There is significant evidence to suggest that buildings do not perform as well when they are completed as was anticipated when they were being designed. This difference between what was anticipated and actual performance is known as the ‘performance gap’. Thermal bridging analysis provides early validation of details and enhanced understanding within the architectural team of the thermal performance of their proposed detailing. It also leads to increased understanding of the performance of materials and components that are being specified for inclusion in the building envelope.
Thermal bridges should be avoided whenever possible. Proper planning, design and construction are essential to help identify and remedy thermal bridges. Care should be taken throughout the construction to ensure thermal bridges are not created at any point during the build process. Linear thermal bridges are assigned a linear transmittance value known as a PSI ( Ѱ) value, measured in W/ mK. Multiplying the Ѱ Value by the linear length (m or ft) of the junction will give the heat loss as a result of thermal bridging for this detail. This is inputted into the energy model to give the correct heat loss from thermal bridging.
Thermal bridges occur within the building fabric where, because of the geometry or the presence of high conductivity materials, heat flows are two or three dimensional. For many situations, simple calculations are no longer sufficient to determine thermal performance correctly and it is necessary to analyse the construction using numerical modelling. A number of numerical modelling software packages are available where these specify the geometry, the materials and the boundary conditions of the model in two- or three-dimensions as appropriate.
To reduce the thermal bridging impact in SAP assessments either accredited details must be used or calculations performed. The improvement in the SAP result for good designs can be substantial, saving in fabric specification and renewables elsewhere. QuinnRoss Energy can complete a Thermal Bridging review to identify whether it is worth completing calculations, and which calculations will yield the best results.
What’s a thermal bridge?
A thermal bridge (sometimes called a cold bridge) is a localised weakness or discontinuity in the thermal envelope of a building. They generally occur when the insulation layer is interrupted by a more conductive material. Such as metal.
Why is that important?
For buildings in general, bridges that conduct temperatures from the outside to the inside can have significant condensation and therefore mould issues, that occur internally in the structure. Once this happens, resolving it becomes very difficult and sometimes impossible.
Equally as significant is their influence on SAP calculations, particularly the Fabric Energy Efficiency (FEE) aspect. A dwelling with unapproved/default thermal bridging is noticeably more difficult to achieve compliance, with compensation required in other areas resulting in a requirement for heavily insulated constructions, expensive triple glazing and/or unfeasibly low air permeabilities.
How does QuinnRoss Energy help?
We have access to and use of industry-leading calculation software, where we build your thermal bridge digitally and analyse its surface temperatures (for condensation issues) and Psi-values (for SAP calculations). If K1 approved thermal bridges are targeted in SAP this calculation is mandatory.