Water leakages are the general cause of moisture stress exceeding the
moisture tolerances of constructions. Moisture stress causes biodeteri
oration (mould, rot and insect damage) in wooden constructions. The de
gree of damage is affected by the severity and duration of moisture ex
posure and temperature, together with the sensitivity of the wooden co
nstruction to environmental stress. The growth of mould fungi on woode
n materials depends mainly on the air humidity, temperature and the su
rface quality of wood. Long term humidity conditions above a relative
humidity (RH) of 80% are a risk for mould growth in pine and spruce sa
pwood when the temperature is between +5-degrees-C and +50-degrees-C.
Between -5-degrees-C and +5-degrees-C the growth of mould fungi is slo
w, and possible only when the relative humidity is above 90%. High nut
rient content of the surface can stimulate the mould growth. In fluctu
ating humidity conditions, the severity and cumulative time of conditi
ons suitable for mould growth are critical factors for the development
of mould fungi in constructions. Humidity conditions must be above RH
95% (around the fibre saturation point of wood, wood moisture content
about 25-30%) for the spore germination and mycelium activation of br
own rot fungi. Under unsuitable conditions, fungi are inactive, but th
ey can become active quickly under suitable conditions. The degree of
decay in wood depends on the total time the decay organism is active.
The natural durability of wood varies with the environmental condition
s, wood characteristics (wood species, sapwood or heartwood, etc.) and
the decay organisms. The service time of wooden materials can be exte
nded by modification of the wood.