**4.1 Causes for decay damages in buildings**

The excessive water into the building structure and materials is the basic cause to different bio-deterioration problems like decay. For instance, in washrooms water often penetrates through inside surfaces or pipe leakage into the structures causing long lasting high humidity conditions. In old wooden buildings, the floor has often been built above a cold ventilated basement or crawl space, where high humidity conditions may exist. If water is penetrated in the crawl space, the ventilation may not keep the floor dry and mould growth is obvious. If ventilation caps are closed, severe decay problems have been found, e.g. dry rot damage (Paajanen and Viitanen 1989, Kääriäinen et al. 1998).

In connection of the decay, also microbial contamination on the surfaces in the crawl space is typically much higher than inside the building. The level of fungal spores in the crawl space is about ten times as high as indoors. In crawl spaces, spore concentrations in a range of 103-105 colony-forming units per gram (cfu/g) of material are common. The levels have usually been highest on wood-based boards and on timber (Hyvärinen et al. 2002). In cases of heavy fungal colonisation, airborne spore concentrations of up to 103-104 cfu/m3 have been detected.

The slab-on-ground structure without thermal insulation below the concrete slab has been used in old buildings. This type a floor is very sensitive for water damage and microbial growth. Especially in detached houses built between 1960 and 1980, wooden beams are often supported on concrete slabs on grade. Partial decay or insect damage is often found in the lower sill plate of exterior walls due to water penetration from the basement (Kääriäinen et al. 1998).

Decay is the more severe result of high moisture exposure of wooden structures when the materials are wet for long periods. According to laboratory studies, the growth of decay fungi and decay development can start when the ambient humidity level in the microclimate remains for several weeks above RH 95 – 100 % and moisture content of pine sapwood above 25 – 30 % (Viitanen and Ritschkoff 1991b, Morris et al 2006). According to experience, decay will develop when moisture content of wood exceeds the fibre saturation point (RH above 99.9 % or wood moisture content 30 %, but also the variation of conditions and temperature has an important effect.

### **4.2 Modeling the decay development**
