What are Thermal Bridging (Psi Value) Calculations?
A thermal bridging calculation is a method used to assess the heat loss along a metre of junction between two thermal elements of a building, such as walls, roofs and floors, due to the presence of thermal bridges. These are areas of the building envelope where the insulation is interrupted or less effective, allowing heat to escape more easily. Thermal bridging calculations are used to determine the additional heat loss caused by thermal bridges, and are included as part of a Standard Assessment Procedure (SAP) calculation for a building.
Thermal bridging calculations are used to determine the additional heat loss due to thermal bridges and this loss is added to the overall heat loss calculations in order to determine the overall energy efficiency of the building.
Why are they important?
Thermal bridging calculations are important for several reasons:
- Energy efficiency: Thermal bridging can significantly increase the heat loss from a building, leading to higher energy consumption and increased carbon emissions. By accurately assessing the thermal performance of a building and identifying areas of thermal bridging, designers can make informed decisions about how to improve the energy efficiency of the building.
- Compliance: In many countries, including the UK, building regulations require designers to assess the impact of thermal bridging on energy performance. By carrying out thermal bridging calculations, designers can demonstrate compliance with these regulations and avoid the risk of penalties or delays.
- Comfort: Thermal bridging can lead to areas of cold or draughts within a building, which can reduce occupant comfort and satisfaction. By identifying and addressing areas of thermal bridging, designers can improve the overall comfort and liveability of the building.
- Cost savings: By reducing the heat loss from a building, designers can help to reduce energy bills and save money for the building owner or occupants. Additionally, by identifying areas of thermal bridging early in the design process, designers can avoid costly remedial measures later on.
- Reducing the gap between design and performance. For too long, SAP calculations have not been representative of the actual performance of the building at completion. Psi Value calculations give a more accurate picture, helping to reduce that gap.
Why will I need them
When Part L of the building regulations was updated at the end of 2021, the new SAP software, SAP 10.0, removed the option for assessors to use Accredited Construction Details (ACDs). ACDs were based on outdated, generic build-ups and requirements, but they gave Psi values, which meant that good results could be achieved in SAP, even when the reality was that they didn’t perform so well thermally.
Now, instead of using ACDs we can do one of the following:
- Use the software’s default values – which will be poor and unlikely to achieve compliance
- Use pre-modelled values
- Model the Psi values ourselves
Some manufacturers provide the pre-modelled values for their products. Please check out these links for some places you might look for these:
- https://www.labc.co.uk/professionals/registration-schemes/registered-construction-details
- https://www.cba-blocks.org.uk/thermal-bridging-details/
- https://unilininsulation.ie/technical/technical-resources/acds/
As long as your build-ups are exactly the same as what you’re able to find online, they can be used in your SAP calculation.
Wherever they aren’t available, we can calculate the thermal bridges for you. Not all SAP assessors are able to do this, so even if you’ve had your calculations completed elsewhere, we still might be able to help you.
Why might I need them
Even if your project is being completed in accordance with the older version of Part L of the building regulations, you may still require thermal bridging calculations. One of the most common reasons that people come to us for thermal bridging calculations is if their SAP is failing at as-built stage. At design-stage, there are a number of elements that can be changed or upgraded to help the SAPs to pass. Once the dwellings are built, there’s very little you can do, but thermal bridging calculations are one of those things!
Even though you can use Accredited Construction Details (ACDs) in the older SAP software, the values given won’t be as good as if we were to accurately calculate the Psi values. This is because ACDs make a lot of assumptions. What we will be doing is inputting the exact specification, which will almost always show a lower heat loss than by using ACDs.
When we calculate the Psi values ourselves, the Fabric (DFEE) element of your SAP Calculation will improve significantly, and we would also expect to reduce (and potentially even remove) the need for solar PV panels. This could save you thousands on your build costs.
Do you need help with Thermal Bridging (Psi Value) Calculations? Whether you’re a developer, architect or even an energy assessor who doesn’t have this skill, please get in touch for a quote.
Contact us today for Thermal Bridging Calculations
We offer the highest levels of service and expertise and are always happy to work with our customers to give you the best possible chance of achieving a pass on your Thermal Bridging Calculations. Additionally, if you need your SAP assessment quickly, we offer a 24-hour service. Please contact us for further information.