A case study is used to introduce a two-step thermal demand calculation for buildings in order to take thermal bridges into consideration. In the first step the outer envelope of a typical apartment is 3- dimensionally modelled and a 3D thermal- coupling coefficient is calculated. In the second step the thermal demands of the whole building is calculated using three variations of the outer envelope design. Each variation is calculated in ID and 3D. The difference between the results of ID and 3D calculations is used to show the influence of thermal bridges upon the overall heat loss of building.