Building Regulations U-Values: Insulation Materials and Thicknesses
The UK Building Regulations set target U-values for walls, roofs, floors, windows and doors under Part L (Conservation of Fuel and Power). A U-value measures how much heat passes through a building element, in watts per square metre per degree Kelvin (W/m²K): the lower the number, the better the insulation. This page covers the current Building Regulations U-value targets for new builds, extensions and existing homes, the thermal conductivity of common insulation materials, and the thicknesses needed to meet them.
Building Regulations U-Value Targets (England, Part L 2021 Edition, as amended 2023)
| Building Element | New Build: Notional Target (W/m²K) | Extension (W/m²K) | Existing Building: Improved Target (W/m²K) |
|---|---|---|---|
| External wall | 0.18 | 0.18 | 0.55 cavity fill, or 0.30 internal/external insulation |
| Party wall (new) | 0.00 (fully filled) | — | — |
| Floor (ground floor) | 0.13 | 0.18 | 0.25 |
| Roof (all types, pitched or flat) | 0.11 | 0.15 | 0.16 |
| Windows and roof windows | 1.2 | 1.4 | 1.4* |
| External doors | 1.0 | 1.4 | 1.4* |
New Build figures are the notional dwelling targets used in the SAP calculation (Table 1.1). Every element must separately meet a higher, worst-case limiting value regardless of how well other elements perform, for example wall 0.26 W/m²K and roof 0.16 W/m²K: see the full Part L U-value targets referencefor the notional and limiting figures side by side. Existing Building figures apply only where the retained element currently performs worse than the Part L threshold (0.70 W/m²K for walls and floors, 0.35 W/m²K for roofs) and the upgrade is achievable within a simple payback of 15 years or less. The wall figure depends on the upgrade method: 0.55 W/m²K where an existing cavity is filled, or 0.30 W/m²K where internal or external insulation is added. *Table 4.3 has no window or door row: replacement windows and doors in an existing dwelling are new elements and must meet the Table 4.2 extension figure of 1.4 W/m²K. Scotland, Wales, and Northern Ireland have similar but slightly different requirements.
Correction
July 2026: The roof row on this table previously split into three sub-rows (pitched ceiling, pitched rafter, flat) with different values per build stage. Approved Document L 2021 (as amended 2023) gives a single roof figure per table with no pitched/flat split; the three sub-rows have been merged into one and the incorrect values corrected: new build 0.11 W/m²K (was wrongly 0.18 on the flat-roof row), extension 0.15 W/m²K (was wrongly 0.16/0.18 on two rows), existing building 0.16 W/m²K (was wrongly 0.18 on two rows). The existing-building external wall figure was also corrected from a single 0.30 W/m²K to the two figures Table 4.3 actually gives depending on upgrade method: 0.55 W/m²K by cavity fill, 0.30 W/m²K by internal or external insulation. Re-verified against the gov.uk Approved Document L Volume 1 PDF and against the already-corrected figures on /reference/part-l-u-values/.
Wall and Roof U-Value: Quick Answers
What wall U-value does Building Regulations require?
For a new-build dwelling, the notional target is 0.18 W/m²K and every wall must also meet a limiting, worst-case value of 0.26 W/m²K. A new wall in an extension must achieve 0.18 W/m²K. For an existing wall being upgraded, the target is 0.55 W/m²K if you are filling an existing cavity, or 0.30 W/m²K if you are adding internal or external insulation. See the thickness table below for how many millimetres of common insulation materials get a typical cavity wall to 0.18 W/m²K.
What roof U-value do Building Regs require?
Since the 2021 update, Approved Document L sets one roof figure covering pitched and flat roofs alike. There is no separate rafter or ceiling figure. The notional target for a new dwelling is 0.11 W/m²K, with a limiting value of 0.16 W/m²K. A new roof on an extension must achieve 0.15 W/m²K. An existing roof being upgraded must reach 0.16 W/m²K if it currently performs worse than the 0.35 W/m²K threshold.
Are Building Regulations U-values the same as Part L?
Yes. “Building Regulations U-values” and “Part L U-values” refer to the same targets: Part L (Conservation of Fuel and Power) is the section of the Building Regulations that sets them, in Approved Document L, Volume 1: Dwellings. This page focuses on the insulation materials and thicknesses that get you there; see the Part L U-value targets reference for the full notional and limiting figures in one table.
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 (mm) | Cost per m² (approx.) |
|---|---|---|
| PIR board (0.022 W/mK) | 100 | £10 – £16 |
| Phenolic (0.020 W/mK) | 90 | £14 – £22 |
| EPS (0.032 W/mK) | 150 | £6 – £10 |
| Glass wool (0.035 W/mK) | 160 | £5 – £9 |
| Rock wool (0.036 W/mK) | 165 | £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
- How many rolls of loft insulation will I need? — worked 48m² example at 270mm
- Part L U-Value Targets Reference: the full notional and limiting figures for every element in one table
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.