Many who work in the wood products industry know that trees are comprised mostly of water, more than 50 percent in some species. Because of this, it is important to understand moisture and how it affects the products our industry produces.
Wood is a hygroscopic material, which means it can absorb or lose moisture. When wood absorbs moisture, it expands; when wood loses moisture, it shrinks.
Moisture can exist in wood as liquid water or water vapor in the cell cavities of the wood. This is called free water. Moisture also can exist in wood as water chemically held within the cell walls. This is called bound water.
The moisture content at which the cell walls are saturated completely, but no water exists in the cell cavities, is called the fiber saturation point (FSP). FSP usually is between 25 percent to 30 percent, depending on the species. Wood fibers are dimensionally stable when the moisture content is above the FSP. When below the FSP, all moisture gained or lost is bound water. As the wood gains or loses bound water, the dimension of the wood begins to change.
Wood also is an anisotropic material, meaning it shrinks and swells differently in each direction. Wood shrinks and swells the most along the circumference of the growth rings (tangentially), about half as much perpendicular to the growth rings (radially), and only in miniscule amounts along the grain (longitudinally). The average range for shrinkage for most wood species from FSP to oven-dry is 5 percent to 15 percent tangentially, 2 percent to 8 percent radially, and .1 percent to .2 percent longitudinally. In some species, juvenile wood from near the center of the tree, called pith, can shrink up to 2 percent or more from FSP to oven-dry.
The moisture content (MC) of wood is measured as the weight of the water in the wood expressed as a percentage of the weight of the wood itself. The weight of the wood is determined by drying the wood to a point where all of the moisture is removed. This is referred to as oven-dried.
This MC affects other properties of the wood, such as its weight, shrinkage, and strength. In living trees, for example, the MC may be as high as 200 percent of the weight of the wood. When wood is harvested and milled, however, it is dried to a MC for its end use. This typically ranges from 6 percent to 9 percent.
The determination of MC is an essential part of quality control, particularly when it comes to wood flooring. It is important to know how much humidity is in the space, as well as the MC of the wood flooring, doorframes, moulding, and substrate. The MC of wood flooring should be adequately aligned with the environment where it will be installed. Each manufacturer should provide temperature and humidity requirements or recommended practices for acclimating their wood. The MC of the flooring and subflooring should be tested and verified using an appropriate meter or test method. A thermo-hygrometer should be used to measure the temperature and humidity in the room. The humidity of the environment also is affected by the region where the jobsite is located. Some areas will be more humid, whereas others will be more arid. All of this testing provides a comprehensive understanding of how the wood flooring will perform in its environment.
The MC of wood below the FSP is a function of both the relative humidity and temperature in the surrounding environment. Optimal performance is achieved when wood is neither gaining nor losing moisture, which is referred to as its equilibrium moisture content (EMC).
Wood performs best when the interior environment is controlled at recommended levels, and the wood is installed at a MC corresponding to the interior conditions. Most wood flooring manufacturers dry their flooring to 6 percent to 9 percent MC, which directly coincides with a relative humidity range between 30 percent to 50 percent and a temperature range between 60°F to 80°F found in most indoor environments.
Dimensional stability is the degree to which a piece of solid wood maintains its original dimensions when subjected to changes in MC. How this change affects wood depends on the species and the grain angle at which the board was cut from the log.
Dimensional Change Coefficient (DCC), published by the Forest Products Laboratory of the U.S. Department of Agriculture, identifies how dimensionally stable a specific species is, measured as tangential shrinkage or swelling within normal MC limits of 6 percent to 14 percent.
The DCC provides a way to calculate approximately how much a particular species of wood may respond to changes in MC. This is helpful for determining species suitability, planning appropriate expansion, and maintaining appropriate climate conditions.
The National Wood Flooring Association has detailed information about moisture content and wood available through NWFA University, an online training platform that is convenient and affordable. More information is available at nwfa.org.
