Energy retrofitting is performed to reduce the energy demand from existing buildings. Different measures can be implemented; however, older buildings will likely never live up to the demands of new construction. Nevertheless, the potential for energy savings must be pursued. Additional insulation is a retrofitting measure that can reduce heat loss through the building envelope and thereby provide energy savings. Internal insulation is introduced when external insulation, a more robust and moisture-safe measure, is not possible. Besides energy savings, internal insulation can provide increased thermal comfort. However, the indoor climate may suffer from internal insulation, if not designed, installed, and managed properly. Internal insulation increases the risk of moisture accumulation behind the insulation, which increases the risk of mold growth and other degradation processes. The current paper presents a case study of a building complex from 1961, where six adjoining apartments were internally insulated in 2015. The external walls were 360 mm (14.2 in.) thick, consisting of yellow brick (externally) and lightweight clinker concrete (internally). The walls were insulated with 50 mm (2.0 in.) (façades) and 80 mm (3.1 in.) (gable) insulation with a semi-diffusion tight and supposedly capillary active system, while the wall in the stairwell (north) was insulated with 50 mm (2.0 in.) of diffusion open and capillary active calcium silicate. For 6 years, the hygrothermal conditions were monitored in the interface between wall and internal insulation, where there is high risk of moisture accumulation. Measurements were performed in both northern (façade), southern (façade), and western (gable) orientations. The paper presents methods used, results from the in-situ measurements, and analysis hereof. The results show acceptable hygrothermal conditions at the interfaces and very little risk of mold growth. Furthermore, 6-12 months after applying internal insulation, the built-in moisture was dried out. Energy retrofitting is performed to reduce the energy demand from existing buildings. Different measures can be implemented; however, older buildings will likely never live up to the demands of new construction. Nevertheless, the potential for energy savings must be pursued. Additional insulation is a retrofitting measure that can reduce heat loss through the building envelope and thereby provide energy savings. Internal insulation is introduced when external insulation, a more robust and moisture-safe measure, is not possible. Besides energy savings, internal insulation can provide increased thermal comfort. However, the indoor climate may suffer from internal insulation, if not designed, installed, and managed properly. Internal insulation increases the risk of moisture accumulation behind the insulation, which increases the risk of mold growth and other degradation processes. The current paper presents a case study of a building complex from 1961, where six adjoining apartments were internally insulated in 2015. The external walls were 360 mm (14.2 in.) thick, consisting of yellow brick (externally) and lightweight clinker concrete (internally). The walls were insulated with 50 mm (2.0 in.) (façades) and 80 mm (3.1 in.) (gable) insulation with a semi-diffusion tight and supposedly capillary active system, while the wall in the stairwell (north) was insulated with 50 mm (2.0 in.) of diffusion open and capillary active calcium silicate. For 6 years, the hygrothermal conditions were monitored in the interface between wall and internal insulation, where there is high risk of moisture accumulation. Measurements were performed in both northern (façade), southern (façade), and western (gable) orientations. The paper presents methods used, results from the in-situ measurements, and analysis hereof. The results show acceptable hygrothermal conditions at the interfaces and very little risk of mold growth. Furthermore, 6-12 months after applying internal insulation, the built-in moisture was dried out.