Thermal Bridging and Psi-Values

Estimated to account for up to 30% of a dwelling’s heat loss, thermal bridges within dwellings contribute significantly to heat loss, energy use and Part L1A compliance.

Where two adjoining materials exist within a construction, e.g. mortar and blocks, or where two constructions come together, e.g. a wall/floor junctions, a thermal bridge is created.

U-value calculations and thermal bridges such as timber studs or mortar joints are not new to Part L1A and they are widely understood. However, a Psi-Value – the heat loss associated to a junction such as a wall/floor junction – is still an inclusion where the impact is still yet to be fully understood by some.

Psi-Values within the current Part L1A

A U-value accounts for the heat loss across surface area and a Psi-Value accounts for heat loss across the length of the junction. In SAP, the proposed building is measured so that all areas and lengths are included.

Currently the heat loss assigned to all the junctions in a dwelling can come from a combination of sources:

1. A government approved Accredited Construction Detail (ACDs)
2. A calculated Psi-Value carried out by a thermal modeller trained in BR497
3. A default Psi-Value
4. A default Y-value of 0.15

To gain compliance with the current Part L1A requirements, a combination of 1) and 2) is often adopted as it has the desired impact on reducing emissions and improving fabric performance.

Psi-Values within future versions of Part L1A

Following the Future Homes Standard Consultation, we now know what will be included in the next versions of Part L. The 31% uplift on current standards, which will come into effect in 2022, will mean there will likely be greater pressures on all elements of the SAP assessment to achieve compliance. More specifically, regarding the Psi-Values, we also know the use of the government ACDs will be removed and the default values used for overall compliance are worse. Overall, this means that there is more emphasis on calculated Psi-Values.

Calculating Psi-Values

Each material of an architectural detail is recreated in a thermal modelling software. A thermal conductivity is assigned along with boundary conditions (similar to a U-value) and the resultant model will provide a Psi-Value. In addition, heat flows and temperatures through the model are recorded so that guidance can be offered on how to further improve the performance of the junction.

The use of Psi-Values in SAP is reviewed with each update to Part L. Whilst this may achieve a more representative assessment again the final dwelling; the tighter requirements will mean calculated Psi-Values become much more common. At C80 Solutions, we have in-house thermal modellers who can offer a fully assisted design service of a junction so that heat loss is minimised.  We can work with architects and design teams alike to provide a cost-effective solution to detail.

How C80 Solutions can help

Thermal Modelling provided by C80 Solutions ensures that the bespoke Psi-Values of all thermal bridging lengths can be measured. Our specialist software delivers a 2D and 3D analysis which allows us to identify, assess and rectify areas of thermal bridging in construction detailing. This means we can make recommendations on how to reduce a building’s carbon emissions, lower energy and construction costs and ultimately ensure that dwellings are SAP 10 compliant.

Contact us for further information and guidance.