Understanding Moisture Content in Wood

A tree requires moisture to live and grow. The roots help anchor the tree to the ground and also collect water and nutrients from the soil. This liquid then travels through the tree’s trunk and branches to the leaves. Like most biological organisms, trees have a high water content. Water can comprise more than 2/3 the weight of a living tree

In order to use wood as a building material, most of this water must be removed. The natural removal of this water begins immediately after the tree is cut down, but is carefully managed during the seasoning process. This seasoning process includes special handling, air-drying, and kiln drying the lumber to get it to a state in which it can be used for flooring.

The wood used in the flooring industry is usually dried to a moisture content between 6-9 percent, which is representative of a level appropriate for interior use. It is important to understand moisture content and its relation to the function of wood flooring. Weight, shrinkage, strength, and other properties depend on the moisture content of wood.
The moisture content of solid 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 itself is obtained by drying the wood to a point where all of the moisture is removed. This is referred to as oven-dried

Moisture can exist in wood in two forms:

  • Free water: Free water is the moisture held in the cell lumina of the wood. This is the water in the wood above fiber saturation point
  • Bound water: Once all of the free water is gone from the cell lumina, only bound water remains. Bound water is held by chemical or hydrogen bond within the cells of the wood. When only bound water remains, the cells have reached the fiber saturation point.

The fiber saturation point (FSP) is the moisture content at which the cell walls are completely saturated (all bound water), but no water exists in the cell lumina. FSP is usually between 25-30 percent, depending on the species. 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.

The moisture content of wood below the fiber saturation point is a function of both relative humidity and temperature in the surrounding air. In a stable environment, when wood is neither gaining nor losing moisture, equilibrium moisture content (EMC) has been reached. Wood and wood products are hygroscopic. A hygroscopic material is a substance that expands with the absorption of moisture, and dimensions become smaller when moisture is lost or thrown off. When wood gains moisture, it swells, and when it loses moisture, it becomes smaller.

Wood flooring is constantly exposed to both long-term (seasonal) and short-term (daily) fluctuations in relative humidity and temperature of the surrounding air. Thus, wood flooring is always undergoing at least slight changes in moisture content. These changes are usually gradual, and short-term fluctuations tend only to influence the surface of both solid and engineered wood flooring. Longer-term fluctuations can have greater effects on flooring, and in some cases, can cause irreversible damage. These changes can be slowed, but not entirely prevented, by protective coatings. Maintaining consistent temperature and humidity levels year-round will minimize moisture and dimensional changes.

In actual practice, shrinkage and swelling may be diminished by the boards’ proximity to each other, installation methods, fastening systems, and moisture interactions from the substrate. All of these factors can influence how an installed floorboard performs when it changes MC.

The physical effects these changes can have on wood and wood flooring products can vary from one floor to the next, based on a few different variables.

Grain Angle/The Cut:
Wood is an anisotropic material, meaning it shrinks and swells differently in each direction within the wood. Different woods exhibit different moisture stability factors, but they always shrink and swell the most in the direction of the annual growth rings, tangentially, about half as much across the rings, radially, and only in minuscule amounts along the grain, longitudinally. This means that solid plainsawn flooring will tend to shrink and swell more in width that solid quartersawn flooring, and that most solid flooring will not shrink or swell measurably in length. Keep in mind, no two trees from the same species are identical, no two boards from the same tree are identical, and properties can vary even within one individual plank of wood.

Engineered Wood:
Engineered wood flooring exhibits different reactions to these changes in moisture. Each layer (lamina) of engineered flooring runs perpendicular to adjoining layers, which creates a more dimensionally stable product. When exposed to longer-term fluctuations in relative humidity and temperature, each layer begins to react. As one layer reacts in one direction, the adjoining layers react with similar forces in the opposite direction. This can result in shrinking and swelling not only along the width of the boards but along the length of the boards as well.

Moisture Testing:
Moisture content from 5-30 percent may be determined using various moisture meters developed for this purpose. Both types of meters will give generally reliable readings somewhere between 5-30 percent MC. The most accurate method (however not practical on an installed wood floor) to determine moisture content is to follow the laboratory procedure for the oven bake method (following ASTM D4442) by weighing the piece of flooring with moisture, removing the moisture by fully drying it in an oven (102 °C – 105 °C or 215 °F – 220 °F) and then reweighing.

The equation for determining moisture content is as follows:
The weight of wood with water minus oven dry weight divided by the oven-dry weight and multiplied by 100 percent equals the moisture content percentage.

Now that you understand some of the basics of moisture and wood, learn more about the importance of moisture testing and the different types of moisture meters on pages 52-56.

Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at brett.miller@nwfa.org.

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