What are Psi Values?
First of all, have you ever come across a u-value? This is a measure of the heat loss through a square meter of thermal element (e.g a wall). Well, Psi values are a measure of heat loss along a meter of junction between two thermal elements, for example the line between a ground floor and an external wall (see green line in adjacent picture), and is measured in W/mK. These junctions are known collectively as thermal bridging.
Why do Psi values matter?
U-values account for the heat loss through thermal elements but not the total heat loss through the fabric. At the junctions, there is additional heat loss referred to as non-repeating thermal bridges.
This is down to the geometry of the junction and also, in many cases, the construction of the junction:
- The geometry because, in SAP, we measure internal dimensions (see dotted red line below) and attribute U-values to thermal elements (e.g. external wall and ground floor) measured this way. We therefore underestimate the total heat loss. If we measured external dimensions, we would overestimate total heat loss (assuming no construction psi-value)
- The construction because materials used at the junction often have a higher thermal conductivity than those used in the thermal elements (e.g. a block of concrete at the junction rather than continuous insulation around the junction).
What to do about PSI Values?
In SAP Assessments, to best model the energy performance of a dwelling, we factor in PSI values for all junctions as well as U-values for all exposed elements. To meet the latest building regs, Psi values need to be kept to a minimum.
In fact, for a realistic shot of achieving compliance, designers need to at least follow the Accredited Construction Details scheme, but if they can adopt Enhanced Construction Details, even better. Some timber frame and SIPS manufacturers provide their own super low Psi values, which can be used by your assessor. On some projects clients design out heat loss at junctions as much as possible and have bespoke Psi values calculated.
There also exist a few schemes set up by the insulation manufacturers, which allow you to demonstrate improved performance by using a combination of their products.
For more reading see Thermal Bridging Explained.
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