Most buildings in Europe were constructed between 1850 and 1960, a period in which energy efficiency was not considered much. However, many of these structures have architecturally valuable facades that should be preserved, making internal thermal insulation the only practical solution. Therefore, internal insulation has grown in popularity despite the potential risk of moisture-related problems behind the insulation and on the external surface of the original wall. To ensure the structure's durability and the residents' wellbeing, insulating solutions must undergo real-life testing to demonstrate their effectiveness and moisture safety. The present paper is a compilation of case studies performed over the last decade in Denmark, it compares across four case studies of residential buildings. The apartments were insulated internally with either diffusiontight or diffusion open and capillary active insulation systems. In some of the cases, internal insulation was applied in combination with hydrophobization of the existing facade. Temperature and relative humidity were measured in the indoor climate, at the intersection between insulation and masonry, and in some cases also at the wooden beam ends or the spandrels. Additionally, the risk of mold growth was calculated. The current study focuses on the wall's hygrothermal performance in relation to the thickness of the masonry and insulation, the wall's orientation, the indoor moisture excess, the effects of hydrophobization and the role of indoor climate. The results indicate that - while internal insulation of masonry more or less result in expected energy savings - areas with thin masonry or very thick insulation give an increased risk for mold growth. In addition, high indoor moisture excess in combination with diffusion open insulation system increases the risk for mold growth, while the other parameters played a less decisive role.
