Biological growth on rendered façades

Detta är en avhandling från Lund University, Division of Building Materials

Sammanfattning: Biological organisms have an incredible ability to adapt to almost any environment and the humans activities on earth have created many new habitats for different kinds of organisms. For example can certain organisms grow on rocks and vertical cliffs, and when humans started building houses with mineral based façades, some organisms found that these were new habitats to live on. Some of these expansions of habitats to our houses are not desirable for the us humans and are considered as “contaminations". Even if this contamination sometimes only is an aesthetically problem, some contamination is highly unwanted because it can be unhealthy for the inhabitants - for example the growth of moulds - or it can degrade the building materials it grows on -as for example wood-degrading fungi. For an organism to grow in a certain environment, different requirements on abiotic (physical and chemical) and biotic (biological) factors have to be fulfilled. Suitable conditions for growth of organisms on façades are certain ranges in temperature and a high moisture level (RH), but also the surface structure, nutrient availability, pH, cardinal direction etc. might be influencing. Different organisms have different demands on these factors and it is a complex interaction of these different factors that decides if an organism can grow in a certain environment. The last decades many houses in Sweden have been built with constructions of thin rendering on thermal insulation, a so called ETICS construction (External Thermal Insulation Construction System). This construction consist most often of a framework of wooden studs with thermal insulation in between, and gypsum boards or cement based boards on both sides. On the outside a thermal insulation layer is applied and the render is then applied directly on the outside of this thermal insulation layer. This is a an efficient and compact construction which is easy to produce. However, many of these constructions have experienced discolourations from growth of algae and moulds on the façades already a few years after construction. It has not always been possible to explain this discolouration. Sometimes one part of the façade had discolorations and another part of the same façade did not. One possible explanation for the fast growth of organisms is the external rendering layer (on thermal insulation that has a low heat capacity and during night the long-wave radiation from the material to the sky can contribute to a lower temperature on the surface than the temperature in the air -on clear nights, when the heat loses through long-wave radiation is high. The lowered surface temperature then causes the RH on the surface to increase, sometimes giving condensation -which increases the risk of biological growth. In this project we have compared temperatures and RH on surfaces on façade elements in a test house with constructions with low heat capacity in the outermost layer (light walls) and constructions with a high heat capacity in the outermost layer (heavy walls). Simulations of the growth risk showed that thin rendering on thermal insulation has a higher growth risk that traditionally render on bricks especially on the north side. On the south side the most important factor was the surface colour. In our study we compared a red and a white surface, and since dark surface colours absorbs more short-wave radiation from the sun they have a higher temperature during daytime and therefore a lower RH on the surface. Another factor which might influence the growth risk is the surface structure of the render. We fabricated specimens with different renders with different surface structures and with a thin and thick rendering layer (3mm and 20mm, respectively) and exposed the specimens outdoors for four years. This study showed that algaes preferred a very rough surface structure while moulds (Cladosporium sp.) also grew on more smooth surfaces. In addition we found that algaes most often grew on the north side whereas moulds rather grew on the south side (Cladosporium has a dark pigment in the cells which protects against radiation from the sun). Furthermore we found a connection between the amount of growth and the season of the year. The biological growth was more clearly seen during spring and especially autumn and occasionally seemed to disappear during summer and winter. It was found that thin (3-4mm) and thick (20mm) render on thermal insulation had the same amount of discolouration. The activity of photosynthetic organisms -algae, lichens and mosseson façades can be measured with Imaging-PAM. This is an instrument that measures the chlorophyll fluorescence and gives an indirect measure of photosynthetic activity. A pilot study was performed where we -during three days in the autumn- studied algae and mosses growing on render. Algae dries out easily and is dependent of moisture from the surroundings and showed the highest activity during mornings before the sun dried them out. The mosses were active a greater part of the day; they are able to some extent store water in their leaves and is not as dependent on moisture from the surroundings as algae. Another method for measuring activity of biological organisms is isothermal calorimetry which measures the produced heat from an organism’s metabolism. In this project we tested a new type of calorimeter that measures activity at four different temperatures at the same time. With measurements of a moss (Tortula ruralis) we found that it was possible to get an activity measure at four different temperatures at the same time, thus being able to get an understanding of how the temperature influences the activity. This method should therefore be very useful in future studies of activity of different types of biological organisms. The aim of this project was to investigate constructions of thin rendering on thermal insulation and the biological organisms growing on the façades of these constructions. With a multidisciplinary approach we have increased the knowledge of the façade as a habitat, the organisms growing, and their interactions with different biotic and abiotic factors.