If you are planning to build or remodel your existing home, always question your local building supply store manager on how the lumber they are selling to the consumer is dried.
When a tree is cut down, the wood may contain from 30 to 300% more moisture than it will after drying. There are two methods of seasoning or drying wood: air drying and kiln drying. Air drying , or seasoning, is done out of doors. The rough lumber is stacked either on edge at an angle or in layers separated by crosspieces called stickers. The wood is allowed to remain stacked usually from one to three months and sometimes longer. After correct air drying, the wood should have an average moisture content of 19%.
In kiln drying, the lumber is stacked in piles also with stickers between the boards. It is then placed in a kiln- an oven in which moisture, air, and temperature are carefully controlled. Steam is applied to the wood at low heat; then the steam is reduced and the heat increased. As the heat increases, moisture is taken out of the wood. Properly kiln-dried lumber has less than 10% moisture content. Green, 1′ lumber can be dried to 6 to 12% moisture content in three or four days in a modern kiln. Moisture content can be checked during the drying process with a moisture meter.
In both kinds of drying, seasoning causes some defects that downgrade the quality of the lumber. Among the most common are checks, honeycombs, warps, loosening of knots, and cracks causes by unequal shrinkage.
Moisture content is a measure of the amount of water contained in wood. In the drying process lumber tends to shrink, both in width and length. However, shrinkage in length is normally so small that in almost all species it is not considered a problem.
Water exists in green wood in two conditions: free in the cell cavities, and absorbed in the cell walls. When the cell walls have absorbed all the water they can hold, but there is no water in the cavities, the wood is at the fiber-saturation point. Water in excess of this amount cannot be absorbed by the cell walls; therefore it fills the cell cavities. Removal of this free water has no apparent effect upon the properties of wood except to reduce its weight. However, as soon as any water in the cell walls is removed, wood begins to shrink. Since the free water is the first to be removed, shrinkage does not begin until after the fiber-saturated point is reached.
Even though lumber is dried to a certain percentage it will continue to pick up or give off water, depending on the condition of the air. If the air is very dry, the wood will shrink. When the moisture content is in balance with the humidity of the surrounding air and neither gains or loses moisture, the wood is said to have equilibrium moisture content.