What U-value is
U-value (units: W/m²K) is the rate at which heat passes through one square metre of building element when there is one degree of temperature difference across it. Lower U-value = better insulation. A wall with U-value 0.18 loses 18% of the heat that a wall with U-value 1.0 would lose at the same conditions. Building elements have different targets: roof loses heat fastest (hot air rises) so target is lowest (0.13); floor loses heat slowest (cold dense earth) so target slightly relaxed (0.18); walls between (0.18); windows highest (1.4 because triple-glazed glass cannot exceed solid-wall insulation thickness).
Part L 2025 targets
Approved Document Part L 2025 (England) sets these limiting values for existing dwellings (renovation/extension): External walls 0.18 W/m²K; pitched roof at rafter 0.16 / at ceiling 0.16; flat roof 0.18; ground floor 0.18; windows 1.4; doors 1.4; rooflights 1.6. New buildings stricter: 0.18 walls, 0.13 roof, 0.18 floor, 1.4 glazing. Achieving values: external wall 0.18 requires 100mm PIR or 150mm mineral wool. Roof 0.13 requires 250mm PIR or 400mm mineral wool. Triple-glazed window achieves 0.85–1.2; double-glazed 1.4–1.6. Verify at building control submission with calculator (SAP or AD-L verified).
Passivhaus and EnerPHit targets
Passivhaus standard (new build): walls 0.10–0.15, roof 0.08–0.12, floor 0.10–0.15, triple glazing 0.85 max. EnerPHit (retrofit Passivhaus): walls 0.15, roof 0.10–0.13, floor 0.15, glazing 0.85 (lower if practical). Achieving requires deep insulation: 200mm K15 EWI for walls 0.15; 400mm K7 for roof 0.10; 200mm TF70 for floor 0.15. Plus thermal bridge management at all junctions (Y-value <0.04) and airtightness <1.0 m³/h/m² @50Pa. Significantly higher build cost (40–80% premium on shell) but transformative performance. London EnerPHit retrofit cost premium £40,000–£90,000 over standard renovation but eliminates heating bills.
