What are Psi Values?

Psi values (or Ψ values) are measures of heat loss along a meter-long junction between two thermal elements. The most common example of this being the junction between the ground floor and an adjoining external wall.

Psi values can be calculated throughout the design phase, and as late as during the construction phase.

Why would I need to calculate Psi Values for a property?

Typically, building projects must be compliant with target U values (representing the overall heat transfer of the property). To achieve these values accurately, thermal bridging must be accounts for through the calculation of Psi values.

Beyond legal requirements, SAP calculations will use the worst-case values for thermal bridging if Psi values are not already calculated. If this is the case, the level of heat lost through thermal bridging may be overestimated, causing higher electricity consumption for heating or cooling the property, and higher costs for the owner.

Beyond this, Psi values can help find localised weaknesses or discontinuities in a property’s thermal envelope, preventing excessive heat emissions.

How are Psi Values calculated?

Psi values can be calculated by identifying common thermal bridges, taking the properties of the building material into account, and then using a combination of modelling software and manual calculations, including applying principles of heat transfer.