Microbial growth on thermally insulated façades
For manufacturers of composite thermal insulation systems, microbial growth on thermally insulated façades still poses a problem. A number of different prevention strategies have been discussed and investigated in recent years. The search is on for ways to limit microbial growth using building physical methods if at all possible, as biocidal solutions work for only a limited period of time and are not environmentally friendly. Approaches to solving the problem using building physics are all based on the idea of reducing moisture on the surface of the façade.
A very promising way of reducing condensation buildup on façade surfaces is to use IR (infrared) paints. By adding special pigments, scientists were able to modify the paints in such a way that the emissivity of long-wave radiation was reduced from over 90 percent to below 60 percent in some cases. At the Fraunhofer IBP’s outdoor testing facility, scientists conducted measurement trials on composite thermal insulation systems painted with IR paint (image 1).
Each type of paint is characterized by determining its radiation values. The degree of short-wave radiation absorption describes the proportion of incident sunlight absorbed by a surface in the wavelength range 250 to 2500 nm (ultraviolet, visible, near infrared). The higher the degree of absorption, the more the surface heats up. The lower the surface’s emissivity, the less long-wave thermal radiation is exchanged with the surrounding environment during the day and night. This means that the surface remains warmer during the day and does not cool as much during the night. Long-wave emissivity is determined by measuring spectral reflectance in the wavelength range between 2.5 and 50 nm using a Fourier interferometer (emission level = 1 – reflectance).
Test surfaces coated with both paints are measured and subjected to analysis. The paints are applied to an existing composite thermal insulation systems consisting of 240 mm sand-lime brickwork and 80 mm mineral-wool insulation. Temperature sensors built into the outer layers of plaster measure the surface temperature. To determine the amount of water adhering to the surface, cellulose pads cut to a specific size are first weighed when completely dry, then applied to the façade and pressed firmly onto its surface. Reweighing the pads allows the scientists to determine how much moisture was on the surface. Drying time can be determined by repeatedly blotting the surface – image 2 depicts this being carried out on a wall surface.
The IR paint under examination was shown to have an absorption level of 0.20 and an emissivity of 0.65, while the comparative paint sample was found to have an absorption level of 0.16 and an emissivity of 0.88. In conjunction with climate data collected at the IBP’s own weather station, the surface temperature fluctuation readings were used to calculate the times during which the façade was covered with condensation. By comparing the amount of time (in hours) during which temperatures dropped below the dew point over the course of the one-year test period, the IR paint was found to reduce these condensation periods by up to 13 percent. The difference is even more obvious if one compares the amount of time spent below the dew point weighted by kelvins per hour. Over the course of the year, this equates to a 28-percent reduction using IR paint.
The diagram shows the fluctuations in surface temperature over time for both paints in relation to the dew point for a specific period of time. The surface temperature of the IR paint is significantly higher than that of any conventional paint during both the day and night. Surface moisture levels were measured several times during the testing period. While the surface coated with the control paint repeatedly yielded between 10 and 25 g/m² of condensation collected by blotting, the surface coated with IR paint remained dry. At no point was any condensation collected from the IR paint by blotting.
The tests showed that applying paint with reduced emissivity significantly reduces the time period in which condensation forms on the surface of the façade. Scientists were able to prove this by measuring the surface temperatures and surface moisture levels at their outdoor testing facility.
Computational tests indicated that the length of time during which temperatures drop below the dew point is some 15 percent lower on the south and east sides of a building than on the north and west sides. Since there was hardly any microbial growth on these two sides, it is logical to assume that reducing the condensation period on the surface of a façade by 15 percent is an effective way to prevent microbial growth.