Q: Your column gives us data on what percentage MC it takes for a given wood to have 1 percent shrinkage. How much RH is that?

A: Although it takes time for an RH change to result in an MC change within the piece of wood (actually the surface fibers change within a few minutes or so, but the entire piece will take a weeks or longer), as a general rule, a 5 percent RH change is 1 percent MC change. So, for many wood species, it takes a 4 percent MC change or about 20 percent RH change to result in 1 percent size change.

If you want to be more precise, using the term EMC as the expression of the moisture in the air that causes a certain MC in wood, 30 percent RH is 6 percent EMC (and results in 6 percent MC in wood as the wood is dried), 50 percent RH is 9 percent EMC, 65 percent RH is 12 percent EMC, and 80 percent RH is 16 percent EMC.

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