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Thermal Bridge Calculations

We have recently seen major changes to the way we assess our new buildings in the UK, and the true impact of thermal bridging within a construction is now better understood.

This means that in many cases it will become a necessity to carry out thermal bridge calculations (Psi-values) on many schemes.

What is a Thermal Bridge?

A thermal bridge occurs where there is a break in the insulation layers within a building, which acts as a path for heat to escape. They can account for approximately 30% of the of the heat loss from a building, so it is important to assess and minimise their impact where possible.

There are several types of thermal bridge and Psi-values are used to properly represent the effect of linear thermal bridges which occur at the junction between building elements, such as:

Thermal Bridge Calculations

A thermographic image showing heat loss through a floor junction.

  • Junction between a ground floor and exposed wall
  • Lintels, sills, window and door jambs
  • Junction between roof and exposed wall

Why are thermal bridge calculations important?

Both SAP Calculations and SBEM Calculations are used to assess for compliance with Part L of the Building Regulations, and they both require that the heat loss associated with linear thermal bridges is included within the calculations.

Where the Psi-value associated with a thermal bridge is unknown, default values should be used which represent a worst-case scenario. Using default values is likely to make complying with the building regulations extremely difficult.

For more standard construction methods, such as Masonry Construction or Timber Frame constructions, there are published thermal bridging values which can be used within calculations as per the links below:

Masonry Cavity

Thermal Bridging Calculations

A 2D junction modelled using Trisco software.

Timber Frame

However, these details do not include all thermal bridges which occur in a building, and other construction types do not currently have published sets of details which can be adopted within the design of your building.

How we can help – Psi-value calculations

Thermal Bridge Calculations

A 3D model showing heat losses around a junction.

Where the proposed construction details do not align with one of the existing published sets above, our experienced assessors can carry out Psi-value calculations in accordance with BR497 methodology. These can then be used within SAP and SBEM calculations to avoid the use of poor default values, and where necessary advise measures which could be adopted to help reduce the thermal bridge.

For modern methods of construction or volume house builders, we can assess your typical construction details which can then be optimised and used for all sites which follow the same details.

For further reading see What are Psi-values?

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