Q. What does "dry" or "kiln dried" mean when referring to hardwood lumber?
. There is no precise definition for the term "dry" or "kiln dry" when referring to lumber. Even the grading rules do not have a specification. In general, within the industry, it means that the lumber is at a fairly uniform moisture content, both within a piece of lumber and between one piece and another. Almost everyone will also think that dry lumber is free of drying stresses (also called "casehardening") and free of insects.
It often means that no lumber will be over 8 percent MC (although a few people think 9 percent MC maximum) which in practice often means under 8.6 percent MC on the average. That is, the core might be a little wetter and the shell a little drier, but the average is under 8.6 percent MC.
The term "dry" also means that the lumber is no drier than 7 percent MC, which often translates into no drier than 6.5 percent MC.
A bit more precise implication of the term "dry" is that the customer buying the dry lumber is expecting that the wood will not move (warp, swell or shrink) significantly when manufactured into a product and the product is put into use in a "normal" home or office.
As movement is caused only by moisture change, the expectation is that the lumber’s MC will closely match the MC in use. However, I find that many manufacturers and most customers have no idea what the precise, correct numerical value on MC should be.
So, the kiln drying people have to guess and go by what they believe is the best average MC. For many, this target is 6.8 to 7.5 percent MC. Unfortunately, this can be too wet for dry locations like Denver, and too dry for much of coastal Florida.
Bottom line: For anyone selling or buying "dry" or "kiln dried" hardwood lumber, specify as closely as possible, the desired or required MC, and other expectations, such as free of drying stress, insect free as it was dried over 150F, uniform shell-core MC and similar.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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