Discusses the accurate evaluation of the effect of thermal bridges on building energy performance using a modelling approach. Sisley software was used first of all to model the heat transfers in the intersections of walls. CLIM 2000 was used to reduce and integrate the models. A comparison was done between these results and the models obtained from thermal regulation values. It appeared that detailed modelling of heat transfers would provide an increased accuracy of around 5% when evaluating the building heat loss.