Damp walls take a long time to dry out, and during this period their increased thermal conductivity leads to higher energy consumption. The drying rate is influenced not only by the type of masonry units but also by the type of thermal insulation and the presence of internal wall coverings such as ceramic tiles. To obtain more detailed information on the drying behavior of exterior walls and the factors in play, researchers at Fraunhofer IBP set up models of various wall structures with different types of insulation and tested them under real-life conditions. Given that, in the worst case, the drying process can last more than ten years, the measurements were complemented with long-term hygro-thermal simulations.
The reference case used in these tests to determine residual humidity was a newly constructed wall made of sand-lime brickwork with a thickness of 24 cm and an external, composite thermal insulation system consisting of mineral fibers and expanded polystyrene (EPS). Extreme drying conditions were tested using a wall built of waterlogged cellular concrete blocks with a thickness of 30 cm, with a composite, external thermal insulation system consisting of mineral fibers combined with EPS and extruded polystyrene (XPS), and various rendering products. To simulate renovation projects, tests were performed on a weathered, 40-cm thick solid brick wall with a mineral fiber and EPS external thermal insulation system. All outer walls were tested in the open air under real-life weather conditions. Drilled core samples were extracted to determine the actual moisture content of the various experimental structures. Hygrothermal simulations were used to extrapolate data from the measured moisture content in the different masonry structures. Based on this data, the researchers were able to predict long-term drying patterns.
The moisture profiles showed that walls insulated with EPS tend to dry from the outside in, whereas walls insulated with mineral wool evacuate a greater portion of their trapped moisture to the outside environment. All in all, walls insulated with products based on mineral fibers dried out within 18 months, whereas walls insulated with EPS took twice as long to reach a stable state. During the drying phase, heat loss through the building fabric is higher due to greater transmissivity, and also because rooms need to be aired more frequently to evacuate the excess moisture in the newly built walls. In the case of sand-lime brickwork, which has poorer insulation properties, the residual moisture resulted in a five-percent increase in the U-value in the first year after construction. By contrast, walls made of lightweight porous bricks with a composite thermal insulation system lose 25 percent of their thermal efficiency in the first year.