The Movement of Wood
Protecting Your Projects
Wood’s natural response to humidity changes is a critical consideration in woodworking, architecture, and construction. As a hygroscopic material, wood absorbs and releases moisture from the air, causing it to expand and contract. Here’s how this works and what it means for wood applications:
How Wood Reacts to Humidity
1. Absorption and Expansion: When the humidity increases, wood absorbs moisture from the air. This moisture causes the wood fibers to swell, leading to expansion. Wood typically expands more tangentially (across the growth rings) and radially (along the growth rings) than longitudinally (along the grain).
2. Release and Contraction: Conversely, when the humidity decreases, wood releases moisture into the air, causing the fibers to shrink and the wood to contract. This contraction can sometimes lead to gaps, warping, or cracking if not properly accounted for in design and installation.
Factors Affecting Wood Movement
Wood Species: Different species of wood have varying degrees of movement. For example, hickory and oak generally exhibit more movement with humidity changes than more stable woods like teak or mahogany.
Cut of the Wood: How wood is cut from the log affects its movement. Quarter-sawn wood, where growth rings are more perpendicular to the board’s surface, tends to expand and contract less and more evenly than flat-sawn wood, where rings are parallel to the surface.
Initial Moisture Content: The condition of the wood at the time of installation also matters. Properly dried wood, acclimated to the environment in which it will be used, will behave more predictably and stable.
Managing Wood Movement
To manage wood’s response to humidity changes:
Allow for Expansion: In construction and cabinetry, it’s essential to leave room for wood to expand and contract. This can mean using floating joints, leaving expansion gaps in flooring, or designing furniture in a way that accommodates movement.
Use Stable Materials: Engineered woods like plywood or oriented strand board (OSB) are designed to have reduced movement and can be more stable in fluctuating humidity.
Control Environment: In spaces where wood is used extensively, controlling the environmental humidity through HVAC systems can reduce the amount of expansion and contraction.
Finish and Seal: Applying sealants or finishes to wood can help reduce the rate at which it absorbs and loses moisture, though it won’t stop the movement entirely.
Understanding and planning for wood’s hygroscopic properties can greatly enhance the longevity and performance of wood products in any setting, ensuring they hold up well against environmental variables.
