Architects want the maximum of design freedom, especially with respect to the outer shell of buildings. This is why they like to employ glass elements that directly adhere to the underlying composite thermal insulation system. In this way, they can create a thin building envelope, but such systems impose special demands in terms of building physics. The impermeability of the glass facade to water vapor can cause moisture to accumulate in the adhesive layer, increasing the risk of frost damage. Researchers at Fraunhofer IBP have conducted tests on such wall structures for over three years in the institute’s open-air test facility. Five different insulating materials were superimposed on glass elements of different colors, two based on diffusion-open sheets of mineral wool, and three with a damp-proof course of rigid EPS foam boards, phenolic resin boards, or vacuum insulation panels. The temperature and humidity in the gap between the glass element and the insulation was measured, first by means of capacitive humidity sensors and second by means of strips of wood placed on the insulation material, which deliver reliable results even at high humidity levels.
Higher temperatures than under plaster of the same color
During the winter season, the researchers maintained the interior of the building at a temperature of around 20 °C and a relative humidity of 50 percent. The temperature curves show the daily and seasonal fluctuations Whereas as the temperature behind clear glass panels ranged between −15 °C and 65 °C, it sometimes rose to as high as 90 °C behind dark glass panels. A noteworthy finding was that the temperatures measured behind the glass panels tended to be higher than those measured under plaster layers of the same color. This can be explained by the presence of a paint coating on the rear surface of the glass panels, with the result that solar heating mostly takes place behind the exterior surface.
Huge variations in moisture content
The five systems also demonstrated wide divergences in their moisture content. The lowest levels were recorded for the vapor-permeable mineral-wool variants. Contrary to expectations based on mathematical calculations, all five systems became drier as years went by. This seems to indicate that the systems were not completely vapor-tight to start with. In conclusion, the accumulation of moisture in the glass elements appears to be less critical than expected, reducing the probability of frost damage. The visual appearance of the glass facades didn’t deteriorate either over the three-year test duration – with the exception of minor discoloration of the silicon joints.