by Professor Gene Wengert
Q.
Some one told me that kiln dried lumber is better to use than air dried. This same fellow told me he made a desk from air dried cherry. The lumber was old stuff (20 years or so) and a meter check revealed about 7% moisture. After it was placed in the house, it started to crack and check badly. If air dried is stickered correctly and then allowed to acclimate inside shouldn't it be ok to use?
A.
The key to any piece of wood is its moisture content. So, IF THE WOOD IS 7% MC (= to 38% relative humidity), it doesn't matter if it was kiln dried (properly) or air dried.
If you air dry, however, the average outside humidity is 65% RH in almost the entire U.S., summer and winter. This is 12% MC. So, air-dried stock would be 12% MC after 1, 20, 200 years. The problem is with the meter check, which was wrong. (Two ways I know: 1) Air-dried is 12%, and 2) if it was really 7% MC, it would not have cracked badly because it wouldn't have changed MC.)
There are two important differences between AD and KD. In a kiln, the wood is usually heated above 130 F, which kills all insects, eggs, and fungi. Second, with resinous softwoods, the heat drives off the resin that would be liquid and runny at room temperature.
Kilns have been used when we started to have central heating, which is about 150 years ago. The early kilns had a fire built under the lumber piles. The stacks had spaces between each piece of lumber, creating multiple vertical convection channels or flues within the piles. I have an old book that discusses some of the old techniques.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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