Wood Moisture Content and Equalization
Understanding how to evaluate wood moisture content when conditions have created a moisture gradient within the wood. April 11, 2008
Question
I have been researching various forums and have found a lot of good information however I want to make sure I understand EMC moisture content. I responded to an ad in the paper for kiln dried quartersawn white oak which the guy told me was dried to less than 6% MC. When I went to look at the lumber, we used his moisture meter which indicated the lumber was at 12%. His kiln is outdoors and he told me the side doors had been open for about a week to allow the lumber to equalize. We are in Michigan where it has been cool and wet recently. Does this sound normal for lumber that has "been dried to 6%"?
Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Maybe and maybe not. If you open the doors, the surface fibers will come up to about 11% MC or so. The core will not change for months. The meter you used, if it has uninsulated pins, will measure the wettest spot along the length of the pins, so with wetter surface fibers, that will be 11% MC. What you need to do it cut a piece cross-wise to expose the core and then measure the core MC. Or, use insulated needles. Incidentally, opening the doors in this manner is not a suggested technique, as you really want to have 6.5% MC lumber and you do not want to have moisture gradients.
The term EMC (equilibrium moisture content) refers to air and not wood. That is, 6% EMC means that very small piece of drying wood exposed to 6% EMC will eventually achieve 6% MC. In more practical terms, the surface fibers of drying wood will reach 6% MC rather quickly, with the core after a long time being a bit higher in MC, especially with thicker wood.
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