Approved Document L 2025 — what changed
The Future Homes Standard transitional measures took effect 15 June 2025 in England, tightening Approved Document L (Conservation of Fuel and Power) for new dwellings and major renovations. For loft conversions (treated as a 'material change of use' under the regulations), the new U-value targets are: roof at 0.11 W/m²K, walls at 0.18 W/m²K, floor at 0.13 W/m²K, windows at 1.4 W/m²K. The previous Part L 2013 standards for new conversions sat at 0.18 W/m²K for the roof — the new target requires significantly more insulation. The principal compliance path remains 'elemental' (meet each U-value target) but a 'fabric energy efficiency' route is also available where overall building performance is calculated via SAP.
What 0.11 W/m²K looks like in practice
On a typical Victorian London loft conversion with 150mm rafter depth, achieving 0.11 W/m²K requires 150mm of PIR between rafters plus a continuous 60–75mm PIR layer under the rafters (insulated plasterboard or separate PIR board with foil-backed plasterboard). Total assembly: 210–225mm of PIR equivalent. Alternative compliant build-ups: 200mm of high-performance phenolic insulation between deeper rafters; 175mm rafters with 100mm rigid wood-fibre insulation plus 60mm internal PIR. Continuous insulation under the rafters is now essentially mandatory because the thermal bridging through timber rafters alone defeats inter-rafter compliance. Always check the build-up with the U-value calculation software — eye-balling thickness is unreliable.
Ventilation, condensation and the warm-roof choice
Two roof construction strategies dominate London loft conversions: 'warm roof' (insulation above and between rafters, no ventilation void) and 'cold roof' (insulation between rafters with a ventilated void above). Warm roof is now the dominant approach because it eliminates thermal bridging, simplifies airtightness and avoids the condensation risk of cold roof construction. Approved Document F 2025 requires controlled ventilation in habitable rooms — typically background ventilators on rooflights and either trickle vents on the staircase to the lower floor or a continuous mechanical extract. Bathrooms in the loft require continuous mechanical extract to AD-F requirements. Builderr's standard loft specification is a warm-roof PIR build-up with foil-faced internal lining, taped airtightness layer and MVHR or constant-extract ventilation.
