How to Best Use Readily Available Psi-Value Details

Part L 2021 allows the use of open thermal bridging (psi value) detail databases, resulting in effective cost savings. However, developers should use them diligently to avoid the risk of non-compliance. The following items can be easily checked to ensure appropriate use and quality of the psi-values:

• Values should be calculated following the requirements of BRE BR497 (the standard used for assessing psi-values for SAP).
• Like our assessors they should be certified by an accreditation body to conduct psi value calculations. There is however no mandatory qualification requirement in the UK yet.
• Associated reports should clearly show all information used in the calculations. This includes information on the model heat flow, construction U-values, list of boundary condition temperatures, among other technical information.
• The list of materials and associated thermal conductivities should be clearly shown in the report, including the respective reliable sources. Any assumptions should be appropriately stated.
• Details with light steel frame, rainscreen brackets and/or bolt-on balcony connectors must be carried out in 3D.
• Party wall to ground details (SAP ref. P1), party inverted ground floors (SAP ref. P6) and inverted ground floors (SAP ref. E19) should also be carried out in 3D.
• Psi-values should be carried out by qualified professionals. There should be available evidence that the case studies from BR497 have been completed successfully by the assessor, or evidence of appropriate training.

When using publicly available details for psi-values, these must be replicated exactly replicated within the design, and photographic evidence provided during construction. This is in opposition to the old ACD checklists, removed from Part L, which had only a couple of design checks to be valid. Discrepancies may risk Part L compliance during SAP audits.