Reducing the risk of microbial growth on insulated walls by improving the properties of the surface materials

Staining of façades due to microbial growth has been risen in recent years. Especially walls with external insulation systems are affected because the low thermal mass of the exterior render combined with a high thermal resistance of the insulation layer leads to frequent overcooling of the render's surface by long-wave radiation exchange with the sky. Condensation forming on the overcooledfaçade is of major importance for microbial growth. Therefore, the best way toprevent growth is to reduce the frequency of condensation by limiting the periods of overcooling. This could be achieved by increasing the thermal inertia of theexterior layer by using massive rendering or the addition of phase change materials (PCM) or by applying low IR emissivity (low-E) paint coats. According to the present state of knowledge not the amount of condensation water is decisive for the risk of microbial growth but the condensation water on the external surface, which is available for micro-organisms in the initial phase of growth. Absorbent sub-strates could offer remarkable advantages in this respect. The performance of novel rendering systems provided with one of these measures or with a combi-nation has been investigated by field tests as well as by hygrothermal simulations.