The benefit of independent psi-value calculations

The benefit of independent psi-value calculations

With Building Regulations becoming increasingly stringent, and the SAP assessment evolving, it’s now imperative that attention is paid to Linear Thermal Bridging.


A psi-value (pronounced ‘si’), or Linear Thermal Bridge is the measurement of heat lost along the length where two building elements meet – for example a wall meeting a floor, a roof, or a window. With U-values of construction elements improving, it is now time to look more closely at the heat loss along these thermal bridges.

So, you’ve spent lots of time and money on designing a building with high-performing U-values and robust architectural detailing. Why undo all that forward-thinking hard work by selecting default values for the thermal bridging? Whilst SAP allows an assessor to apply default values to thermal bridging lengths, this can have a huge detrimental effect on the assessed performance of a dwelling. An assessor also has the option to select ‘Accredited Details’, if the builder signs up to building to an approved set of construction details. The Accredited Details scheme is now quite dated and only applies to a limited number of construction methods, and so is likely to be taken out of use with the next update of the Building Regulations. By calculating the specific heat loss through a thermal bridge, you can accurately reflect the performance of a dwelling which will afford increased design flexibility.

We have spent a lot of time analysing linear thermal bridges for Clients, and identifying the areas in which they can gain benefit over the SAP defaults. Once a set of independently calculated psi-values is applied to a dwelling, we have seen improvements in SAP performance of between 8-10%. This has enabled clients to explore the possibilities of reducing their PV requirements, removing MVHR completely, relaxing their air permeability target, or value engineering their fabric specification. The cost savings across any size of scheme can be incredibly significant.

As well as significant cost savings across a project, accurately analysing the thermal bridging can give confidence that there won’t be any post-occupancy issues relating to condensation or mould growth. By running complex thermal calculations, any potential condensation risk can be identified, and mitigated before a single brick is picked up on site.

Case Studies

Working with a medium sized developer in Central London, we analysed their specification, calculated four specific psi-values, and enabled the removal of MVHR units from all 121 apartments across their development. It’s estimated that this provided a saving of between £100K and £120K. This was in addition to relaxing their air permeability target back from 3 to 5, meaning a much-decreased likelihood of the delay and costs associated with retesting failing plots.

At a site of large executive housing, priced between £1m and £2m, a client was finding difficulty achieving the design air permeability of 3. The nature of the dwellings meant that there was a large number of penetrations in the building fabric, as well as several areas of complex construction where air leakage was difficult to mitigate. To avoid the necessity of very costly and disruptive remedial works, the calculation of only three psi-values enabled the relaxation of the air permeability target back from 3, to as much as 7.

For a Client building a relatively small number of houses in Kent, confidence in achieving the site wide energy target, whilst staying within budget was of paramount importance. We identified the junctions that occurred most often across the buildings, and those that would be of most benefit, and calculated 5 psi-values. The benefit from applying these calculations meant that the PV requirement across the site was reduced by 60%, providing them with a huge time and cost saving across the site. This client then went on to use the same construction detailing on another site, which meant that these psi-values could then be applied to those SAP calculations, and reap the same benefit with no further costs. Once the calculations are done and paid for – they’re yours to use across as many sites as you like, as long as the detail matches!


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