Wood is a natural, porous material classified as hygroscopic, meaning it absorbs moisture from its surrounding environment. This exchange is governed by the relative humidity and temperature of the air, which determines the wood’s moisture content. When wood becomes significantly wet, the excess moisture initiates a dual process: immediate physical distortion and, if saturation is maintained, long-term biological degradation.
The Physics of Water Absorption and Swelling
Water enters the cellular structure of wood through two primary mechanisms: capillary action and molecular diffusion. Capillary action draws liquid water into the large, hollow spaces within the cells, known as lumens, which is referred to as “free water.” Molecular diffusion causes water vapor to be absorbed into the cell walls themselves, becoming “bound water.”
The wood’s dimensional stability is dependent on the amount of bound water present in the cell walls. A threshold known as the Fiber Saturation Point (FSP) is reached when the cell walls are completely saturated with bound water, but the cell lumens still contain no free water. This point occurs when the wood’s moisture content is between 28% and 30%.
Swelling and shrinkage only occur when the moisture content fluctuates below the FSP. As the cell walls absorb water, they expand, and as they release it, they contract. This movement is not uniform in all directions, a phenomenon known as anisotropy.
Swelling is most pronounced across the grain, occurring tangentially (parallel to the growth rings) by about 8% from a dry to a fully saturated state. It is approximately half as much in the radial direction (perpendicular to the growth rings), around 4%. Longitudinal swelling, along the length of the wood, is minimal, often less than 0.5%. This differential movement creates internal stresses, leading to damage such as warping, cupping, and the development of checks or cracks.
Biological Consequences: Mold, Mildew, and Fungal Decay
When the wood’s moisture content remains high for an extended period, it allows biological organisms to flourish. Fungi, including both mold and decay species, requires oxygen, a suitable temperature, and a moisture content generally above 20% to become active.
Mold and mildew are surface fungi that primarily consume simple sugars and starches present on the wood’s surface. These organisms usually cause discoloration, such as staining, but do not significantly compromise the structural integrity of the wood itself. Mold growth can lead to health issues for occupants, including respiratory problems like asthma and various allergic reactions.
Decay fungi cause the most serious damage by actively breaking down the wood’s structural components: cellulose, hemicellulose, and lignin. They require a prolonged period of moisture content above 20%, though colonization often begins above the FSP. Decay is broadly classified into different types based on the wood components they digest.
Brown rot fungi attack and consume cellulose and hemicellulose, leaving behind a dark, crumbly residue composed mostly of lignin. This causes the wood to shrink and fracture into small, cubical pieces, resulting in substantial strength loss even early in the decay process. White rot fungi consume all three components—cellulose, hemicellulose, and lignin—often leaving the wood with a white, spongy, or stringy texture. Soft rot is a third, less common type that occurs in harsh environments, creating microscopic cavities near the surface.
Practical Steps for Drying and Prevention
Mitigating the damage from wet wood involves carefully controlling the drying process to prevent further physical distortion. Rapid drying, especially with high heat, should be avoided because it can intensify the uneven shrinkage between the surface and the core, leading to severe checking and cracking. The process must be slow and controlled to allow the moisture gradient to equalize gradually.
The most effective drying techniques involve increasing air circulation and reducing ambient humidity. Using fans to promote airflow across the wet surfaces and dehumidifiers to pull moisture from the air can significantly hasten the process. Wood should be stacked off the ground and separated with “stickers,” small wooden strips that ensure air circulates around all sides of the material.
Preventing moisture absorption is simpler than remediation. Protective measures include applying water-repellent preservatives, paints, or sealants to the wood surface to block liquid water intrusion. Ensuring proper drainage away from wooden structures and maintaining low indoor humidity levels are important preventative steps. By controlling the environment and protecting the surface, the wood’s moisture content can be kept below the 20% threshold, inhibiting fungal decay.