Thermal Bridge Calculator — Psi Values & y-Factor

A thermal bridge (cold bridge) is a localised area of the building envelope where heat flows at a higher rate than the surrounding construction. Common examples include steel lintels, window reveals, balcony connections, and wall-to-floor junctions. Thermal bridges can account for 20-30% of total fabric heat loss in well-insulated buildings.

A psi-value (ψ-value) measures the additional heat loss per metre length of a linear thermal bridge, expressed in W/mK. A lower psi-value means less heat loss. Typical values range from around 0.04 W/mK for a well-detailed window sill to 0.50 W/mK for an uninsulated balcony or a standard open-back cavity steel lintel. For a Part L 2021 / SAP 10 assessment, psi-values should come from project-specific thermal modelling (BS EN ISO 10211) or from a BRE-certified manufacturer detail — the indicative figures in this tool are for early estimating only.

The y-factor represents the overall thermal bridging in a building as a proportion of the total envelope area, in W/m²K. Under SAP 10.2 (Approved Document L 2021, Appendix K) the default y-factor used when no junction details are calculated is 0.20 W/m²K — it was raised from the old SAP 2012 figure of 0.15 to penalise the no-calculation route and encourage assessors to model real junctions. If you demonstrate better detailing through calculated psi-values, you can achieve a much lower y-factor (typically 0.04–0.08), improving the SAP rating.

Key strategies include: using thermally-broken (insulated) lintels instead of bare steel, insulating window reveals, fitting thermal breaks at balcony connections, keeping insulation continuous through junctions, using BRE-certified manufacturer details with low published psi-values, and avoiding metal fixings that bridge the insulation. Passivhaus construction aims for virtually zero thermal bridging through careful detailing.

ACDs were a set of standardised government junction details with pre-calculated psi-values that once let you claim a y-factor of around 0.05 in SAP without bespoke modelling. They were withdrawn under Approved Document L 2021 — judged out of date and incompatible with modern fabric specifications — so the 0.05 shortcut no longer exists and they are no longer offered for download. For a current Part L 2021 / SAP 10 assessment you now need project-specific psi-value calculations (BS EN ISO 10211) or details from an independently-assessed library such as a BRE-certified manufacturer detail.