Moisture Variations in Dried Lumber
"Wet pockets" in kiln-dried lumber, or occasional wet pieces of wood in a load of dried lumber, are an unacceptable defect.June 13, 2014
Question
What is the common moisture range in a single piece of lumber? I have heard of wet pockets in lumber. How common are these pockets? Often times I hear kiln operators say that a certain probe is reading high because it is in a wet pocket. Any info on this would be greatly appreciated.
Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
The moisture range can be a few percent to 20 percent or more, depending on the species and the presence of bacteria (I am not counting injured wood.) The drier the wood, the smaller the variation tends to be. Wet pockets are not common in most species, but can be found in eastern white pine, hemlock, cottonwood, aspen, and some others, but would not be found in oak, maple, yellow-poplar, etc.
From the original questioner:
Gene - thank you very much for your answer. It really clears things up for me. So lumber with a moisture range as high as 20% will not have an effect on the wood worker (problems due to being wet), as long as the majority of the wood is kiln dried to a satisfactory level?
From contributor J:
Gene was talking about green wood. After it comes out of the kiln it should not have more than .5% variation within a board.
From Gene Wengert, forum technical advisor:
Wet pockets can show up in KD lumber. They can be a small zone (several inches wide and long at most and maybe only 1/2" thick, typically) that has a MC of 12 to 25% MC, while the rest of the piece is 7% MC. Wet pockets cause delayed shrinkage. The piece, or a small region within the piece, will shrink during or after manufacturing. Wet pockets are not acceptable. Often, wet pockets indicate the presence of quality-reducing bacteria. Note that my first posting referred to wet lumber and that the variation will become smaller as the wood dries, as I stated.
Regarding your comment about a few wet pieces, even a few high MC pieces after drying (not talking about wet pockets now) are not acceptable. All the (hardwood) lumber should be within 1.5% MC of the average, at the least, and even less variation is better. A mediocre hardwood lumber kiln load will have 95% of the pieces within 1.5% MC; a good drying job will have 95% of the pieces within 1/2% MC (or, as Contributor J says "0.5% MC").
From contributor S:
I am having trouble with kiln dried 8/4 Padauk lumber. It arrived at 8-14% moisture and the ends started checking as soon as we started cutting it. We have ripped some of it into 2 1/2" wide strips and stickered it with fans to circulate air. After ten days the moisture reads between 6-12%. I cut up some of the 6% boards and they end checked overnight. After re-sawing them I could see that they were around 10% inside. What is the best way to dry this lumber and how long will it take? This has been an ongoing problem for us.
From Gene Wengert, forum technical advisor:
Are you using a pin moisture meter? The pins with insulation will allow you to get a core MC value rather than just an average. The pinless meter may not be the best in this case; the same for a pin meter with needles that are uninsulated. Do you know which Padauk you have (African, Burma, Andaman, or even another type)? Most are known to dry slowly and cracking is often an issue. The first step is to get the wood to a uniformly low MC. This will require (I am guessing) a change in moisture measurement. Also, if using a pin-less, due to the high sensitivity to density, you will likely get some poor readings unless you accurately know the density, as there are different types of Padauk with different densities.
