PSI Value Calculations with AutoPSI

Not every project needs a bespoke finite-element model for every junction. In the UK, where thermal bridging feeds into SAP and Part L, teams often need reliable psi-values for a set of fairly standard details, quickly. AutoPSI is an online tool built for exactly that — a template-driven psi-value calculator aimed at SAP assessors, architects and retrofit coordinators.

What AutoPSI is

AutoPSI is a cloud-based thermal-modelling tool (developed by BRS Technology, launched in 2019) that calculates PSI values — linear thermal transmittance — for construction junctions. Instead of drawing every detail from scratch, it offers a library of editable, accredited junction templates: you pick a junction type and adjust the materials, insulation, bridging and frames to match your build-up via a drag-and-drop interface.

Standards and outputs

Calculations follow BS EN ISO 10211 and the UK BR 497 conventions, so the results are suitable for compliance work. For each junction AutoPSI returns:

• The psi-value (W/m·K) for use in the SAP thermal-bridging calculation;
• The temperature factor (fRsi), so you can check surface condensation and mould risk at the same time.

Why not just use default psi-values?

SAP lets you fall back on default or "accredited construction detail" psi-values, but those defaults are deliberately conservative — they assume worse performance than a well-detailed junction usually delivers. On a whole dwelling, thermal bridging can be a meaningful slice of heat loss, so relying on defaults can make a compliant design look like it fails, or force unnecessary fabric upgrades. Calculating custom psi-values for your actual details often unlocks the margin you need — and AutoPSI makes that fast enough to be worth doing.

Where AutoPSI fits alongside THERM

AutoPSI and full finite-element tools like THERM are complementary. For standard, template-friendly junctions on SAP projects, AutoPSI is quick and consistent. For unusual geometry, bespoke details, complex 3D effects or window install lines — see WINDOW 8 — a purpose-built THERM model gives you full control. Choosing the right tool for each junction is part of doing thermal bridging well.

The takeaway

AutoPSI lowers the barrier to proper psi-value calculation: instead of defaulting to pessimistic values, you can model your real junctions to ISO 10211 / BR 497 quickly and get both psi and fRsi. For SAP and Part L projects with mostly standard details, it is an efficient way to sharpen the numbers.

This article is general guidance and reflects information available at the time of writing. AutoPSI is a third-party tool; always follow the calculation standard and conventions required for your project. We are not affiliated with AutoPSI.