This page brings together some of the main limiting U-values used in England under the 2013 (previous) and 2021 (current) editions of Part L, covering both dwellings and buildings other than dwellings.
A U-value tells us how easily heat passes through part of the building fabric. It is measured in W/m²K. The lower the U-value, the less heat is transferred through the element.
The figures below are limiting values under Part L. They should not automatically be treated as recommended design U-values. For historic NCM based u-values click here.
New dwellings: 2013 vs 2021
| Element | Part L 2013 | Part L 2021 |
|---|---|---|
| Roof | 0.20 | 0.16 |
| Wall | 0.30 | 0.26 |
| Floor | 0.25 | 0.18 |
| Party wall | 0.20 | 0.20 |
| Swimming pool basin | 0.25 | 0.25 |
| Windows | 2.00 | 1.60 |
| Rooflights | 2.00* | 2.20 |
| Doors | 2.00* | 1.60 |
U-values are in W/m²K.
In the 2013 guidance, windows, roof windows, glazed rooflights, curtain walling and pedestrian doors were grouped together at 2.00 W/m²K.
The limiting air permeability for new dwellings also reduced from 10.0 m³/(h·m²) @ 50 Pa under the 2013 guidance to 8.0 m³/(h·m²) @ 50 Pa under the 2021 guidance.
New non-domestic buildings: 2013 vs 2021
| Element | Part L 2013 | Part L 2021 |
|---|---|---|
| Flat roof | 0.25 | 0.18 |
| Pitched roof | 0.25 | 0.16 |
| Wall | 0.35 | 0.26 |
| Floor | 0.25 | 0.18 |
| Swimming pool basin | 0.25 | 0.25 |
| Windows / curtain walling | 2.20 | 1.60 |
| Rooflights | 2.20 | 2.20 |
| Pedestrian doors | 2.20 | 1.60 |
| Vehicle access doors | 1.50 | 1.30 |
| High-usage entrance doors | 3.50 | 3.00 |
| Roof ventilators / smoke vents | 3.50 | 3.00 |
U-values are in W/m²K.
The limiting air permeability for new non-domestic buildings reduced from 10.0 m³/(h·m²) @ 50 Pa under the 2013 guidance to 8.0 m³/(h·m²) @ 50 Pa under the 2021 guidance.
What about existing buildings?
Existing buildings need to be treated slightly differently.
Part L distinguishes between new or replacement thermal elements, renovated thermal elements and retained thermal elements.
For example, under the 2013 guidance for existing dwellings, new thermal elements had the following maximum U-values:
| Element | Maximum U-Value |
|---|---|
| Wall | 0.28 |
| Pitched roof, insulation at ceiling level | 0.16 |
| Pitched roof, insulation at rafter level | 0.18 |
| Flat roof | 0.18 |
| Floor | 0.22 |
| Swimming pool basin | 0.25 |
U-values are in W/m²K.
For retained thermal elements, Part L instead provides a threshold U-value and an improved U-value.
| Element | Threshold | Improved |
|---|---|---|
| Wall, cavity insulation | 0.70 | 0.55 |
| Wall, internal or external insulation | 0.70 | 0.30 |
| Floor | 0.70 | 0.25 |
| Pitched roof, insulation at ceiling level | 0.35 | 0.16 |
| Pitched roof, insulation at rafter level | 0.35 | 0.18 |
| Flat roof | 0.35 | 0.18 |
U-values are in W/m²K.
The same broad distinction between new, replacement, renovated and retained elements continues in the 2021 Approved Documents.
Limiting U-value does not mean design U-value
This is an important distinction.
If Part L gives a limiting wall U-value of 0.26 W/m²K, it does not mean that 0.26 W/m²K is necessarily the U-value that should be used when designing the building.
The limiting value is essentially a backstop (i.e. worst U-value you are allowed to use for an individual building element).
For new buildings, individual elements may need to perform significantly better than the limiting values for the building to meet its overall Part L energy performance targets.
The appropriate design U-value therefore depends on the building as a whole and the relevant Part L calculation.
A note on calculating U-values
It is also important to check how the U-value has been calculated.
The 2013 Part L guidance refers to the calculation conventions in BR 443.
For windows and doors, the U-value generally refers to the whole unit, including the frame, rather than simply the centre pane of glass.
Rooflight U-values also need to be interpreted carefully because the value can depend on the test position and the slope at which the rooflight is installed.
When comparing a specification against a Part L table, make sure you are comparing the same type of U-value.
Sources
- UK Government, Approved Document L1A: Conservation of fuel and power in new dwellings, 2013 edition incorporating 2016 amendments.
- UK Government, Approved Document L1B: Conservation of fuel and power in existing dwellings.
- UK Government, Approved Document L2A: Conservation of fuel and power in new buildings other than dwellings, 2013 edition incorporating 2016 amendments.
- UK Government, Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments.
- UK Government, Approved Document L, Volume 2: Buildings other than dwellings, 2021 edition incorporating 2023 amendments.
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