Thermal bridging: a primer and an update

Hopefully you are all aware that the details for thermal bridging using aircrete blocks have been updated and are now available. Partial fill cavity wall details were updated just eight days ago. If not, read on!

What is Thermal Bridging?

Thermal bridges occur at junctions: for instance, where walls meet, where a wall meets a floor, or where a wall meets a roof, window or door. Generally speaking, they occur in a building where a material with poor insulating properties passes through an insulation system, causing a ‘bridge’ for heat to escape. There are two main types of thermal bridge. A repeated thermal bridge, such as the studs in a timber frame wall, is dealt with in a conventional U-value calculation. A non-repeated thermal bridge occurs, for example, around openings, using a section of steel in a timber frame wall, or the junction between different elements types, such as a wall and a floor.

It’s worth noting that it is important to get your assessor to calculate the junctions rather than putting in the default value: this can have a big effect on the overall build costs and whether you choose full fill cavity wall insulation or partial fill. For a recent, real-world example, please see our engagement with Celotex and Cumbrian Homes, here.

What is a ψ-Value?

The heat loss associated with the non-repeated thermal bridge is called linear thermal transmittance, or psi-value or ψ-value (pronounced ‘si’). This is the rate of heat flow per degree per unit length of the thermal bridge that is not accounted for in the U-value of the plain elements. This is illustrated below where the heat loss path is indicated by the arrows. The larger arrow in diagram (a) indicates a greater heat loss.

The concept of a ψ-Value

Repeated thermal bridges such as timber studs or mortar joints are already covered by U-value calculations, but it is the heat loss at junctions between the various building elements that will be addressed by Constructive Details.

This extra heat loss, which is over and above the heat flow through the adjoining plane elements in a junction, is the linear thermal transmittance or ψ-value expressed in W/mK. To calculate this value one needs to subtract the heat flow through the plane elements (expressed by the U-value multiplied by the length of the element) from the total heat flow from the external to internal. This principle is illustrated in the drawing of the wall–floor junction. The image alongside showing the temperature distribution for the same junction is taken directly from thermal modelling software.

thermal bridging

The evaluation of thermal performance of both elements and whole constructions is becoming increasingly important. Another factor that is becoming increasingly significant is linear thermal bridging and the need for accurate and reliable calculations.

How can I calculate a ψ-Value and what are the relevant standards?

This is done with the use of computer programmes. All these programmes are adequate in estimating the heat flow through different types of junctions, in both simple (2D) and the more complex (3D) cases. The current acceptable methodology for calculations is the one described in BR 497 : 2007 Conventions for calculating linear thermal transmittance and temperature factors, and the documents Information Paper IP 1/06: Assessing the effects of thermal bridging at junctions and around openings and BS EN ISO 10211 Thermal bridges in building construction- Heat flows and surface temperatures-Detailed calculations.

Alternatively, if you are building with a lightweight internal skin of block (aircrete block), ψ-values can be sourced from the latest release from A set of ψ-values has been calculated for various common junctions for partial-fill and full-fill cavity walls. If you adhere to the requirements outlined in the documents, these enhanced ψ-values can be incorporated into SAP calculations. These values follow the same principle as accredited construction details; however, they consider more variables and are subsequently more accurate.

Why is it becoming important now?

The Approved Documents L1A and L2A (sections 5.12a and 5.7a) addressed the issues of thermal bridging, introducing the concept of a scheme for quality assurance of thermal bridging details. The context of this scheme is yet to be determined and in the meantime designers, energy assessors and specifiers tend to depend on the default values of the existing accredited construction details. These are potentially higher and less accurate than those that can be achieved by performing bespoke calculations, and they don’t cover all types of construction. Consequently, the heat loss estimated in a SAP calculation in such cases may be higher than expected based on the details used. As an alternative to using default values, individually calculated psi values can be used, but building control officers still need to satisfy themselves that the psi-values used have been created by a person with suitable expertise and experience. These values can be included within the BBA Certificate rather than having to rely on less accurate default values. Clients own existing values can also be accommodated following validation of the work. We can advise on individual client requirements for elements such as roof windows, roof lights, conservatory roofs, or even PassivHaus.

If you have any queries or projects that need quotes for any of Part L, please feel free to send them to Claire Beard at, or call her on 01454 317940 .


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