Moisture Content

by Gregory Jackson

Originally published as Guild of American Luthiers Data Sheet #296, 1984 and Lutherie Woods and Steel String Guitars, 1997



Equilibrium moisture content (EMC) is the point at which wood is not losing or gaining moisture. This occurs when the wood is in balance with its environment. Since the environment changes from day to day, the EMC normally considered is the average EMC. It is very important to understand that this is a delicate balance between the wood and the environment. EMC is not a universal moisture content (MC) for all conditions. As conditions change, the EMC will also change. The water has a tendency to leave the wood and become airborne moisture, just as does the water in clothes hung out to dry. At the same time the wood has an attraction to water and will tend to absorb any available moisture. Water spilled on unfinished lumber can be observed to soak into the wood. The water in humid air, while not so obvious, is also available to the wood and will sometimes be drawn into the lumber. The two forces — 1) for water to be drawn into the air; 2) for water to be drawn into the wood — are opposing forces. The net effect is to create a balance which is called an equilibrium.

Equilibrium is affected by both humidity and by temperature. As the humidity in the air is increased the wood will gain moisture. If the humidity is lowered the wood will give up water to the air. Higher temperatures will force water into the air while lower temperatures will let the wood gain moisture.