Targeting tougher U-values under new Part L

What are the technical implications of the changes to Approved Document L for building fabric and energy efficiency?


  • Craig Newbound

06 June 2023

Solar panels on house roof

The Part L uplift now in force in England means that all new domestic buildings have to reduce operational carbon dioxide emissions by 30% and commercial buildings by 27%.

The tougher energy efficiency requirements will support the transition towards The Future Homes Standard, due to be introduced in 2025. Along with The Future Buildings Standard for non-domestic properties, this was proposed as part of the government's two-part consultation on stringent changes to the Building Regulations.

The changes are designed to guarantee that all new homes ultimately generate only 20–25% of the carbon emissions associated with housing built under the previous regulations with the aim of eventually achieving an operational and embodied reduction of 75–80%.

However, those regulations will still apply to any project for which an initial notice, full plans or building notice was submitted before 15 June last year, provided a meaningful start is made on every plot before 15 June this year, such as putting drainage or foundations in place.

Projects registered after 15 June last year or on which work has not commenced before the June 2023 deadline, irrespective of the notification date, will need to comply with the new standards under the revised Part L.

Metrics introduced to assess energy usage

The recent revisions focus on reducing the passage of air through building fabric – referred to as infiltration and exfiltration – while improving the thermal resistance of the materials selected.

They also require more energy-efficient building services, including reduced flow temperatures for heating systems, lower-energy lighting and heat recovery from shower wastewater.

Photovoltaics (PV) should be installed as well, in proportion to the gross ground area of the building. At the most basic level, the larger the house then the more PV panels will likely be necessary to pass the assessment.

All new dwellings will be assessed under Standard Assessment Procedure (SAP) 10.2. This will be used to compute the target emissions rate (TER) along with the design-stage predicted energy assessment (PEA), a rating that tells building control surveyors that a new dwelling can attain the necessary standards. It will also inform a new performance metric, the target primary energy rate (TPER) as part of the Building Regulations England Part L (BREL) compliance report.

Primary energy is defined in a BRE briefing note as 'energy from renewable and non-renewable sources [that] has not undergone any conversion or transformation process'. TPER, which is measured in kilowatt–hours of primary energy per square metre, has been established to take better account of the total annual energy consumption.

Both the energy used by the dwelling and the initial input to generate that energy are combined to calculate the TPER using SAP 10.2, or the annual limit of primary energy used for a dwelling.

To narrow the gap between the modelled energy use and the completed works, both the Building Regulations England part L report (BREL) and similar reports in the other home nations – and the Building Regulations UK Part L (BRUKL), which apply to buildings other than dwellings – require a body of geolocated or date-stamped photographic evidence to prove compliance. Any plans, air permeability reports, U-value certificates for windows and doors and other performance-related documentation will all need to be included as well.

Related article

What the Part L changes mean

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New U-values encourage fabric-first approach

The revised Part L also sets new limiting U-values for building fabric. Measured in watts per square metre–kelvin, U-value represents the potential for heat to move from warm zones through the fabric to cooler areas, either within the dwelling or outside.

It stands to reason that the better insulated a building element such as wall, floor, door, roof or window, the lower the U-value will be and the greater that element's thermal resistivity. If less heat escapes through the fabric, then the potential space heating requirement of a given building will be lowered, reducing the associated carbon dioxide emissions.

Those undertaking new work to existing dwellings will find that the tougher U-values required under the new Part L ought to encourage designing and building in line with the notional specifications.

These specifications are based on a theoretical structure that is to all intents and purposes an identical copy of the proposed works, and the SAP performance data of the actual and notional dwellings will be compared at both the design and end stages of the build.

Embracing the notional specifications and designing using a fabric-first approach – that is, trying to optimise the building envelope's thermal performance long before considering the building services – should result in a compliant structure, given that the new Part L already takes account of renewable energy generation on site. This will also discourage the practice of offsetting poor U-values by installing features such as photovoltaic panels.

Finally, it is worth noting that poor construction detailing while working to the limiting U-values would make compliance with the Building Regulations highly unlikely, given that those in the revised Part L are considered the minimum acceptable performance.


Craig Newbound is an assistant building control surveyor at Assent Building Control
Contact Craig: Email

Related competencies include: Legal/regulatory compliance

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