Why do we often get biological growth on thin rendering on thermal insulation constructions?

If one takes a closer look at almost any building facade one will find biological growth. Although this can be seen as an integral part of an antique building, for modern buildings it is in most cases seen as an aesthetical problem. All building facades will most likely be discolored by biological growth by time, but in Sweden we have had many cases of newly built rendered facades that have been discolored much more rapidly than usual. The problem is believed to occur mainly on thin rendering applied on thermal insulation -which is a common building facade construction in Sweden -but apart from this, it is difficult to find clear indications of which facades that will be affected. As it is well known that different biological organisms has different requirements in physical factors (especially temperature and relative humidity) for germination and growth on any substrate, it is important to define which conditions the organisms needs to grow on a building fasade. We have studied the temperature and relative humidity conditions on the surfaces of different rendered facade constructions. We find that the surface temperature on a facade of thin rendering on thermal insulation drops quickly during nights, as these constructions have low heat capacity in the rendering layer. This leads to high relative humidity and even condensation, during clear nights. Other abiotic factors that seem also to influence the risk for growth are surface color and rendering structure. From the temperature and relative humidity measurements in this study, we aim at giving insight of the surface climate on a building façade in order to fully understand the rendered façade as a biological habitat.