Insulation U-Values — Part L Targets & Thicknesses
U-values measure how much heat passes through a building element — the lower the U-value, the better the insulation. This reference page covers the target U-values required by Part L of the UK Building Regulations, the thermal conductivity of common insulation materials, and the thicknesses needed to achieve compliance.
Part L Target U-Values (England, 2025 Update)
| Building Element | New Build (W/m²K) | Extension (W/m²K) | Existing Building (W/m²K) |
|---|---|---|---|
| External wall | 0.18 | 0.18 | 0.30 |
| Party wall (new) | 0.00 (fully filled) | — | — |
| Floor (ground floor) | 0.13 | 0.18 | 0.25 |
| Pitched roof (insulation at ceiling) | 0.11 | 0.13 | 0.16 |
| Pitched roof (insulation at rafter) | 0.13 | 0.18 | 0.18 |
| Flat roof | 0.13 | 0.18 | 0.18 |
| Windows and roof windows | 1.2 | 1.4 | 1.4 |
| External doors | 1.0 | 1.4 | 1.4 |
Values shown are the maximum elemental U-values for Approved Document L (England). Scotland, Wales, and Northern Ireland have similar but slightly different requirements.
Insulation Material Conductivities
| Material | Lambda (λ) W/mK | Notes |
|---|---|---|
| PIR board (Celotex, Kingspan) | 0.022 – 0.023 | Best performance per mm thickness |
| Phenolic foam (Kooltherm) | 0.018 – 0.021 | Highest performance, premium cost |
| EPS (expanded polystyrene) | 0.030 – 0.038 | Good value, used in cavity walls and under floors |
| XPS (extruded polystyrene) | 0.029 – 0.036 | Moisture resistant, good compressive strength |
| Glass wool (Knauf, Isover) | 0.032 – 0.044 | Cheapest option, excellent acoustic performance |
| Rock wool (Rockwool) | 0.034 – 0.038 | Non-combustible, excellent fire performance |
| Spray foam (open cell) | 0.035 – 0.040 | Fills irregular cavities, requires specialist installer |
| Spray foam (closed cell) | 0.021 – 0.025 | Also acts as vapour barrier |
| Wood fibre board | 0.038 – 0.043 | Sustainable option, good breathability |
Required Thicknesses to Achieve 0.18 W/m²K (External Wall)
Approximate insulation thickness needed as part of a typical cavity wall build-up (brick outer leaf, cavity, block inner leaf) to achieve a U-value of 0.18 W/m²K:
| Insulation Material | Approximate Thickness | Cost per m² (approx.) |
|---|---|---|
| PIR board (0.022 W/mK) | 100 mm | £10 – £16 |
| Phenolic (0.020 W/mK) | 90 mm | £14 – £22 |
| EPS (0.032 W/mK) | 150 mm | £6 – £10 |
| Glass wool (0.035 W/mK) | 160 mm | £5 – £9 |
| Rock wool (0.036 W/mK) | 165 mm | £6 – £11 |
Thicknesses are approximate and depend on the full wall build-up. Always calculate the actual U-value using the complete construction details.
Calculate Your U-Values and Insulation Needs
- U-Value Calculator — calculate U-values for your wall, floor or roof build-up
- Insulation Thickness Calculator — determine the depth needed to meet Building Regulations
- Heat Loss Calculator — room-by-room heat loss based on U-values
- Cavity Wall Insulation Calculator — cavity wall insulation quantities and savings
- Loft Insulation Calculator — loft insulation rolls for your loft area
U-value targets reference Approved Document L of the Building Regulations for England (2021 update, effective June 2022). Scotland (Section 6), Wales (Part L), and Northern Ireland (Part F) have their own equivalent requirements. Always verify current requirements with your local building control body.