PSI value calculations

Thermal Bridging Calculations

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.

If you require a SAP or SBEM Assessment of your new property and do not include a separate Psi Value calculation, then the assessor will have to adopt default Psi- Values.  Assuming default Psi Values can typically be worse than a properties actual Psi-Value and may affect whether your new property complies with regulations. 

Conducting separate Psi Value calculations as part of your assessment will provide much greater accuracy than using the default Psi values and will (in all likelihood) increase the chances of your property achieving compliance the first time round.  Additionally, they can also help to reduce the need for more costly additional measures to attain compliance.

In the newest version of SAP, the option to use ACDs (accredited Construction Details) were removed, and therefore it has become more difficult to achieve compliance using assumed values, leading to a much greater emphasis on calculated Psi Values.

What is a Psi-Value?

Also written as Ψ-Value, a Psi Value Calculation is used to measure linear heat loss across a junction between two thermal elements. Psi values are measured in W/mK (Watts per meter-Kelvin) and show the energy lost per metre between two thermal elements, for example, the join between a ground floor and an external wall.

Psi Value Calculations are carried out to determine the actual Psi Values which are used to assess the overall thermal bridging performance of a building. 

A thermal bridge is an area of a building that can have significantly higher heat transfer than the surrounding materials and occurs where two thermal elements meet, thermal bridging is one of the most significant sources of heat loss within a building and the use of bespoke Psi Value can lead to better performance in SAP results, please see our case study showing benefits of calculating bespoke psi values.

Via the use of Psi Value Calculations, junctions can be well designed, then once built and properly insulated, can significantly reduce the amount of heat escaping through air leakage, which means less energy is wasted through the fabric and therefore reducing fuel bills overall. 

Which junctions should we be concerned with?

The typical junctions that present the greatest opportunity for heat loss include;

  • Where a ground floor meets an exposed wall
  • Where an external wall meets a balcony
  • Lintels and sills
  • Window or door jambs

How can Melin help you?

Psi Value Calculations are desk based and are created by our in-house team of dedicated technical experts, using specialist thermal modelling software. As well as the standard calculation sheets, we will produce a ‘jargon free’ guidance and findings report which summarises the results and highlights the performance in a clear and readable way.  Should any of the junctions fall short of the target, we will include a recommendations report, describing how the thermal performance of the junction could be improved.

Producing your own, bespoke psi-values can really pay dividends if you regularly build to the same specifications.  Once you have a set of thermal bridging calculations, you can use them again and again on subsequent projects, leading to potentially significant cost savings on insulation.  For those looking to go ‘above and beyond’, the calculations can lead to improved SAP/EPC results. We can help you produce these figures. See our case study here.

For those looking to achieve Passivhaus certification, it is essential to achieve thermal bridge free construction, so these calculations effectively become part of the design process.

So, whether you’re a self-builder wanting to maximise your building’s energy performance credentials, if you want to achieve SAP10 with minimal fuss, or you’re a developer seeking to future-proof your buildings design across multiple sites - don’t delay, put us to the test by contacting us here.

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Melin Energy Consultants

01554 571010

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