Free and bound water
Water lives in two places in wood. In the cell cavities it is free, as in pipes. In the walls themselves it is bound: water molecules cling to the hydroxyl groups of cellulose and hemicelluloses.
Free (capillary) water fills cell lumina and intercellular spaces and is held weakly. Bound (hygroscopic) water is sorbed by hydroxyl groups of the cell wall polymers, above all hemicelluloses and less ordered cellulose. Moisture content is expressed as the mass of water divided by the oven-dry mass of wood, so it can exceed 100%.
Fibre saturation point
When wood dries, free water leaves first and the size does not change. Once the cavities are empty but the walls are still full, the fibre saturation point is reached, at about 30% moisture. After that water leaves the walls and the wood shrinks.
The fibre saturation point (FSP) is the moisture content at which walls are saturated with bound water and no free water remains; on average it is close to 30%. Below the FSP, dimensions, strength and other properties change with moisture; above it they barely change. In a real board the surface dries before the core and starts shrinking first.
Equilibrium moisture content
Leave wood in one room long enough and its moisture stops changing. It adapts to the air: at 20 °C and 65% relative humidity wood holds about 12% water.
Equilibrium moisture content (EMC) is set by air temperature and relative humidity and depends little on species, at least at low and moderate relative humidity. At 20 °C and 65% RH it is about 12%. The sorption isotherm is sigmoid and shows hysteresis: EMC on desorption is higher than on adsorption. The calculator on this page uses the Hailwood-Horrobin equation with Wood Handbook coefficients.
Shrinkage and swelling
Wood shrinks differently in different directions. Along the grain it hardly changes. Across, along the rings, tangentially, it shrinks about twice as much as along the radius. Hence end checks and warped boards.
Longitudinal shrinkage from green to oven-dry is usually only 0.1-0.2%, because S2 microfibrils run almost along the cell. Tangential shrinkage is roughly twice the radial one; radial shrinkage is restrained, among other things, by rays. The difference between tangential and radial shrinkage causes radial checks and cupping of flat-sawn boards.
Why it matters
Wood is dried to the moisture it will live at. Then the product will not crack or warp. Moisture also matters for chemistry: in pulping, gluing and impregnation.
Lumber for heated interiors is dried to a moisture content close to the expected EMC. Moisture affects density (which is therefore given at a standard moisture content or as basic density: oven-dry mass per green volume), impregnation rates and chemical reactions, and bond quality in gluing.
Key facts
- FSP
- about 30%
- EMC at 20 °C, 65%
- about 12%
- longitudinal shrinkage
- 0.1-0.2%
- tangential : radial
- about 2 : 1
- moisture basis
- oven-dry mass
- hysteresis
- desorption higher
Sources for this chapter
- Ross R.J. (ed.) Wood Handbook: Wood as an Engineering Material. General Technical Report FPL-GTR-282. Madison, WI: USDA Forest Service, Forest Products Laboratory, 2021.
- Sjöström E. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego: Academic Press, 1993.
- Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.