Resilience of biocide-free ETICS to microbiological growth in an accelerated weathering test

Biocide-free renders and coatings for external thermal insulation composite systems (ETICS) are now long-established on the market and contribute notably to the improvement of ETICS’ environmental properties. A low likelihood of microbiological growth is a relevant criterion when choosing ETICS. We examined whether a previously developed accelerated weathering test would be able to determine the ETICS’ resilience to microorganisms and be suitable as a standard instrument to check the fitness for use of biocide-free ETICS renders and coatings. 15 different combinations of renders and façade coatings, including both new and established products, were tested for 15 weeks under accelerated weathering. The used lifelike weather conditions derived from climate data on an average autumn day in Holzkirchen, Bavaria are favourable for microbiological growth. Specimens were inoculated with a spore suspension containing fungi and algae typical for buildings derived from the institutes' own collection of building-relevant organisms. The results clearly show that biocide-free ETICS’ coatings can be highly resistant to microbiological growth. Of the tested product variants, only two showed low or medium resilience. Additionally, we measured the formation of surface water condensation. Knowledge of the amount of condensation water accumulated during dew point shortfall usually confirms the results of the accelerated weathering test. However, contradictory results also exist. This can be explained by different evaporation times of condensation water on the surface of renders. The evaporation times depend on the render's inherent properties. Our results confirm that the used accelerated weathering setup is a suitable standard instrument to assess and compare the resilience of biocide free ETICS to microbial infestation.