The following provides facts about the moisture-shrinkage relationship of wood. A table of shrinkage values for North American hardwoods and softwoods is provided after the text.

Moisture in wood can exist as water or water vapor in cell cavities, or lumens, or as water bound chemically within the cell walls. Green wood is deigned as wood in which cell walls are completely saturated with water; however, green wood usually contains additional water in the cell cavities. The moisture content at which the cell walls are completely saturated but the cell cavities contain no water is called the "fiber saturation point." The fiber saturation point of wood averages about 30 percent moisture content, but individual species and individual pieces of wood may vary by several percentage points from that value.

The fiber saturation point often is considered as that moisture content below which the physical and mechanical properties of wood begin to change as a function of moisture content. In other words, once the moisture content of wood reaches 30 percent no more dimensional changes will occur as more water is added to the wood.

Wood is dimensionally stable when the moisture content is above the fiber saturation point. Wood changes dimension as it gains or loses moisture below that point. It shrinks when losing moisture from the cell walls and swells when gaining moisture in the cell walls. This shrinking may result in warping, checking, splitting, or performance problems that detract from the woods usefulness.

Wood is an anisotropic (not homogeneous in terms of directionality) material in shrinkage characteristics. It shrinks most in the direction of annual growth rings (tangentially), about one-half as much across the rings (radially), and only slightly along the grain (longitudinally). The combined effects of radial and tangential shrinkage can distort the shape of wood pieces because of the difference in shrinkage and the curvature of the annual rings.

The shrinkage of wood is affected by a number of variables. In general, greater shrinkage is associated with greater wood density. The size and shape of the wood may also affect shrinkage, as may the temperature and rate of drying for some species. Longitudinal shrinkage of wood (shrinkage parallel to the grain) is quite small. The longitudinal shrinkage from green to oven-dry condition is only 0.1 to 0.2 percent for most species of wood...so small, that it can usually be ignored.

The percentages are expressed as a percentage of shrinkage FROM the green dimension, NOT the swelling from the oven-dry dimension.

Sources:

Anonymous. 1987. Wood Handbook: Wood as an Engineering Material. USDA, Agricultural Handbook No. 72, Revised Edition.U.S. Gov't Printing Office. Washington, D.C. 700+ pages.

(This is an excellent reference book that costs about $25 from the U.S. Printing Office. Just call their number and say that you'd like to order the USDA, Ag. Handbook, No. 72)

Panshin, A.J. and Carl de Zeeuw. 1980. Textbook of Wood Technology: Structure, Identification, Properties, and Uses of the Commercial Woods of the United States and Canada.4th Edition McGraw-Hill Book Company. New York. 722 pages.