Thermal Bridge Calculations in LBNL THERM 8

THERM is the free two-dimensional heat-transfer tool from Lawrence Berkeley National Laboratory (LBNL), and it is one of the most widely used programs for analysing thermal bridges. Version 8 uses a finite-element engine to model exactly how heat moves through a construction detail — a wall-to-floor junction, a lintel, a balcony — so you can quantify the extra heat loss and the condensation risk at that junction. This article walks through how it works.

What THERM 8 actually computes

THERM solves the temperature field across a 2D cross-section. From that solution it gives you three things that matter for thermal bridging:

• The heat flow through the detail, from which a thermal coupling coefficient is derived;
• The psi-value (ψ, linear thermal transmittance in W/m·K) — the extra heat loss along a junction beyond the plain wall areas;
• The surface temperatures, which give the temperature factor fRsi used to check condensation and mould risk.

The workflow, step by step

1. Build the geometry. Draw the cross-section with THERM's polygon tools, or import a scaled DXF from CAD (SketchUp, Rhino, AutoCAD) and trace it — far faster for complex details.
2. Assign materials. Give every polygon a conductivity from THERM's material library (or a custom value), including any air cavities modelled per the standards.
3. Set boundary conditions. Apply interior and exterior film coefficients and temperatures, and tag the U-factor surfaces so THERM knows where to measure heat flow.
4. Mesh and solve. THERM auto-meshes and runs the finite-element solution, refining until the energy-error estimate is within tolerance.
5. Read the results. View colour isotherms and heat-flux vectors, read the U-factor/heat flow, and export the numbers you need.

From heat flow to a psi-value

THERM gives you the 2D thermal coupling coefficient (often called L2D) for the modelled section. The psi-value is what remains after you subtract the one-dimensional heat loss of the plain elements meeting at the junction: ψ = L2D − Σ(U × length). Getting this right depends on measuring lengths and choosing internal or external dimensions consistently with the convention you are reporting to — which is why standards matter.

Condensation and the temperature factor

Heat loss is only half the story. THERM's surface temperatures let you calculate fRsi — the temperature factor at the coldest internal surface. If fRsi falls below the threshold for the building type and climate, the junction is at risk of surface condensation and mould, regardless of how good its psi-value is. THERM lets you catch that at the design stage.

Standards and conventions

Credible thermal-bridge modelling follows BS EN ISO 10211 for the calculation method and, in the UK, the BR 497 conventions for how junctions are defined and dimensioned. Two identical-looking THERM models can produce different psi-values if the conventions differ, so the discipline is as important as the software. THERM also underpins window work — see how it feeds WINDOW 8 for window U-factors — and for UK SAP junctions many teams use the streamlined AutoPSI approach.

This article is general guidance and reflects information available at the time of writing. THERM is developed by LBNL; always follow the calculation standard and conventions required for your project.