Thermal Bridge PSI Calculations

Thermal bridging is a major contributor to heat loss within buildings, typically as much as 30% heat loss occurs through thermal bridges in typical construction methods. Our assessors are able to design out thermal bridges and provide solutions to any junction or thermal bridge that will help lower heat loss, improve your SAP rating and of course improve the indoor thermal comfort levels for the occupants.

With the removal of Accredited Construction Details (ACD’s) in 2022 designers and builders are now required to provide independent third party calculations for the proposed junctions and thermal bridges within a dwelling. Failure to do so will result in poor default values having to be assumed within the SAP calculation which will most likely cause most SAP assessments to fail the minimum fabric efficiency standards.

Typical thermal bridges included in most newly constructed homes include:-

• Ground Floor / Wall Junction
• Window & Door Openings (head, jambs & cill)
• Intermediate Floor Junctions
• Roof at Eaves
• Roof at Gable
• Flat Roof Perimeters
• Internal & External Corners
• Party Wall Junctions
• Party Floor Junctions

What is a Psi Value?

A Psi Value is a linear thermal bridging that occurs usually in the main building fabric and is a measure of heat loss expressed as W/mk and is used within the SAP or SBEM calculation. A Psi Value is often referred to as a thermal model and are calculated in accordance with BRE BR 497 (conventions for calculating thermal linear transmittance and temperature factors).

What’s the difference between a Psi Value & U Value?

A Psi Value is linear heat loss measurement which is measured in linear meters (m) and U-Value is a surface heat loss measured in meters squared (m²). Both measure heat loss through the building fabric but Psi Values usually occur at junctions around the building fabric and where different materials abut or change direction.

When should I get them calculated?

Psi Values should be calculated once a specification has been selected which will enable the designers and builder to check the proposed details are fit for purpose.

Obviously, should a proposed detail be preforming poorly our modelers can provide help and advice on how to improve the thermal performance of the detail.