Criteria defining rain protecting external rendering systems

In the 1960s and 70s extensive experimental investigations were carried out at the open air test site of the Fraunhofer-Institute of Building Physics concerning the driving rain protection of Autoclaved Aerated Concrete (AAC) walls coated with innovative synthetic resin renders. Since some of the exposed test walls did not perform as well as others, a correlation between the water absorption and the vapour diffusion properties of the façade coatings was established. This correlation was subsequently introduced into the German Standard for exterior rendering systems with a slight modification to account for the special characteristics of mineral renders. When models to simulate transient heat and moisture transport processes in the building envelope were successfully applied in the middle of the 90s, parametric studies confirmed that the empirical correlation, established 30 years ago, is appropriate to define the rain protection characteristics of façade systems. However, the magnitude of the parameters depends on the climate conditions and the wall assembly. While the existing specifications are suitable for moderately insulating masonry, walls made out of well-insulated blocks may get too wet. Similarly, masonry structures with interior insulation may accumulate too much moisture behind the insulation layer unless the existing limit for the water absorption coefficient is reduced by 60% for high wind driven rain zones. Recent calculation results show that the local driving rain load and intermittent drying conditions as well as the characteristics of the substrate are the principle factors for selecting limit criteria for rain protecting rendering systems