Otherwise referred to as PSI value calculations, thermal bridging calculations are carried out to measure the heat loss at junctions in the fabric of a building. These are the weak points in the construction of a building and are a key consideration when building to an energy-efficient standard as they are prone to heat loss.
PSI values are included in SAP and SBEM calculations and can account for 20-30% of the heat loss from a building.
What is thermal bridging?
Thermal bridging occurs at junctions in the fabric of a building i.e. corners, wall-floor joins and window jambs, sills and lintels.
This type of thermal bridging is known as a non-repeating thermal bridge. This is the type of thermal bridge that we consider when calculating PSI values.
Weak points also occur throughout the fabric element, such as timber studs in a wall or roof build-up. This is referred to as a repeating thermal bridge however this is accounted for within the elements U-Value calculation.
Thermal Bridging in SAP and SBEM Calculations
Properly accounting for thermal bridging in energy modelling can potentially reduce the overall cost of the build.
When assessing commercial buildings with SBEM there is only the option to use a default figure or a calculated thermal bridging calculation. Opting for the default leads to the development being penalised, impacting the accuracy of the SBEM calculation. This can also lead to increased insulation levels throughout the build to ensure overall compliance is achieved.
For domestic buildings in SAP there is currently the option to select ‘Accredited Construction Details’ instead of default values or a calculated value. These are however due to be removed in the next version of SAP (SAP10) which will result in thermal bridging calculations becoming a key part of the design process.
Two figures will be detailed on your thermal bridge calculation certificate:
1. The PSI (ψ) value which is used in energy calculations to declare the heat loss from the junction
2. The temperate factor (ƒRsi) which determines the risk of surface condensation. This should be above 0.75.
All of our calculations are carried out using the latest Physibel Trisco modelling software.
Frequently Asked Questions
Your architect can provide us with a drawings (preferable in CAD format) showing the build up of a junction. Alongside this we will need to know the materials that will be used (including the thermal conductivity of each material). This means we need to be specific about insulation and blockwork performances.
No. For construction a residential building you can opt for the use of Accredited Construction Details until the next Building Regulations update when the option is due to be removed. You will still need to follow the details and sign off the certificates at completion to confirm they have been used. For a commercial building the default options needs to be selected if no calculation has been carried out. This is detrimental to the result leading to higher levels of insulation being required for compliance. It may be more cost efficient to carry out the calculations.
For developers building the same specification across multiple units it can lead to significant cost savings on insulation for those who are looking to achieve Building Regulations compliance. This is because the calculation only needs to be carried out once and can be used on each building where that junction type is used. For those looking to go ‘above and beyond’ the calculations can lead to improved SAP/EPC results. When aiming to achieve Passivhaus certification it is essential to have a thermal bridge free construction so these calculations will inevitably become part of the design process.