Coupled 2D thermal bridge coefficient, used to gauge the numeric impact of thermal bridges. Ψ, upper case psi (English: sigh), is the most commonly used symbol. The unit for psi is W/(m K). It applies to a length, i.e. the length of the thermal bridge. It denotes the thermal conductivity of an assembly of materials. The higher the value, the higher the additional heat loss through that joint. When external dimensions are used to assess the heat loss through the thermal envelope, psi values can be negative (=calculatory heat gain), as with external dimensions convex corners are considered twice (or even thrice, if assessment is made in 3 dimensions). When internal dimensions are used for heat loss calculations (like it is done by default in NZ), every convex corner has to be analysed for thermal bridging effects. Using external dimensions overestimates heat loss, and thus compensates for some geometrical thermal bridge effects. Thus, when external dimensions are used, an assessment of thermal bridge effects at convex corners is usually not necessary.