Crack size in coating and moisture problems comparing thermally modified and native spruce window frame profiles using hygrothermal simulations

Fungal growth and degradation of wood may be caused by damage in the surface coating. The larger the cracks, the greater in principle the possibility of moisture-induced problems. Measuring basic unknown material parameters and employing hygrothermal simulations, the suitability and the maximum acceptable vertical crack size in the surface coating for a given bottom window profile made of thermally modified (TM) spruce(wood) with that made of native spruce were compared for location Ljubljana. Validation with the field test data was the second objective of the respective research. The average calculated maximum moisture content in TM spruce is about 4% (kg/kg) lower than that of native spruce. The 3 mm wide crack in the surface coating of a window frame made of native spruce is of the highest concern, whereas a 9 mm wide crack in the coating of a TM spruce profile is still acceptable. As far as moisture content is concerned in our study the TM spruce window frames were proved to be significantly more suitable for installation than the corresponding frames made of native Norway spruce. It was shown that isopleth, VTT and biohygrothermal models for mould growth do not properly capture the comparison between both materials, mainly because they classify both in the same material class/substrate category and they do not consider the material moisture content.