This paper describes the modelling approach used to accurately evaluate the effect of thermal bridges on the energy performance of buildings. The heat transfers in the intersections of walls were initially modelled in Sisley software. These models were then reduced and integrated in Clim 2000. The simulation results were compared against the models obtained from thermal regulation values. For standard wall configurations, it was seen that the detailed modelling of heat transfers provides an additional accuracy of about 5% in terms of the evaluation of the building's heat losses.