The moisture behaviour of building components is becoming a major durability issue worldwide because it is almost impossible to create perfectly tight building assemblies. Alerted by numerous severe damage cases caused by rain water penetration through stucco clad walls in North-America, the American Society of Heat Refrigeration and Air-conditioning Engineers (ASHRAE) has developed a standard on moisture design criteria. This standard will form the basis for hygrothermal performance evaluations by numerical simulation which should ultimately lead to better moisture management in buildings. In this paper the moisture behaviour of walls with leaks in the exterior thermal insulation system is simulated by applying the procedure described in the ASHRAE draft standard 160. In a parametric study the different assembly modifications and outdoor climate conditions in North America and Europe are considered. While everything works well when there is no rain water leakage, the results show severe problems dealing with leaks in polystyrene insulation systems. Only at warm locations, an increase of the drying potential toward the interior by replacing the vapour barrier by a humidity controlled retarder may solve this problem. However, when the polystyrene insulation is exchanged by high density mineral wool slabs the penetrating rain water rapidly dries out through the exterior insulation and the humidity conditions in the sheathing material serving as substrate will remain below the critical threshold.