Dehumidifiers and Lumber Storage

A dehumidifier in the storage room will keep lumber at the proper moisture content, and will do no harm. December 6, 2012

I operate a small millwork shop and stock several thousand feet of material at any given time between my shop and barn here in the Southeast, where humidity can be anywhere from moderate to high during the year. Wood, unless fresh out of the kiln, will settle at 9-13% in unconditioned spaces, which is too much for many projects.

I was thinking about using the commercial dehumidifier I use in my shop to dry lumber (in a tent) down a few percent (target 6-8%), which begs the following questions: Will woods with high tannins (oak and others) or even low-tannin woods damage the dehumidifier even though all wood will be sub-14%? Can I create stress in the wood by drying this way? What time-frame would make a meaningful difference for a few hundred to a thousand feet of lumber? The dehumidifier is a 360 cfm/6600 btu/56 pints per day unit.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
“Will woods damage the dehumidifier?” There is no risk of damage at this low MC level. “Can I create stress in the wood by drying this way?” No. “What time-frame?” At room temperature of 80 to 85 F and with air movement around the wood to assure fairly uniform RH, a couple of weeks will bring the MC down in 4/4 in pine and similar density woods; longer with oak and hickory. Check it with a moisture meter. Run about 30% RH in the wintertime and 38% RH summertime.

From contributor E:
I also had the same general question since I'm getting ready to unload my kiln to start another load and I don’t have the convenience of a "controlled" atmosphere for storage. I have a 28'x32'x10' DRY basement under an incomplete (not finished or heated) house. My plans are to have upright/standing storage around outside walls so I could use center as work space, but have concern air flow would be hindered. I could stack in center racks but limits work area to smaller portion but better air flow. Would be a DH running 24/7 due to no heat or insulation yet.

From Gene Wengert, forum technical advisor:
If dried lumber is wrapped in plastic, even a whole bundle, then no moisture can enter or leave the wrapped bundle, so the MC will not change, even if the temperature changes or outside RH is high. If the wrapping is pretty good, not perfectly sealed, it will work.

From contributor E:
Actually being wrapped won't work for me since I need the ability to look through lumber for that "special" custom piece. Most of mine is live edged (some curved) and to go through a stack each time wouldn't be feasible. I guess that's where custom cuts get custom storage and custom prices.

From contributor W:
Most commercial dehumidifiers on the market will not be able to maintain 30% humidity because the dew point is too low. You will have to heat the area somewhat. I think I would try that first. Heat the storage area until the RH drops to 35% or so. That will keep the wood dry and there may be only a few days a year where, if you run the heater with a humidistat instead of a thermostat, when the temperature will get too high to work in the space. I would try heat first. It is very, very difficult to keep an area at a lower absolute humidity than the surroundings. A commercial dehumidifier will cost more to run and may not work. Be sure you understand the difference between absolute and relative humidity. The result you need is relative humidity but the challenge is the surrounding absolute humidity.

From contributor Y:
At some point is the RH within a chamber affected by the dry wood (sponge) we put in it? If the wood is drier than the chamber's RH the wood dries the chamber by absorbing the H, right? I thought there were times in my small solar kiln that I was getting false hygrostat readings. Seemed the wood was drying the chamber (sucking the moisture from the air) once it bottomed out with the gear and methods I was using.

From Gene Wengert, forum technical advisor:
If you put 500 bf of 7% MC oak lumber in a chamber at 80% RH, it will take about two gallons of water to change the MC of this wood by 1% MC. If the chamber is tightly closed, then the dry wood will act as a dehumidifier and drop the RH to 38% RH. The small amount of moisture that the wood extracts from the air will lower the RH but will not affect the wood's MC by more than a small amount (under 0.1% often).