Biological growth on stucco

In recent years, microbial growth on stucco facades has become an increasing problem in Central Europe. Apart from the effect of a cleaner environment and less poisonous additives in the stucco, this is mainly due to better hygrothermal growing conditions for algae and fungi. Essentially, this means that the facade surface retains enough moisture throughout a certain period of time to allow the formation of vegetation. The increase in surface humidity of exterior walls can be largely attributed to better thermal insulation and lower thermal capacitance of the facade, which leads to frequent condensation of moisture from outdoor air caused by longwave sky radiation. Another effect is the slower drying of precipitation moisture due to the diminished exterior surface temperature of energy-efficient buildings with high thermal insulation. In order to prevent or at least reduce algae and fungi growth without polluting the environment, methods have to be devised to improve the hygrothermal surface conditions. These methods include a reduction of the water absorption of the stucco by innovative water repellents that rely on the lotus effect and exterior coatings with a low longwave emissivity. Both methods are supposed to reduce the wetting of the facade resulting from precipitation and condensation. Another method would be the staining of the stucco, which leads to a higher surface temperature and better drying during daytime. Alternatively, increasing the suction tension of the stucco substrate could also result in a faster reduction of the stucco surface humidity. The different methods described are currently being examined at a test site in Germany. First results should be available by the end of 2001.