Repeated thermal bridges such as timber studs or mortar joints are already covered by U value calculations, but it is the heat loss at junctions between the various building elements that will be addressed by Constructive Details.

This extra heat loss, which is over and above the heat flow through the adjoining plane elements in a junction, is the linear thermal transmittance or ψ-value expressed in Wm-1K-1.

To calculate this value one needs to subtract the heat flow through the plane elements (expressed by the U value multiplied by the length of the element) from the total heat flow from the inside to the outside. This principle is illustrated in the drawing of the wall–floor junction. The image alongside showing the temperature distribution for the same junction is taken directly from thermal modelling software.