Guide
Meet building envelope thermal performance requirements
How to comply with Part L thermal performance requirements for walls, roofs, floors, and windows. Covers U-value targets for domestic and non-domestic buildings, when renovation triggers insulation upgrade under Regulation 23, thermal bridging, air tightness testing, and the Future Homes and Buildings Standards.
When thermal performance requirements apply
If you are constructing a new building, extending an existing one, or carrying out renovation work that affects your building's walls, roof, or floors, you must meet the thermal performance standards set out in Part L of the Building Regulations 2010. These standards exist to limit heat loss through the building envelope, reducing energy consumption and carbon emissions.
Part L applies to both domestic and non-domestic buildings in England. It sets maximum U-values (a measure of heat loss through a building element - the lower the U-value, the better the insulation) for each part of the building envelope: walls, roofs, floors, windows, and doors. Your building must also meet overall energy performance targets for carbon dioxide emissions and primary energy consumption.
These requirements apply whether you own or lease your premises. If you are a tenant carrying out fit-out work that affects thermal elements, you need to comply with Part L for those elements.
U-value targets: domestic buildings
The 2021 edition of Approved Document L Volume 1 sets the current U-value limits for new dwellings. These apply to new-build houses, flats, and conversions creating new dwellings. Extensions must meet the same fabric standards as new builds.
U-value targets: non-domestic buildings
Approved Document L Volume 2 sets separate U-value limits for non-domestic buildings such as offices, retail premises, warehouses, and industrial units. Non-domestic buildings also have specific requirements for building services (heating, cooling, lighting) that interact with the fabric performance.
When renovation triggers an insulation upgrade
You do not only face thermal performance obligations when constructing a new building. Under Regulation 23 of the Building Regulations 2010, renovating a thermal element beyond certain thresholds triggers a legal obligation to upgrade the insulation of that element to current Part L standards. This catches many routine maintenance and refurbishment projects that building owners may not initially consider as "building work" requiring compliance.
The trigger depends on how much of the element's surface area you are renovating. The following snippet sets out the precise thresholds and U-value targets.
Thermal bridging at junctions
A thermal bridge is a localised area of the building envelope where heat flows at a higher rate than the surrounding construction - typically at junctions between building elements such as wall-to-floor, wall-to-roof, and around window openings. Thermal bridges can account for up to 30% of a building's total heat loss if not properly addressed.
Part L requires you to account for thermal bridging in your design. You can demonstrate compliance by either:
- Using Approved Document L Appendix R default values - these assume a standard level of thermal bridging at each junction type. They are conservative, so your building may perform better than these defaults suggest.
- Calculating junction-specific psi-values - a thermal modelling specialist calculates the actual heat loss at each junction, giving you credit for better detailing. This approach can make it easier to meet overall energy targets.
Accredited Construction Details (ACDs) provide standard junction details that, when followed correctly, achieve acceptable thermal bridging performance without bespoke calculation.
Air tightness testing
Reducing uncontrolled air leakage is as important as insulation for thermal performance. Part L requires mandatory air permeability testing for all new dwellings and non-domestic buildings. The design target is 8 m3/(h.m2) at 50 Pa, but lower (better) values may be needed to meet your overall energy performance targets.
Air tightness testing must be carried out by a tester registered with a UKAS-accredited body such as ATTMA (Air Tightness Testing and Measurement Association). The test is conducted with a blower door that pressurises or depressurises the building to measure how quickly air leaks through the fabric.
Common areas of air leakage include service penetrations, poorly sealed window and door frames, junctions between walls and floors, loft hatches, and recessed downlighters. Address these during construction rather than attempting to seal them after completion.
Future Homes Standard and Future Buildings Standard
The government has consulted on significantly more stringent thermal performance requirements through the Future Homes Standard (for new dwellings) and the Future Buildings Standard (for new non-domestic buildings). These standards represent a step change in fabric performance and heating system requirements.
If you are planning a development, consider these forthcoming standards carefully. Transitional arrangements typically allow developments with building control approval before the implementation date to use current Part L 2021 standards, but new applications will need to meet the higher requirements.
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1. Identify which U-value standards apply to your project
Determine whether your building work involves new construction, an extension, or renovation of existing thermal elements. Check whether Regulation 23 thermal element upgrade triggers apply. Use the domestic or non-domestic U-value table as appropriate.
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2. Commission a thermal performance assessment
Engage a competent building designer or energy assessor to calculate target U-values, thermal bridging psi-values, and overall energy performance. For new builds, this forms part of the SAP (dwellings) or SBEM (non-domestic) calculation.
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3. Notify building control before work starts
Submit a building control application to your local authority or an approved inspector before starting work. This applies to new construction, extensions, and renovation work that triggers Regulation 23 insulation upgrades.
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4. Arrange air tightness testing
For new buildings, book an air tightness test with a UKAS-accredited tester. The test must be carried out before the building is occupied. Address any air leakage paths identified during construction.
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5. Obtain completion certificates and retain records
Once work is complete and passes building control inspection, obtain your completion certificate. Retain this certificate, air tightness test results, SAP or SBEM calculations, and any thermal element upgrade documentation. These are needed when selling or letting the property.