of its area and U rises from 0.357 to 0.476 W/m²K — one-third more heat loss from framing alone, and the quiet math behind continuous-insulation code options. Even the US IECC concedes the point with its Table R402.1.2 U-factor alternative, because U is what the whole assembly, framing included, actually delivers; BR 443 and ASHRAE 90.1 govern the same calculation in their jurisdictions.