This International Standard sets out the specifications for a three-dimensional and a two-dimensional geometrical model of a thermal bridge for the numerical calculation of: — heat flows, in order to assess the overall heat loss from a building or part of it; — minimum surface temperatures, in order to assess the risk of surface condensation. These specifications include the geometrical boundaries and subdivisions of the model, the thermal boundary conditions, and the thermal values and relationships to be used. This International Standard is based upon the following assumptions: — all physical properties are independent of temperature; — there are no heat sources within the building element. This International Standard can also be used for the derivation of linear and point thermal transmittances and of surface temperature factors.