Thermal modelling for Psi-values involves using specialised software to simulate heat flow through complex building junctions. This 2D or 3D numerical analysis calculates the temperature distribution and heat flux, allowing for the precise determination of the Psi-value for a specific junction detail.

To perform thermal modelling for Psi-values, we require annotated detailed dwg drawings of the junction with conductivity and thickness of any materials detailed.

While default Psi-values are sometimes provided in building regulations (e.g., in the UK, Approved Document L often provides tables of default Psi-values), these are often conservative and may not accurately reflect the thermal performance of a well-designed or specific detail. Using calculated Psi-values from thermal modelling can often demonstrate a better performance and help achieve compliance more cost-effectively.

U-value (W/m²K): Represents the rate of heat loss through a uniform building element (e.g., a wall, roof, or floor) per unit area. It's a measure of overall thermal transmittance for a flat element.

Psi-value (W/mK): Represents the additional linear heat loss that occurs at a junction where two or more building elements meet, beyond what would be calculated from their individual U-values. It accounts for the 'extra' heat loss at these linear thermal bridges.

It is highly recommended to have Psi-values calculated for projects that:

• Need to demonstrate compliance with stringent building regulations.

• Feature complex or unusual construction junctions.

• Want to optimise insulation levels and reduce overall construction costs.

• Are concerned about condensation risk and internal surface temperatures.