PSI Value - The heat loss

When modelling a thermal bridge detail the main result we look for is a Psi-Value. This value, measured in watts per meter Kelvin (W/mK), expresses the amount of heat loss that will be lost through this junction.

These additional heat losses at each thermal bridge are then multiplied by their linear length at which they occur and added to the combined  losses of each fabric elements surface area and its associated U value.

Together these will result in the Total Fabric Heat loss of the building.

On a well insulated new build these Thermal Bridges alone can account for up to 30% of the entire fabric heat loss

The illustration below provides an example of how the same detail can be modelled, one with a standard lintel, the other a thermally broken lintel. It highlights just how much lower a Psi Value can be if detailed correctly and the energy savings associated with this.