This report summarizes basic wood-moisture relationships, and reviews conditions conducive to adverse consequences of wetting, such as staining, mold growth, decay, strength reduction, and dimensional change and distortion. It also outlines solutions and available resources related to on-site moisture management and design measures. Sorption, including desorption (i.e., loss of moisture) and adsorption (i.e., gain of moisture), is the interaction of wood with the water vapour in the ambient environment. The consequent changes in the amount of bound moisture (or “hygroscopic moisture”) of pre-dried wood affect the physical and mechanical properties. However, the core of a mass timber responds slowly and is well protected from fluctuations in the service environment. Mold growth and fungal staining may occur in a damp environment with a high relative humidity or sources of water. Sorption alone does not increase the moisture content (MC) of pre-dried wood above the fibre saturation point and does not lead to decay. Wood changes its MC more quickly when it absorbs water compared with sorption. This introduces free water (or “capillary water”) and increases the MC above the fiber saturation point. Research has shown that decay does not start below a MC of 26%, when all other conditions are favourable for fungal growth. Decay can cause significant strength reduction, for toughness and impact bending in particular. For a wood member in service, the effect of decay is very complicated and depends on factors, such as the size of a member, loading condition, fungi involved, location and intensity of the attack. Appearance of decay does not reflect true residual stiffness or strength. For wood-based composites severe wetting without decay may affect the structural properties and performance due to damage to the bonding provided by the adhesive inside.
There are large variations among wood species, products and assemblies in their tendency to trap moisture and maintain durability. For a given wood species, the longitudinal direction (vs. the transverse directions) and the sapwood (vs. heartwood) absorb water more quickly. Capillaries between unglued joints (e.g., some CLT, glulam), exposed end grains, and interconnected voids inside a product increase the likelihoods of moisture entrapment, slow drying, and consequently decay. Many mass timber products, composites in particular, may be modified to reduce these issues. Measures should also be taken in design, during construction, or building operation to reduce the moisture risk and increase the drying ability. It is also important to facilitate detection of water leaks in a mass timber building and to make it easier to repair and replace members in case damage occurs. Preservative-treated or naturally durable wood should be used for applications that are subjected to high moisture risk. Localized on-site treatment may be appropriate for specific vulnerable locations. Changing environmental conditions may cause issues, such as checking, although it does not compromise the structural integrity in most cases. Measures may be taken to allow the timbers to adjust to the service conditions slowly (e.g., through humidity control), particularly in the first year of service.
Overall there is very little information about the potential impacts that various wetting scenarios during construction and in service could realistically have on mass timber products and systems. The wetting and drying behaviour, impacts of wetting and biological attack on the structural capacity, and the behaviour under extreme environmental conditions, such as the very dry service environment that occurs during the winter in a northern continent, should be assessed to improve design of mass timber buildings.