Evaluation of the performance of different internal insulation systems in real-life conditions ‐ A case study

Thermal retrofitting of historic buildings is essential to reducing heat loss in European buildings. Although exterior insulation often is hygrothermally a better solution, internal insulation is the only option in facades worthy of preservation (e.g., solid masonry external walls). However, internal insulation might involve hygrothermal risks for the original walls. Mold growth has frequently been discovered in buildings with internal insulation at the interface between the insulation and the original wall, while simulations and lab tests show minimum risk, and vice versa. Therefore, real-life testing is required so that the building owners are more likely to accept these measures.
This paper presents a case study of an 1837-built typical historic structure. The study focuses on the building's top floor, a residential area (commune). Eight different rooms have four different types of internal insulation installed. The performance of the insulation systems was determined by monitoring the interior and exterior climate and the temperature and relative humidity at the intersection of the insulation and the existing wall for 20 months. The risk of mold growth was calculated based on the measurements. In the wall interfaces, the Mold Index indicated that the risk for mold growth is not severe, if the materials can be assumed resistant to mold growth. Hygrothermal simulations were also performed. To increase simulation accuracy, the original exterior masonry bricks underwent laboratory testing to determine their precise properties. The main outcome from the simulations was that vapor-open systems are more susceptible to indoor moisture load changes and might demonstrate elevated risk of mold growth if they cannot be assumed resistant to mold growth, whereas vapor-tight systems are more tolerant of insulation thickness and internal moisture level changes, and in this case demonstrated the best performance in respect to moisture safety.