When it comes to energy efficiency, U-values play a crucial role in Part L compliance. Part L sets specific targets for U-values, which are the maximum allowed heat transfer rates for building elements like walls, roofs, windows, and doors.
These targets ensure that these components meet minimum insulation standards, reducing heat loss or gain.
We've collated the latest recommended U-Values straight from Appendix B of Approved Document L Volume 1 (with 2023 amendments)
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- Summary of notional dwelling specification for new dwelling
- U-values for party walls
- Limiting U-values for new fabric elements and air permeability in new dwellings
- Limiting U-values for new fabric elements in existing dwellings
- Limiting U-values for existing elements in existing dwellings
Part L Building Regulations U Values
Disclaimer: These U-values provide general guidance, but it's important to refer to the most recent version of Part L and any associated documents, as well as consult with building professionals, architects, or local building control authorities to ensure compliance with specific project requirements.
Summary of notional dwelling specification for new dwelling:
Element or system |
Reference value for target setting |
|
Opening areas (windows, roof windows, rooflights and doors) |
Same as for actual dwelling not exceeding a total area of openings of 25% of total floor area |
|
External walls including semi-exposed walls |
U = 0.18 W/(m²·K) |
|
Party walls |
U = 0 |
|
Floors |
U = 0.13 W/(m²·K) |
|
Roofs |
U = 0.11 W/(m²·K) |
|
Opaque door (less than 30% glazed area) |
U = 1.0 W/(m²·K) |
|
Semi-glazed door (30–60% glazed area) |
U = 1.0 W/(m²·K) |
|
Windows and glazed doors with greater than 60% glazed area |
U = 1.2 W/(m²·K) Frame factor = 0.7 |
|
Roof windows |
U = 1.2 W/(m²·K), when in vertical position |
|
Rooflights |
U = 1.7 W/(m2·K), when in horizontal position (for correction due to angle, see specification in SAP 10 Appendix R) |
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U-values for party walls:
|
Party wall construction |
U-value W/(m2·K) |
|
Solid |
0.0 |
|
Unfilled cavity with no effective edge sealing |
0.5 |
|
Unfilled cavity with effective sealing around all exposed edges and in alignment with insulation layers in abutting elements |
0.2 |
|
A fully filled cavity with effective sealing at all exposed edges and in alignment with insulation layers in abutting elements |
0.0 |
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Limiting U-values for new fabric elements and air permeability in new dwellings:
Element type |
Maximum U-value W/(m2·K) |
|
All roof types |
0.16 |
|
Wall |
0.26 |
|
Floor |
0.18 |
|
Party wall |
0.20 |
|
Swimming pool basin |
0.25 |
|
Window |
1.6 |
|
Rooflight |
2.2 |
|
Doors (including glazed doors) |
1.6 |
|
Air permeability |
8.0m3/(h·m2) @ 50Pa 1.57m3/(h·m2) @ 4Pa |
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Limiting U-values for new fabric elements in existing dwellings:
Element type |
Maximum U-value W/(m2·K) |
|
Roof |
0.15 |
|
Wall |
0.18 |
|
Floor |
0.18 |
|
Swimming pool basin |
0.25 |
|
Window |
1.4 or Window Energy Rating Band B minimum |
|
Rooflight |
2.2 |
|
Doors with >60% of internal face glazed |
1.4 or Doorset Energy Rating Band C minimum |
|
Other doors |
1.4 or Doorset Energy Rating Band B minimum |
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Limiting U-values for existing elements in existing dwellings:
|
Element |
U-value W/(m2·K) |
|
|
|
(a) Threshold |
(b) Improved |
|
Roof |
0.35 |
0.16 |
|
Wall–cavity insulation |
0.70 |
0.55 |
|
Wall – internal or external insulation |
0.70 |
0.30 |
|
Floor |
0.70 |
0.25 |