Attic drying and over-drying
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Can wood be over-dried in your attic? October 15, 2001
Q.
Is it possible to over-dry wood in your attic? I am assuming my attic gets to about 130 to 140 degrees on a hot summer day. What would the symptoms of over-drying be? I recently dried some walnut in my attic, and there was considerable twist in the dried boards. Could this be from over-drying?
Forum Responses
Assume it cools down to 70F in the morning. The air then is 100% humidity. The air then heats up to 130F in the attic. The EMC then is 1/2%. Over-dry, yes. If you put it up there when it was green, you my have severely checked wood, also.
I used to get the MC below 20% and then poke the lumber in the attic. It dried below 7% in two weeks during this time of the year (July).
I suspect the twist is from drying too slowly, or the wood gaining moisture after it has started to dry.
You can over-dry lumber (I don't know if you can do that in your attic but it can be done) but that doesn't necessarily make it twist. The twisting may be from many other factors, such as stress in the tree or lack of proper weight on top of the drying stack of wood. The effects of over-drying are different for every species.
You can over-dry the wood somewhat, but if you check the average EMC in the attic, it will be about 6% (at least that is what mine is) and that would not really over-dry the wood very much, if at all. Sure, it is dry when the sun is shining--today it would have been about 2% EMC at the hottest time of the day, but right now (11PM) it is close to 25% EMC (90% RH). So, the average EMC is not too dry (in spite of what another person indicated above; specifically, a 60 F depression is not 1/2% EMC). Incidentally, I have about 300 BF in my attic and use it straight from the attic and check the MC too. It is okay.
The main point is that twist is not a drying defect, but occurs because of spiral grain or diagonal grain (which can be a natural defect in the tree or can be a crooked log or can be a log placed on the carriage at an angle). Over-drying will result in cupping and poor machining.
Gene Wengert, forum technical advisor
I use attic heat to dry lumber also, except I sticker the stack, then cover it air tight, install a couple window fans on the front of the pile, weight it down, then install a couple drain pipes into the stack, up to the inside of the peak. Set my timer to click on at 9 am and off at 8 pm. The gable vents are closed off, the lumber is air dried to 20% before stickering, and the pile is on the second floor of my workshop instead of up in the ceiling joists. You could do this on the ground if you ran your drain all the way up to the peak. It's a great way of storage, also.
Very original and effective. One word of caution about using an attic (in addition to the weight of the lumber breaking a rafter or joist)--the water added to the attic's atmosphere (water from the drying lumber) can be a problem in the winter, as it may freeze in the attic when it hits a cold spot. Then when it melts, the water can run down into the ceiling. Most attics are not designed to carry away large amounts of water--most (but not all) kitchen and bath fans are vented to the outside and not into the attic for this reason.
Gene Wengert, forum technical advisor
