Wood and Moisture Relationships

Even after being dried, solid wood products shrink and swell as they lose or gain moisture. Loss or gain of moisture also may interfere with paint adhesion and increase susceptibility to stain, decay, and insects. Many of these problems are preventable if the manufacturer or user understands wood and moisture relationships.

Structure of wood cells

Through photosynthesis*, a tree produces the sugar glucose. Long chains of glucose form cellulose. Cellulose molecules combine to form elementary fibers, which in turn are grouped into bundles called microfibrils. These microfibrils form the major structural component in cell walls and play an important role in the wood-moisture relationship.

(*Technical terms are printed in bold at their first use and are defined in the glossary.)

Amount of water in wood

The weight of water in living trees, freshly cut logs, and freshly sawn lumber can exceed the weight of the wood. The total amount of water in a piece of wood is called its moisture content. MC is defined as the weight of the water in the wood divided by the weight of the wood. This number is then multiplied by 100 to become a percentage.

If the weight of the water equals the weight of the wood, the MC is 100%. If the water weighs more than the wood, the moisture content is greater than 100 percent. Moisture content for freshly cut logs and undried lumber ranges from 45% to greater than 200%.

Fsp occurs at the cell level. At any given time, some cells in a wood product may be at the fsp while others are not. It would be unreasonable to expect all cells in a large piece of wood to be at their fsp at the same time.

As wood is dried further, bound water leaves the cell wall, and cells start to lose moisture below the fsp. As water leaves and the microfibrils come closer together, shrinking occurs (Figure 3).

When moisture is added to wood, the process is reversed. First, water enters the spaces between the microfibrils in the cell wall. The wood swells as the microfibrils are forced apart.

Once the fsp is reached, excess moisture re-enters the wood lumens and is held like water in a bucket. This water does not force the microfibrils apart since they are as far apart as possible at the fsp. Wood does become heavier as moisture is added above the fsp but it does not swell any further.

Fsp for most wood species falls in the range of 25% to 30% MC. Some wood species have higher extractive contents (organic compounds in the lumens and between the microfibrils of the cell wall). These woods have lower fsp. For example, redwood has an fsp of about 22%, but birch has an fsp as high as 35%.

The important thing to remember is that wood shrinks as the moisture content of most cells falls below the fsp. If moisture is added, wood swells until the moisture content of most cells reaches the fsp.

For example, if water is removed from a wood product with an MC of 50%, it will not start shrinking until most of its cells are at the fsp or below, probably about 30% MC. If water is added to a wood product with an 18% MC, it will continue to swell until most of its cells are at the fsp, about 30% MC.

Temperature and humidity

Relative humidity is the ratio of the amount of moisture in the air to the total amount of moisture the air can hold at that temperature. Absolute humidity is the actual measure of moisture in a given volume of air (measured in English units of grains/cubic foot).

Heating air increases its ability to hold moisture. When we heat our home or office, the air can hold more water. Therefore, the relative humidity decreases. Absolute humidity, on the other hand, remains the same; we have not changed the actual amount of moisture in the air.

Cooling air decreases its ability to hold moisture. Therefore, as air cools, absolute humidity remains the same, but relative humidity increases.

Humidity is important because wood products exchange water molecules with the surrounding air according to the level of relative humidity.

How wood dries

Wood reaches an equilibrium moisture content in relation to the relative humidity of its surroundings. The EMC is defined as that MC where the wood neither gains nor loses moisture. As relative humidity increases, the EMC also increases; that is, more moisture is held in the wood. Table 1 shows how relative humidity affects the EMC.

As an example, consider a wood product dried to an MC of 12% and then introduced into an environment with relative humidity of about 20%. The wood eventually will reach an MC of about 4%. This means the wood will lose moisture from 12% MC to 4% MC. Since the wood already is below the fsp, it will shrink.

As wood dries, it shrinks differently in different directions (Figure 4). The greatest shrinkage occurs parallel to the growth rings. This is called the tangential surface. Wood shrinks about 8% along this surface as it dries from fsp to 0% MC. Thus, a 10-inch wide quartersawn board dried from fsp (assume 30% MC) to 15% MC would shrink about 0.4 inch in width. This same board would shrink almost 1 inch (0.8 inch) in width if dried to 0% MC.

An intermediate amount of shrinkage occurs perpendicular to the growth rings. This is called the radial surface. Wood shrinks about 4% along the radial surface as it dries from fsp to 0% MC. A 10-inch wide flatsawn board dried from fsp (assume 30% MC) to 15% MC would shrink about 0.2 inch in width. This board would shrink about 0.4 inch in width if dried to 0% MC.

The least shrinkage occurs along the long axis of the tree, that is, along the grain of the wood product. Wood shrinks about 0.1% longitudinally as it dries from fsp to 0% MC. A 20-foot board dried from fsp to 0% MC would shrink only about 0.24 inches, or less than one-quarter inch.

Because wood put in use is not dried to 0% MC, there is even less shrinkage. Thus, for most applications, longitudinal shrinkage is negligible.

However, juvenile wood (first wood produced in new stems) and reaction wood (in leaning trees) can exhibit as much as 2% longitudinal shrinkage (up to 20 times that of normal wood). Lumber with juvenile or reaction wood often exhibits warp, especially bow, crook and twist, as it dries or after it is put in use (Figure 5).

Remember, the shrinkage percentages are approximate. They differ somewhat among species and even within the same species. Also, because moisture never is removed completely from wood to be put in use, shrinkage is somewhat less than would occur with drying to 0% MC.

Table 2 shows shrinkage values for common Oregon species. Only radial and tangential shrinkage values are shown. Longitudinal shrinkage is minimal unless juvenile or reaction wood is present.

How to measure moisture content

The easiest way to determine moisture content is to use a moisture meter. However, the most accurate way is to use the oven-dry method.

Using the oven-dry method

Follow these steps to use the oven-dry method to determine moisture content:

1. Before you dry the wood, weigh a sample to obtain the combined weight of the wood and the water. Record the weight.
2. Dry the sample in an oven at 218°F (103°C). This is just above the boiling point of water. Dry the wood for approximately 24 hours. Be careful not to burn or char the wood.
3. Weigh the wood sample again and record the weight.
4. Repeat steps 2 and 3 until you get the same weight twice in a row. The wood sample now is oven dry (OD), sometimes referred to as bone dry.
5. Use the following equation to determine the MC percent of the wood:
   (Weight of wood before drying ÷ oven-dry weight) – 1 x 100 = MC%

For example, if the wood weighs 200 grams before drying and 75 grams after drying, the equation would be:
(200g ÷ 75g) – 1 x 100 = 166.7%

If the water and wood weigh exactly the same, the MC is 100%. If the water weighs more than the wood, the MC is greater than 100%; if the water weighs less, the MC is less than 100%.

You can find more information on the oven-dry method in the Dry Kiln Operator’s Manual (see “For more information,” below).

Using a moisture meter

There are two general types of moisture meters — conductance-type and dielectric-type.

Conductance-type meters have prongs that are driven into the wood. They register the wettest point between the prongs.

Many conductance-type meters use insulated-pin electrodes. All but the tips of the prongs are covered with a tough insulating resin. You can start at the surface and take successive readings as you hammer the prongs into the board.

Dielectric–type moisture meters have a flat plate on the bottom. The wood surface is penetrated by an electric field as you run the plate slowly across the board. The average moisture content registers on a meter. Depending on design, different dielectric-type meters read to different depths.

Dielectric-type meters are more accurate for thinner lumber such as 4/4 (1 inch). In thicker lumber, there can be a wide variation in moisture readings throughout the wood. In these cases, it usually is more important to know the wettest areas than an average. Therefore, a conductance-type meter is more useful for thicker lumber.

The characteristics affecting all moisture meter readings are species, density, moisture distribution, thickness and temperature. Manufacturers explain how to adjust for these factors based on the meter you use. A partial list of suppliers of moisture meters is listed at the end of this publication.

Some guidelines for working with wood

1. Use wood dried to the proper MC. It should have an MC equal to the EMC where it will be used.
   • For products to be used inside heated buildings, use wood with an MC of about 7%.
   • Wood used for framing or other construction purposes should be dried below 20 percent MC. Wood below 20% MC is less susceptible to fungi and insect attack and generally is stronger.
   There is a trade-off, however. Drying wood to a lower MC may cause greater shrinkage and, especially if the piece contains juvenile or reaction wood, a greater potential to warp.
2. After manufacturing, protect wood products from gaining or losing moisture while they are stored or shipped.
3. Always finish interior wood products with a good coating of paint or varnish. Although this will not completely stop moisture exchange between the air and the wood, it will slow it considerably and greatly reduce the MC variation. Remember to seal all surfaces, not just the top and sides.
4. After the wood product is put in use, avoid direct sun and precipitation if possible. For outside uses, roof overhangs and adequate foundations are important. If wood will receive direct sun or precipitation, allow for shrinking and swelling in the design of the product.

Unfinished wood exposed to direct sunlight will “weather.” The wood turns grayish in color as ultraviolet rays cause the surface of the wood to break down.

Applying wood finishes and sealers containing UV inhibitors can help prevent this breakdown. For added protection, use a finish that contains UV inhibitors, a fungicide, and a water repellent. These finishes and sealers are sold under several brand names at most home centers.

For more information

• Schniewind, Arno P., ed. Concise Encyclopedia of Wood and Wood–based Materials (New York: Pergamon Press, 1989).
• Dry Kiln Operator’s Manual (USDA Forest Service, Forest Products Lab, 1988).
• Panshin, A.J. and C. de Zeeuw. Textbook of Wood Technology (San Francisco: McGraw-Hill Book Co., 1970).
• William, J.L. Electric Moisture Meters for Wood (USDA Forest Service, Forest Products Lab, 1988, FPL–GTR–6).

Partial list of moisture meter suppliers

This is only a partial list of suppliers. This list does not mean the Oregon State University Extension Service either endorses these companies’ products or intends to discriminate against companies not mentioned.

• Bailey’s
  Phone: 888-465-8227
  PO Box 1877
  Woodland, CA USA 95776
• Delmhorst Instrument Company
  Phone: 877-335-6467
  51 Indian Lane East
  Towaco, NJ 07082
• Forestry Suppliers Inc.
  Phone: 800-647-5368
  205 W. Rankin Street
  P.O. Box 8397
  Jackson, MS 39284-8397
• Lignomat USA Ltd.
  Phone: 503-257-8957; 800-227-2105
  14345 N.E. Morris Court
  P.O. Box 30145
  Portland, OR 97230
• Wagner Electronic Products
  Phone: 855-454-6238
  326 Pine Grove Road
  Rogue River, OR 97537

Glossary

• Absolute humidity — The actual quantity of moisture in the atmosphere. Often expressed as grains/cubic foot.
• Bound water — Water held within the wood cell wall material. Wood does not shrink until after bound water starts to leave the cells.
• Bone dry — Wood at zero moisture content. Not a natural state for wood. As soon as bone-dry wood is exposed to air, it will take in moisture.
• Bow — A form of warp. Bow describes a deviation flatwise from a straight line drawn from end to end of a board. If the board is laid flat on a wide face, the ends of the board will be off the ground.
• Cellulose — Principal component of wood. Cellulose is made up of long strings (about 10,000 units) of glucose molecules.
• Crook — A form of warp. Crook describes a deviation edgewise from a straight line drawn from end to end of a board. If the board is laid on its edge (narrow face), one or both edges will be off the ground.
• Cup — A form of warp. Cup describes a trough-like shape with the board edges remaining approximately parallel to each other.
• Elementary fibers — The smallest structural organization of cellulose molecules in a wood cell wall.
• Equilibrium moisture content — The balance of moisture content wood attains in response to the relative humidity and temperature of the surrounding atmosphere.
• Extractives — Chemically diverse organic compounds found in cell lumens and between microfibrils in the cell wall. No one knows exactly how extractives form.
• Fiber saturation point — The stage in the drying or wetting of wood when the cell walls are saturated with bound water, and the cell cavities are free of liquid water. Fiber saturation point for most wood species occurs at moisture contents of about 25 to 30 percent.
• Free water — Liquid water and water vapor in the cell cavities of wood.
• Hygroscopic — A material that exhibits such a strong attraction to water it is impossible to prevent moisture gain. Dry wood is hygroscopic.
• Juvenile wood — Wood formed during the first years of tree growth and extending out several rings from the pith. It is related to the age of the cambium layer and not to a certain area of the cross section of the tree. Juvenile wood exhibits more longitudinal shrinkage than mature wood. This natural flexibility allows a young stem to bend rather than to be rigid and possibly break.
• Lumen — Cavity of a wood cell where free water is held.
• Microfibril — A bundle of cellulose chains joined into a lattice-like structure. It is the smallest natural unit of cell wall structure distinguishable with an electron microscope.
• Moisture content (of wood) — The weight of the moisture in wood, usually expressed as a percentage of its oven-dry weight.
• Oven dry — See bone dry, above.
• Photosynthesis — Manufacture of simple sugars by green plant cells using solar energy. The sugars are formed from carbon dioxide and water. Oxygen is a by-product.
• Relative humidity — The ratio of the amount of moisture in the air to the maximum amount of moisture it could hold at that temperature.
• Reaction wood — Wood formed in leaning trees. In softwoods, it forms under the lean and is called compression wood. In hardwoods, it forms above the lean and is called tension wood. Reaction wood is formed by the tree to upright itself.
• Twist — A form of warp. Twist describes a lengthwise “twisting” of a board in which one corner twists out of the plane of the other three.
• Warp — Distortion in lumber and other wood products causing departure from its original plane. Common forms of warp are bow, crook, cup and twist.