Wood undergoes changes in size and shape due to variations in moisture content and temperature. Understanding these changes is important for anyone working with wood, from construction to furniture making, to ensure the longevity and stability of wooden products.
Moisture: The Main Driver of Wood Expansion
Wood is a hygroscopic material, meaning it readily absorbs and releases moisture from the surrounding air. This behavior is due to its cellular structure, which contains components that attract and bind to water molecules. When water molecules enter the cell walls, they cause the cell walls to swell, leading to the overall expansion of the wood.
The amount of moisture wood holds depends on the relative humidity and temperature of the air around it. Wood will continue to exchange moisture with its environment until it reaches a state of equilibrium, known as the equilibrium moisture content (EMC), where it balances with the surrounding atmospheric conditions. Changes in moisture content below the fiber saturation point—the point at which cell walls are fully saturated but cell cavities are empty—are what cause dimensional changes in wood.
Temperature’s Subtle Influence
Temperature also plays a part in wood’s dimensional changes, though its impact is less significant than that of moisture. Wood generally expands when heated and contracts when cooled, a process known as thermal expansion. In wood, this occurs because increased temperature causes a slight increase in volume.
However, wood’s thermal expansion coefficient is considerably lower than its hygroscopic expansion coefficient. When moist wood is heated, it tends to expand due to thermal expansion but also shrinks due to moisture loss. Unless the wood is very dry, the shrinkage from moisture loss often outweighs the thermal expansion, resulting in a net contraction rather than expansion.
Understanding Wood Movement: Direction and Impact
Wood does not expand or contract uniformly in all directions. Wood movement is most pronounced tangentially (parallel to the growth rings), followed by radially (perpendicular to the growth rings), and is almost negligible longitudinally (along the grain). This differential movement occurs because of the arrangement of wood fibers and the tree’s growth rings.
Uneven wood movement can lead to various issues. Warping, which includes bowing (a warp along the length of the face), cupping (a warp across the width of the face), and twisting (a distortion where ends do not lie on the same plane), commonly arises from uneven moisture absorption or release. Cracking and splitting can also occur as wood shrinks excessively, particularly if the moisture content drops too rapidly.
Practical Steps to Manage Wood Expansion
Managing wood expansion involves controlling its moisture content and allowing for its natural movement. Acclimatization is an important step, requiring wood to adjust to the temperature and humidity of its intended environment before installation or use. This process helps the wood reach its equilibrium moisture content, minimizing subsequent dimensional changes.
Applying sealants and finishes, such as paint, varnish, or oil, can slow the rate at which wood exchanges moisture with the air. While these finishes do not completely prevent moisture absorption, they create a barrier that reduces rapid fluctuations, mitigating extreme expansion or contraction. Another strategy is to incorporate expansion gaps during installation, particularly for flooring. These spaces allow wood to expand without buckling or cupping.
Controlling indoor humidity is also important for preserving wood products. Maintaining relative humidity levels between 35% and 60% can help stabilize wood’s moisture content, preventing excessive swelling or shrinkage. Using humidifiers in dry conditions and dehumidifiers in humid environments can help regulate indoor air moisture. Selecting wood species known for their dimensional stability, such such as mahogany or teak, can reduce the likelihood of significant movement.