Jon Ducker from Kingspan Insulation explains the nuts and bolts of the Part L update – and why designers now need to pay even closer attention to the details
In June, England introduced its first update to the energy efficiency requirements in the Building Regulations for eight years. These changes – contained in Part L of the Building Regulations – mean that new buildings will need to be more energy efficient and have lower carbon emissions. Updates are also due to be introduced for domestic properties in Wales on 23 November with further changes expected for non-domestic properties in Wales, and all properties in Scotland, over the coming months. In each case, these should include a much greater focus on designing details for insulation measures to prevent heat loss from thermal bridging. To explore what this will mean for architects and specifiers, let’s take a look at the requirements for new homes in England.
Key changes
Part L 2021 is supported by Approved Documents which provide technical advice on how to achieve compliance with the Building Regulations. The requirements for domestic properties are contained within Approved Document L Volume 1 (ADL1 2021). The changes, which came into force on 15 June, are designed to act as a stepping-stone to the Future Homes Standard due in 2025 where all new homes should utilise low-carbon heating technologies such as heat pumps and be ‘net-zero ready.’
ADL1 2021 has four key performance metrics for new homes:
- Carbon emissions 31% lower than previously required
- ‘Primary energy’ which limits the total energy used by a dwelling, and considers the energy used to produce and transport fuel to it
- Fabric Energy Efficiency Standard (FEES) – only related to the thermal performance, thermal bridging and airtightness of the property
- ‘Worst-case U-values’ – which sets hard limits on insulation performance.
To calculate the targets for the first three of these metrics, SAP assessors carry out modelling on a proposed dwelling, comparing it with targets set by a notional dwelling of the same dimensions using a set ‘notional’ specification (shown below). Critically, while previously the FEES target was relaxed by 15%, this is not the case in ADL1 2021. This makes it much harder to relax any of the U-values from the notional dwelling specification, without significantly upgrading other aspects of the building envelope such as increasing airtightness and adding mechanical ventilation.
These more exacting U-values mean the thermal conductivity (lambda value) of the insulation material takes on greater importance. The lower the thermal conductivity, the more effective the product is at preventing heat loss. This means a slimmer thickness can often be used to achieve the desired U-value. Rigid insulation materials such as phenolic boards can achieve thermal conductivities as low as 0.018 W/mK, making it possible to reduce the depth of constructions, and so realise more internal space, without compromising on thermal performance.
Thermal bridges
In addition to setting tougher FEES targets, ADL1 2021 also makes key changes to how heat losses from thermal bridges are considered. Thermal bridges are areas where materials which are more conductive to heat than the insulation layer form a path between the inside of the property and the outside. These commonly occur in areas such as around windows and doors and at junctions between building elements. These paths can form a fast-track for heat to escape and have been shown to account for as much as 30% of total losses.
To improve practice in this area, the Accredited Construction Details (ACDs) have been removed as it was felt they had become outdated. The default heat loss values given where no detail is supplied (either for the junction or globally for the building) have also been tightened. This makes it much more difficult to reach compliance with the FEES target without either having bespoke details calculated, or making use of details from manufacturers or industry libraries.
Compliance reports
In addition to encouraging the adoption of bespoke details, the revised regulations also include new BREL Compliance Reports with similar BRWL to be introduced in Wales. Project teams need to provide both a design stage and ‘as-built’ report.
The BREL design stage report requires key aspects of the property SAP model to be documented and signed off by an energy assessor. This includes the details and psi-values for key junctions within the building such as sills, jambs and eaves. The as-built report should detail any changes from the original specification or details and be supported with photographs of the insulation measures taken by the installers before junctions are covered, to prove that the materials fitted match the specification and have been installed correctly. The finished report will be submitted to Building Control, and deviations could lead to a building being found to be non-compliant, resulting in costly remedial work.
This is a key change for architects, as it can help to strengthen initial specifications. Factors such as the insulation product’s thermal conductivity and emissivity are incorporated into the Psi-value calculation where bespoke details are used. As a result, the value is non-transferable to a different material. This means that if a different material is going to be installed, the detail would need to be reworked and the SAP model recalculated, making it a much more involved process.
Closing the gap
At a time when heating costs are rapidly rising for everyone, the updates to the Building Regulations are an opportunity to ensure new homes deliver excellent energy performance. By paying close attention to details, architects can strengthen specifications and close the performance gap, providing the best results for occupiers and the environment.
Jon Ducker is head of regulatory affairs at Kingspan Insulation GB