Simulation water leaks in external walls to check the moisture tolerance of building assemblies in different climates

The moisture tolerance of building assemblies is becoming a major durability issue worldwide because it is almost impossible to create perfectly tight building assemblies. Alerted by numerous cases of severe moisture damage caused by rain water penetration through stucco clad walls in North-America, the American Society of Heat Refrigeration and Air-conditioning Engineers (ASHRAE) has recently put out a standard on moisture design criteria for public review. This new standard forms the basis for hygrothermal performance evaluations by numerical simulation that should ultimately lead to better moisture control in buildings. In this paper the moisture behaviour of stud walls with leaks in the exterior thermal insulation system (EIFS or ETICS) is simulated by applying the procedure described in the ASHRAE standard. In a parametric study the different assembly modifications and outdoor climate conditions in North America and Europe are considered. The results show severe problems dealing with exterior insulation systems based on polystyrene insulation. Only in some cases increasing the drying potential toward the interior by replacing the vapour barrier by a smart retarder helps to solve the problem. However, when the polystyrene insulation is exchanged by mineral wool the penetrating rainwater rapidly dries out through the exterior insulation system and the humidity conditions in the wall always remain below the critical threshold.