Biological growth on mineral façades

Sammanfattning: During the last years there has been reported considerable and rapid contamination on newly built rendered façades in Sweden due to biological growth. The problem is believed to occur mostly on façades of render applied on thermal insulation, but apart from this it is difficult to find clear relationships indication which façades might be infected. The aim of this project is to explain why certain façades gets heavy biological growth while other seemingly similar do not. We will do that with comparing biological knowledge with building physical and -technical knowledge.

One of the major approaches in this project is to investigate building façades. We have investigated several buildings with discolored façades, mostly in Skåne, the southern most part of Sweden, and tried to identify the organisms growing. The most dominating organisms we found growing on the façades were lichens and algae. In addition we have investigated building façades with organized patterns of small round areas where no growth occur on otherwise quite fouled façades. These small round areas appears then on the façade as “white dots”. Further investigations of the façades show that the “white dots” appear at each fastener placed under the rendering to fasten the thermal insulation to the structural wall behind. We believe that the fasteners are heat bridges, with higher outer surface temperature and therefore a lower surface relative humidity at which no microbial growth occurs. Results from computer simulations showed that the fasteners are heat bridges and in cases with thin layers of thermal insulation and rendering give a 1-2 K higher night-time temperatures than the rest of the façade which made a difference in relative humidity at 11%. This small difference in temperature gives high enough differences in moisture state to almost prevent microbial growth above the fasteners. As we know that organisms need a high relative humidity to grow, these results could explain the uneven distribution of biological growth on the façades with “white dots”.

Temperature and relative humidity measurements on façades of thin rendering on thermal insulation (light façades) and brick wall with rendering (heavy façades) shows that light façades are more directly influenced by the weather conditions than the heavy façades. The temperature varied much more on the light façades than the on the heavy façades; in a day of October 2005 the temperature varied from 60.4°C during daytime to -0.4°C during the next night. During nights there is no difference between the lowest temperatures on the light façades on the south- and north-facing façades. The light façades gets colder in the night then the heavy façades due to the lower heat capacity in the construction, whereas the heavy façades with the higher heat capacity can store the heat from daytime during night. The red-colored façades, which had a higher temperature than the white-colored façades, have a significantly lower relative humidity with a difference of 20% relative humidity if it was not fully overcast. The relative humidity during night on the south-facing façades shows that the white-colored heavy façade had a higher maximum relative humidity with approximately difference of 10% relative humidity during night than the red-colored light façade in spite of the higher minimum temperature during night on the white-colored heavy façade.

The results from the temperature measurements above the fasteners show that the fasteners are heat bridges that increase the façade surface a few degrees during night, but also in the winter on the north-facing façade during daytime.

We will continue our measurements of temperature and relative humidity on heavy and light façades in addition with investigations of biological growth on both the above façades and on newly built light façades. Also biological growth will be studied on samples with thin rendering on thermal insulation where we will investigate different abiotic factors on the samples. By this we hope to understand the mineral façade as a biological habitat and learn more about the organisms living there and which abiotic and biotic factors that control the growth.