The subject building of this case study had high counts of actinobacteria in insulation layer of sandwich facade and in indoor air of specific flats. The aim was to resolve the air draft routes that might have conveyed microbes from external wall indoors, and find and prevent a cause to high microbial contamination in a specific flat. The study included pressure difference measurements, airflow measurements, microbial air samples, structural investigations and follow up measurements. The main contamination routes were located to the bushings in the exterior wall and after reparations the indoor air actinobacteria counts were clearly decreased.