Algal or fungal growth on façades is caused by moisture from rain or condensation on the rendered surface not evaporating quickly enough. The higher the humidity of the outside air and the lower the temperature of the rendered surface, the higher the condensation level. The humidity lasts for a particularly long time on the sides less exposed to the sun (north and west) and when trees or plants cast shade on the façade.
The growth of algae and fungi is favoured by the absence of protection against rainwater, such as eaves or a window sill, or against water splashes at substructure level.
Algae and fungi can form on both insulated and non-insulated façades. As a rule, thin-layer exterior renderings on insulated façades and coarse-textured façades are more vulnerable to algal growth. On the other hand, mineral coatings covered with a suitable paint are less vulnerable.
