In case of historical buildings, where the facade is to be preserved for aesthetical and preservationist reasons, the application of an interior insulation system is the only possibility to reduce the transmission heat losses of external walls. However, the application of an interior insulation always causes certain risks. Because of the thermal separation from the interior, the temperature level in the construction decreases, what might cause a reduced drying potential of the wall after driving rain and leads to a raised danger of condensation problems. The risk of moisture problems increases with the insulation hickness. To guarantee an adequate failure security of the construction as well as the maximization of the energy saving potential, the assessment of interior insulation systems by means of numerical simulation methods is state of the art. In these simulation methods the coupled transport of heat and mass is taken into account as well as unsteady climate data (temperature, air humidity, driving rain, solar radiation). As in static calculations, the choice of a representative load determines the result. In case of a hygrothermal simulation, the inside and outside climate conditions are the loads for the construction. The choice of a representative outdoor climate for a hygrothermal simulation was made by a concept, which has been developed within the framework of this thesis. In this concept, the effects of different outdoor climate records on the construction were compared to each other and to the results of longtime simulations based on continuous outdoor climate records of several decades. The studies were divided according to the water absorptive capacity of the facade. While it can be shown, that in case of a hydrophobic exterior rendering the choice of a representative outdoor climate record is mainly determined by the temperature level, in case of facades with a higher water absorptive capacity (which is normal for historical buildings), a representative outdoor climate contains a specific combination of the temperature level and the driving rain load. Based on the insights of this thesis, it was also tried to give recommendations for the preselection of a reference year.