Whether or Not to Condition Lumber after Drying

It's not always necessary to condition a load of dried lumber at the end of the drying cycle — only sometimes. February 22, 2011

Question
I'm curious about the need for conditioning lumber at the end of your kiln drying cycle. Is it necessary in all species? I have a Nyle unit, and I don't remember seeing anything about it in the manual. I have definitely had stress problems when I dry oak, as I have to finish drying the cycle at 160 degrees to get the MC down to 6%-7%. It does relieve the stress to add water at the end of the cycle for 24 hours. Do I need to do this with all my lumber loads? It seems that the lumber relaxes with time after you pull from the kiln. Does the conditioning just speed up the process?

Forum Responses
(Sawing and Drying Forum)
From contributor D:
Nyle has several humidification options for conditioning. Whether you need to condition depends on a lot of factors such as when and what the lumber is going to be used for.

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From contributor Z:
I would love to know about the whys and why nots to condition lumber.

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From Professor Gene Wengert, forum technical advisor:
Conditioning (called stress relief) is done to remove or reduce stress that develops early in drying. The stress will cause the pieces to warp instantly when machining heavily. We check the stress levels by cutting prongs and also ripping, as discussed in Drying Hardwood Lumber. The customer's needs determine how much stress can be tolerated in their manufacturing process. If you do not know the customer, then removing all or most all would be prudent.

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From contributor W:
I tend to condition based on my dry bulb/wet bulb depression. I have been drying lumber long enough to know when a particular species is getting close. Typically my schedules are not very aggressive and most of the drying occurs between 95-135 degrees. Once I get to the point of a 36 degree depression (38-40 in oaks), I close all my vents, increase the heat to 150 degrees and run that for 48-72 hours for 4/4 lumber. Typically my depression will adjust to the 32-35 degree range. What that means is that the increased heat has drawn a little more moisture out of the lumber and that moisture, to a certain degree, is reabsorbed through the surface of some of that lumber. I turn my kilns off and typically let it set in the kilns overnight. The temp doesn't drop incredibly low - usually will be 120 in the morning. I have no problems with lumber doing erratic things except those occasional boards that have strange grain patterns, like compression wood in a log that was growing crooked out of the side of a hill. I do a lot of resawn lumber. This is how I know the lumber is stable - they lay flat on each other when coming out of the resaw instead of bowing out into rocking chair rails.

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From contributor D:
Stress relief is something you do when you need to do it. And you don't always need to do it. Obviously if there is no stress, you don't need do it. Also stress relief is less important in some products than others. For example, if you are making a fancy moulding and some of the finished product will be from the center or the stock and some near the surface, then stress relief is important. But if you are making flooring and all the outside is going to be planed away evenly, it is less important. Also, if the lumber is being shipped a long way or there will be a long time between coming out of the kiln and being used, you may not be as concerned. But it isn't hard to do if you decide you need to condition. There are lots of solutions.

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From contributor Z:
Thanks for the information. I air dry my stock and don't seem to have any problem with crazy wood 99% of the time. As I understand when air drying or even solar drying, the wood fibers will relax at night, hence, not much stress.