Importance

SAP and SBEM calculations both incorporate heat loss from thermal bridges while checking for compliance with Part L. Where ever there’s a break in the insulation layer, or a point where two or more building units meet, that area can be classified as a thermal bridge which is a path for heat to escape.

Thermal bridges have several classifications depending on the type of junction, and each thermal bridge has a specific Psi-value associated with it. However, instead of using default Psi-values, they can be calculated to give a more accurate measurement of the heat loss through the thermal bridges.

Where do thermal bridges occur?

The general categories of thermal bridge are:

Repeating thermal bridges

These happen when there are regular interruptions in the building fabric, like studs or wall ties. They are factored into U-Value calculations

Linear (non-repeating) thermal bridges

These occur around openings like windows or doors or where a more conductive material penetrates the insulation layer. Heat loss through these bridges is estimated using Psi-values (Ψ-values), which measure heat loss through the linear joint. Multiplying the Psi-value by the junction length gives an estimate of the total heat loss.

Geometrical thermal bridges

These occur where different planes meet, like junctions between building elements. They're common in areas where the heat loss area exceeds the internal surface, such as corners; and

Point thermal bridges

These are single penetrations in the building's thermal envelope, like fixings or fasteners, brackets for a canopy / or in a rainscreen, flues passing through walls or isolated steel beams / columns. Heat losses through point thermal bridges are typically calculated using Chi-values (χ), which is typically considered as an adjustment in U-value calculation.