Our AI calculator handles all common UK roof constructions with correct surface resistances and ventilation allowances
Insulation at ceiling level with cold loft space above. Well-ventilated loft treated as external. Surface resistance Rsi = 0.10 m²K/W for upward heat flow.
Typical U-value: 0.10 - 0.16 W/m²K
Insulation between and/or below rafters (warm roof). Requires ventilated cavity between insulation and underlay. Thermal bridging from rafters included.
Typical U-value: 0.12 - 0.18 W/m²K
Insulation above the structural deck with waterproofing on top. No ventilation required. Preferred solution for avoiding condensation.
Typical U-value: 0.12 - 0.18 W/m²K
Insulation between joists with ventilated cavity above. Must include thermal bridging from joists. Requires careful condensation analysis.
Typical U-value: 0.15 - 0.20 W/m²K
For roofs with upward heat flow, BR 443 specifies the following surface resistances:
Rsi = 0.10 m²K/W
Internal surface resistance (upward flow)
Rse = 0.04 m²K/W
External surface resistance
Note: For well-ventilated lofts, treat the loft space as external (R = 0 for the cavity).
For new builds in the UK under Part L 2021, roofs should achieve a U-value of 0.11 W/m²K. For extensions with pitched roofs at ceiling level, the target is 0.16 W/m²K, or 0.18 W/m²K for flat roofs and pitched roofs at rafter level.
In a warm deck roof, insulation sits above the structural deck, keeping it warm. In a cold deck roof, insulation sits between or below the joists, with a ventilated cavity above. Warm deck is generally preferred as it avoids condensation issues and thermal bridging through joists.
For ceiling level insulation (cold roof), use Rsi = 0.10 m²K/W for upward heat flow. Include the loft space as a well-ventilated cavity (R = 0). For rafter level insulation (warm roof), calculate through all layers including the roof covering, ventilated batten cavity if present, and internal finish.
Yes, timber rafters create thermal bridges. For insulation between rafters, use the combined method with typical rafter fractions of 12-15%. The rafter conductivity (0.13 W/mK for softwood) is much higher than insulation, increasing the overall U-value.