Building Regulation changes: Using lintels to reduce thermal bridging heat losses and meet Part L 2021

With the publication of the technical detail for Part L 2021, we have seen the first step towards the much-heralded Future Homes Standard being adopted as part of the Building Regulations in England. The government consulted on changes to Part L (conservation of fuel and power) and Part F (ventilation) for new dwellings and has now finalised the proposals based on the responses received.

High standards of building fabric performance are required, including reducing thermal bridging heat losses. The good news is that solutions to achieve that, such as thermally broken cavity wall lintels, already exist. Rather than waiting until 2022, these lintel solutions can be used now to help achieve compliance with current as well as future regulations and make an immediate difference to the carbon emissions from dwellings.

This post describes the introduction of Part L 2021 and the Future Homes Standard, which is specific to the Building Regulations in England. While different updates will be made in the other countries of the UK, the principles of good building fabric and junction detailing will be no less relevant.

The Covid-19 pandemic has meant a delay to the planned updating of the Building Regulations in England. In confirming the new requirements for new-build dwellings, the government also published the timetable for introducing the Future Homes Standard.

At the time of writing, a consultation is ongoing for the ‘uplift’ to requirements for existing dwellings and non-domestic buildings, as well as regulations relating to overheating. The government will confirm its response to that consultation during 2021, with Part L 2021 (covering all building types) coming into force in June 2022.

Establishing these interim ‘uplift’ regulations is considered essential to give supply chains within the construction industry time to mature. The Future Homes Standard will be implemented in 2025, and it is necessary to ensure that there is enough expertise to deliver the low carbon heating systems that will replace traditional gas boilers.

As has been the case since 2006, new dwellings will be assessed for compliance with Part L using the Standard Assessment Procedure (SAP). A new version of SAP - SAP 10 - has been developed alongside the new proposals. It will be updated further in response to the consultations, with the intention that SAP 10.3 is implemented at the same time as Part L 2021.

In SAP, the performance of the proposed dwelling is compared to the performance of a theoretical ‘notional’ building. For Part L 2021, the proposed dwelling must better the notional dwelling in the following metrics.

• Primary energy (which measures the regulated energy use of the dwelling, taking into account decarbonisation of the grid).
• CO₂ emissions (an existing metric).
• The fabric energy efficiency standard (FEES, and an existing metric).

The use of SAP allows flexibility in the building fabric specification, although minimum standards for fabric and fixed building services must continue to be met. If the designer chooses, however, they can copy the notional specification like-for-like in the knowledge that the dwelling will comply. In Part L 2021, the notional dwelling specification uses the following U-values.

• External walls - 0.18 W/m²K
• Floors - 0.13 W/m²K
• Roofs - 0.11 W/m²K
• Doors - 1.0 W/m²K
• Windows and rooflights - 1.2 W/m²K

These values do not represent a significant change over the values in Part L 2013, meaning the 31% overall reduction in carbon emissions will come from other parts of the specification, such as improvements in airtightness.

It will also come from reductions in thermal bridging heat losses

Thermal bridging heat losses have been a feature of Part L compliance since SAP was introduced. As building fabric U-values have improved over time, thermal bridging has become responsible for an increasing proportion of total heat loss. The treatment of junction details - such as around door and window openings, or at floor/wall and wall/roof junctions - has not typically kept pace.

Some projects still choose not to properly account for the heat loss through these linear thermal bridges. Rather than inputting specific psi values (measures of thermal bridging heat loss) into SAP, default values are used. Default values penalise the performance of the proposed dwelling and require it to compensate elsewhere to achieve compliance.

Not only that, but when poorly performing thermal bridges are constructed on site, they cause issues for the occupants of the finished building. The rates of heat loss at thermal bridges compared to the surrounding fabric cause discomfort. And, if a thermal bridge gets cold enough, condensation can form on the surface and eventually lead to mould growth.

The option to use default values, and subsequent penalty to performance, remains in Part L 2021. With the tighter overall standards, however, it will be even harder to achieve compliance through compensating elsewhere.

The message from the publication of the finalised Part L 2021 proposals is clear: it will be even harder to justify not inputting accurate psi values into SAP.

Even for designers who do use psi values, some things are changing. Accredited construction details (ACDs) have long been available as a source of typical junction constructions, with accompanying calculated psi values. Those values are no longer seen as being sufficiently high performance for the new requirements, however, meaning ACDs are being removed as an option.

Psi values for compliance calculations will now have to come from bespoke thermal bridging models or other libraries of details. One option is to use values supplied by construction product manufacturers to demonstrate the performance of their solutions.

For example, one-piece lintels are the preferred solution for structural support over openings in masonry cavity walls. Different designs of lintel offer different levels of performance. Even in those lintels that feature integral insulation, if the steel body spans the complete width of the wall, then it is providing a path for heat loss through the wall’s insulation layer

Thermally broken lintels feature a design where the steel body is not continuous, and the amount of heat energy conducted is significantly lower. The thermal bridging heat loss through the door or window head detail featuring the lintel is significantly reduced.

Catnic’s Thermally Broken Lintel (TBL) offers low psi values of between 0.02 and 0.05 W/mK, ensuring that door and window head details make the best possible contribution to building fabric heat loss calculations.

The default Psi value for an open back cavity lintel with base plate is 0.50 W/mK. By guaranteeing a psi value of 0.05 W/mK or less, the Catnic TBL allows no more than one tenth of the heat loss through the junction compared to a traditional one-piece cavity lintel.

Thermal bridging heat losses have been part of Building Regulation compliance for some fifteen years now. Part L 2021 is making them even harder to ignore. Choosing not to deal with psi values is no longer an option, and it is possible to make a significant difference to building fabric performance through simple product specification choices - whether working to Part L 2013 or Part L 2021.

To find out more about the difference that the Catnic TBL can make to both current and future building fabric specifications, download our white paper. For all types of wall construction, the Catnic product selector can help you to choose the best lintel for your design. Alternatively, download technical literature or contact us to find out more about Catnic can help meet your project requirements.