Stored Lumber, Ambient Moisture, and Lumber Moisture Content Changes

Kiln-dried lumber stored in a non-conditioned space will equilibrate with the natural relative humidity conditions of the ambient air. April 21, 2011

I store my kiln dried lumber in a semi trailer on racks in the summer. It picks up 2 to 3% points. I solar dry down to 8%. In the winter I don't notice that kind of difference. I live in central Indiana. What can I do to stop this increase - vent, fans, I'm at a loss?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
The outside RH in your area in the summer is 65% RH. This means that lumber stored outside, protected from rain, will try to achieve 12% MC. (Actually, 11.5% MC due to the hysteresis effect.) The change in MC will be faster in the summer due to the heat.

So, what can you do? Why not paint the trailer black (at least the roof), as when air is heated, its RH and EMC will drop. You might even include a small fan to stir the air. Without solar, just use a heater that is turned on when the RH is above 38% RH. In any case, monitor the RH; try to get 38% RH.

From the original questioner:
Gene: The lumber stored in my shop does the same thing. By November it starts to come back down to 9 or 10. I sell hardwood in small amounts to woodworkers a couple hundred bd ft at a time. I try to explain this to customers who think you're trying to pull something... if you know what I mean. I've thought about dehumidifiers, but that's more expense and trouble. Guess I'm just stuck with Mother Nature's humidity levels, like it or not! Thanks.

From contributor B:
The MC will change no matter who has it, or when; it cannot stay kiln dry. Your customers need to understand this, and also that they need to acclimate the wood to their own environment before they work with it.

From Professor Gene Wengert, forum technical advisor:
Kiln drying refers to a process and not to a moisture level. So, once kiln dried, the wood is always kiln dried. Wood kiln dried to 6% MC and then stored at 12% EMC so that the MC changes is still kiln dried. Note that wood dried to 7% MC that is stored and is used at 7% EMC condition so that the wood's MC does not change appreciably is common. The MC of wood does not always have to change when a customer gets the wood.