Building Standards require the insulation envelope of any heated building to be designed and constructed to limit heat loss through thermal bridging. Furthermore, Section 6 (Energy) and Part L1A of the Building Standards / Regulations also require thermal bridging to be taken into account as part of energy assessments. Whilst repeating thermal bridges are accounted for within U-value calculations, a separate psi-value calculation is required to assess non-repeating thermal bridges.
Thermal bridging accounts for 20-30% of total building heat losses, which is significant. As thermal modellers, we can work closely with clients and design teams to identify effective measures to reduce thermal bridging. This process also involves calculating the Temperature Factor for each junction, which helps to mitigate the risk of condensation and mould growth at the junction.
Developers currently have three options to account for thermal bridging within SAP Calculations:
1. Use the governments Accredited Construction Details
Currently the most common method of accounting for thermal bridging is to use the psi-values from government Accredited Construction Details; however, these psi-values are average and can be improved upon without difficulty. It is also worth noting that in order to use the ACDs, the proposed construction must be aligned to the key criteria listed for each individual junction.
2. Ignore thermal bridging and use ‘default’ values
Using default psi-values will result in poor thermal performance, making it difficult to comply with the Target Emission Rate without a significant reliance on renewable technology. Applying default psi-values will also significantly increase the energy demand for space heating within SAP making it almost impossible to achieve compliance with Aspect 2 under Section 7 (Sustainability).
3. Calculate bespoke psi-values based on the proposed construction detailing
If your proposed construction does not align to the Accredited Construction Details and using default psi-values is not viable (or desirable), then you must calculate bespoke psi-values. You may also choose to calculate bespoke psi-values if you repeatedly build using the same details and specification. Calculating bespoke psi-values will provide a more accurate energy assessment resulting in improved SAP and EPC ratings whilst also delivering significant savings on specification.
As accredited thermal modellers, we have worked closely with a variety of clients in order to improve thermal bridging on projects.
Architects and Designers
Clients who engage with us at concept design stage can also utilise our in-house thermally modelled construction detail packages which have been created to offer an alternative to Accredited Construction Details. The timber frame packs come in two grades and can provide up to a 50% improvement in thermal bridging performance on a typical dwelling (when compared against Accredited Construction Details). This service is particularly beneficial to architects and designers who are keen to achieve an enhanced ‘fabric first’ solution through improved thermal bridging and U-value performance.
Contractors and Housebuilders
We have supported many leading building contractors by producing bespoke psi-values aligned to their preferred methods of construction. This process involves working closely with design and procurement teams to review various aspects of the building fabric, from material specification to U-value performance, resulting in a highly efficient and cost-effective fabric design.
Local Authorities and Housing Associations
Compliance with Section 7 (Sustainability) is often a funding requirement of Scottish Government and Local Authorities for affordable housing. Bespoke psi-value calculations help to improve the Dwelling Emissions Rate (DER), whilst significantly reducing the energy demand for space heating, making compliance with Aspect 1 & Aspect 2 easier to achieve.