Moisture-Related Properties of Wood and the Effects of Moisture on Wood and Wood Products

MOISTURE IS ARGUABLY THE MOST IMPORTANT factor affecting the performance and service life of wood and wood products. Moisture affects the dimensional movement of wood and wood products; under certain conditions, mois­ ture change can result in major dimensional change. The in­ tegrity and strength of adhered (bonded) wood products can be compromised by swelling-induced stresses that accom­ pany wetting. Progressive deflection over time of wood members under load is influenced by moisture conditions, particularly by large repetitive fluctuations in moisture con­ tent. Mechanical connections between wood members can be compromised by exposure to elevated moisture condi­ tions or by significant moisture cycling. It is widely recog­ nized that the structural integrity of wood can be irreversibly degraded by biological attack. In some cases biological infes­ tation does not influence structural integrity but neverthe­ less influences serviceability. For many insect pests and all fungi, moisture conditions higher than the preferred inservice conditions are either required for infestation, or in­ crease the likelihood of infestation. Although wood, wood products, and wood construction can be degraded by elevated moisture levels or by greatly fluctuating moisture conditions, the vast majority of resi­ dential structures built in North America over the past three centuries were constructed primarily of wood, and most of these have performed reliably. Wood and wood products dried to an appropriate level, and maintained within a rea­ sonable range of fluctuating moisture conditions will per­ form nearly indefinitely. In contrast, wooden buildings con­ structed without consideration of moisture control may rapidly suffer moisture-induced damage, leading to exces­ sive repair and maintenance costs; in extreme cases the dam­ age may even justify premature demolition. The central topic of this chapter is how moisture affects the properties and behaviors of wood and wood-based prod­ ucts used in building construction. Physical properties and behaviors are discussed, as are structural behaviors, and what may be considered biological behaviors (specifically, the likelihood of biological infestation by microbes and in­ sects). The order of discussion is that outlined in the previ­ ous sentence, namely physical first, then structural, then biological. Before the chapter delves into its central topic, it provides background information on wood and on adhered (bonded) wood products. The information presented on wood includes discussion of wood structure, composition and basic characteristics, with an emphasis on structure. The information presented on bonded wood products fol­ lows a similar discussion path, but inasmuch as these are manufactured products, the emphasis is on product classifi­ cation, composition, fabrication, and characteristics. Infor­ mation concerning contemporary wood products is pre­ sented, as is information on wood products produced in past decades. The intent is to provide information applicable to buildings of various ages, not just to recently-constructed buildings. The chapter also presents recommendations and guide­ lines for in-service moisture content. The recommendations are reiterated values from the literature, and take the form of not-to-exceed values. The recommendations are applicable to prevention of biological infestation; there is evidence that these recommended values also roughly correspond with values that have structural performance implications, irre­ spective of biological infestation. The guidelines, in contrast to the recommendations, relate to limitation of fluctuation in moisture content. The guidelines provide a basis that al­ lows the reader to develop their own project-specific limita­ tions on moisture fluctuation.