All change with Psi-Values in SAP 10.2

In this article, we look at the impact of psi-values and why they matter. Ψ-values (Psi-values) are a measure of the heat loss of a junction between two fabric elements, e.g., floor and external wall. These junctions are called non-repeating thermal bridges.

?Previous versions of the UK Building Regulations have raised awareness of the importance of fabric u-values and air tightness, the incoming changes are set to refocus on the ‘non-repeating’ or ‘linear’ thermal bridges that occur at junctions between fabric elements. The easy option of using Accredited Construction Details (ACD) have now disappeared from SAP in the new version. This will force developers to really consider junctions when choosing their building fabric choice.

?As dwellings have become better insulated, the importance of thermal bridging has increased. In well insulated dwellings, the effect that thermal bridging can have on the overall thermal performance can be very significant. Recent research suggests that thermal bridging can be responsible for up to 30% of a dwelling's heat loss in highly insulated buildings.

The heat loss associated with thermal bridges is usually expressed as a linear thermal transmittance (‘Ψ-value’, W/m2.K; also & commonly written as ‘psi-value’). It is the rate of heat loss per unit length of the thermal bridge, and per degree kelvin temperature difference between the inside and outside. This article considers the way in which thermal bridges were treated in SAP 2012 and describes improvements made for SAP 10.2.

?In SAP 2012, assessors had three options for thermal bridging:

1. The use of a global factor (a ‘y-value’ of 0.15 W/m2K), which is multiplied by

the dwelling’s total exposed surface area, as described in SAP Appendix K. (worst case)

2. Entering the length of each junction only, which is multiplied by a default Ψ value for that junction type from SAP Table K1.

3. Use pre-calculated Psi-values supplied by manufacturers.

?Under the Building Regulations, it is also possible to use Accredited Construction Details (ACDs). These are standard junctions designed to simplify the process of calculating Psi-values for a property. However, it is now considered that the ACDs are outdated and as part of the changes to Part L and Section 6 in England, Wales, and Scotland, they have been removed from SAP.

??So, what’s changed with the latest update - SAP 10?

?The ACD scheme has been criticised as being open to abuse by developers and of simply being outdated. SAP 10 will remove the option to use ACD, forcing developers to take one of two alternative approaches in order to avoid the defaults. The first is to use certified make-ups from manufacturers which have corresponding predetermined psi-values. The second is to have their own bespoke psi-values calculated by an assessor who has specialist training.

In summary, the two key changes to Appendix K from SAP 9 (or SAP 2012) to SAP 10 are the removal of the ACDs and adjustments to some of the default psi-values. These changes tend to give rise to a significant increase in the default y-value which is a building specific sum of the applied psi-values expressed per unit of surface area. The y-value can be directly compared with the u-values because it has the same units of W/m2K which allows the heat loss from the junctions to be evaluated as a proportion of the total heat loss of a building.

?DG Timber Solutions have recognised this change and have carried out our own thermal modelling analysis on all our timber frame standard details and created a full set of Ψ-values (Psi values) these can then be used by your assessor into your SAP calculations for each of your plots.

So, you can with confidence involve us in your early design and with the low u-values available from a closed panel timber frame and the potential of a higher PMV (pre manufactured value) score with items such as factory assembled floor and roof cassettes.

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