Thermal Bridging

A thermal bridge defines an area of increased heat loss in comparison to the surrounding elements. There are two distinct types of thermal bridge – ‘repeat’ and ‘non-repeat’.
Repeat Thermal Bridging is where a thermal bridge occurs at regular intervals, for example a rafter every 600mm in a roof. Repeat thermal bridging is accounted for in U-value calculations.

Non-repeat Thermal Bridging usually occurs at junctions of building elements such as a wall/floor, wall/roof or wall/window junction, etc. This type of thermal bridge is not accounted for in U-value calculations and the heat loss through non-repeat thermal bridges should be included in fabric heat loss calculations. The amount of heat loss through a non-repeat thermal bridge is referred to as its thermal transmittance or Psi (Ψ) value and is expressed in W/mK.

As U-values of floors, walls, roofs and windows tend to improve; the Ψ value at junctions tends to increase, resulting in more heat loss through the thermal bridges and increasing the risk of surface condensation at these locations.

The total heat loss due to non-repeat thermal bridging in the DEAP software is determined by the ‘y’ fraction. This ‘y’ fraction expresses the heat loss through these thermal bridges as a fraction of the total exposed surface area. The lower the ‘Y’ value, the less heat loss through thermal bridging. A default ‘y’ value of 0.15 is used in the DEAP software. An improved ‘y’ fraction of 0.08 can be used if Accredited Construction Details are used. However, a ‘y’ fraction of 0.08 still represents a considerable amount of heat loss. There is also the option to model alternative construction details to calculate lower Ψ values and hence lower ‘y’ fractions.

Our technical department can model, calculate and provide guidance on reducing or even eliminating thermal bridging at these junctions. Please contact our technical department for further information.