This paper presents a real-scale experimental test field (3.0 m × 3.2 m) which consists of red brick masonry (29 cm) and hydrophobic mineral wool (16 cm) on the interior side. The interior air is conditioned, while the external side of the wall is exposed to real climatic conditions representative to colder months at semi-continental climatic locality. Since the masonry is cold and less permeable for water vapor than mineral wool high relative humidity occurs at the interface of materials. In colder periods, the relative humidity is sufficient for mold growth on organic materials. To investigate mold development, small plates from pine and spruce sapwood (30 mm × 50 mm × 3 mm, tangential surface) were inserted into slots glued on the internal surface of masonry. Half of specimens was inoculated in the microbial laboratory. The second half of specimens was stored in contact with interior air so that natural inoculation was possible. Visual inspection of wood specimens by naked eyes, photographic documentation, Mycometer fungi tests, identification of molds, and microscopic observations of collected wood samples were conducted. Preliminary results from the first half of cold season 2024/2025 are presented. Measured hygrothermal conditions at the location of wooden plates, mold growth observations, the results of Mycometer fungi test, and mold growth calculated by a mathematical model are presented and discussed.