The removal of accredited construction details from the new Part L building regulations 2021 in England places added emphasis on the role of PSI values in achieving compliance.
Standard Assessment Procedure (SAP) calculations have taken linear thermal bridging into account since 2006, in recognition of the fact that heat loss cannot be measured by U-values alone. Using calculated PSI values in SAP calculations has therefore been beneficial to Part L compliance for some time now.
Nevertheless, many people have chosen to use only default PSI values or ignore linear thermal bridging entirely. While previously, it has been possible to compensate elsewhere in the specification to still achieve PSI compliance, the higher standards required by Part L 2021 make that an even more difficult approach to take.
Read on to discover the role of PSI values in Part L building regulations.
Accredited construction details, or ACDs, were standard construction details demonstrating how continuity of insulation and airtightness could be achieved at different accredited construction junctions. Accredited details for insulation were grouped according to common construction types (e.g. masonry or timber framed construction).
Checklists aimed to give designers and installers the key points of attention that would help to ensure compliance and improve energy performance. By meeting all of the checklist items for a given junction in approved documents, the PSI value published in Appendix K of SAP could be used in the compliance calculations for the project.
With the introduction of Part L 2021, however, ACDs can no longer be used for heat loss calculations. For improved thermal performance and to meet energy efficiency requirements, accurate PSI values are required.
The UK government has set out a timetable for introducing overall energy efficiency standards (and accompanying ventilation and overheating standards). These are aimed at delivering building stock capable of achieving net zero carbon without the need for retrofitting. The Future Homes Standard and Future Buildings Standard are both scheduled for introduction in 2025.
Part L 2021 in England took effect in June 2022 and has been created as a stepping stone or ‘uplift’ to the 2025 standards of energy performance. It proposes a 31% reduction in carbon emissions for dwellings and a 27% reduction for buildings other than dwellings, compared to Part L 2013.
These new building regulations in the UK will require designers to make huge reductions in carbon emissions. This is to ensure both existing buildings and new buildings are energy efficient by cutting down on necessary fuel and power, whether in domestic or non-domestic buildings.
The uplift is intended to give the construction industry time to prepare for the intended 70 to 80% reduction in carbon emissions required in 2025, set by the Future Homes Standard timetable.
Since their introduction in 2002, ACDs have not been updated and are now generally seen as outdated methods for building regulation calculations. To reflect the higher standards of thermal performance needed in Part L 2021 and beyond, ACDs have therefore been removed.
If you have applied for building or initial notice before the 15th of June 2022, your project will still work under the old regulations as long as building work begins by the 15th of June 2023.
With the removal of ACDs, PSI values for junction details must come from other libraries of details or be calculated specifically for the project. Default values for junctions can still be used, but achieving the required compliance metrics will be significantly more difficult when doing so without bespoke PSI values.
To make getting project-specific PSI calculations easier, BRS Technology has developed an online modelling tool called AutoPSI. It uses a drag-and-drop interface to provide quick and easy PSI value calculations in accordance with relevant standards.
PSI values measure the overall performance of thermal elements with linear thermal heat transmittance. These heat values are measured in watts per Kelvin metre (W/mK), and they indicate how much heat is lost between two thermal elements in a building envelope. For example, this could be a thermal bridge between an external wall or party walls and the ground floor or window frames.
For Part L Building Regulations, U-values alone are not as accurate a portrayal of overall heat loss as PSI (ψ) values are. As such, builders cannot create sufficiently energy-efficient buildings with these values.
By taking into account different temperature factors, air leakage, and thermal bridging, PSI values can measure the actual performance of a building and give the construction designer or builder an idea of possible methods for reducing heat loss in windows, walls, or elsewhere. In turn, improving heat loss will improve energy efficiency.
This makes being compliant with Part L construction standards much easier, as thermal bridges are taken into consideration. BR497 also requires the use of 3D PSI values for high-rise construction sites.
While a U-value is beneficial to measuring heat loss in fabric thermal elements in metre kelvin, it cannot identify areas of non-repeating thermal bridges (cold bridges). Because of this, U-values are not as relevant as PSI values.
Without the use of thermal modelling in SAP assessments, your heat loss calculations may be inaccurate and result in poor insulation. Consequently, you will inhibit your building’s performance.
To find out how you can create customisable junction details using an AutoPSI PSI value calculation, request a demonstration.
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