Drying wood in a tent
Recommendations on how to use the sun's rays and a plastic tent to dry lumber. November 7, 2001
Question
Would I likely have success placing stickered 1" planked wood in a heavy plastic, tightly sealed 'tent' in the sun with a dehumidifier inside, say for a month?
Forum Responses
Ventilation will be the key to your success. With some tinkering you should be able to find a good way to dry small quantities of lumber. But you don't want to keep the tent too tight--air needs to circulate through the lumber stack. If you have a dehumidifier, you might as well put a box fan in there, too.
You don't want direct light on it, or it will dry too fast and crack or warp on you. You will also want the air to circulate through the pile. The stack should be evenly weighted down, with weights on the rows of stickers.
I agree that you need some good air flow through the lumber piles. I also think that you may have trouble controlling the temperature and RH, which could be serious for some species, like oak.
Is the tent sealed on all 6 sides? Is the DH big enough to dry the wood at a good safe rate? Many other questions involved before I can answer "Yes--it is a good idea."
Gene Wengert, forum technical advisor
From contributor L:
The dehumidifier probably isn't a good idea. If it is a residential type, and the tent is tight, it won't tolerate the conditions inside the tent. It won't do any good if the tent is loose. If it is a commercial dehumidifier designed for drying lumber, this would be a poor application.
From the original questioner:
What size dehumidifier is good for what size pile? The tent would be sealed on all 6 sides. I do have a choice in taking it all to a kiln, but I understand that the slower drying conditions of a 10mil poly tent and a residential dehumidifier would offer better conditioning that a kiln. This wood has been stickered and air-dried for close to two months now. I won't need to use it for another month or more. I won't be using red oak. It is all honey locust, Kentucky coffee bean, black walnut and some burr oak.
From contributor L:
Your assumption that slower drying in a solar kiln produces better lumber than a kiln is incorrect. I take it that this is not a business, but a hobby, so you can try some things with the tent and vents. The use of a residential dehumidifier is a poor idea, as it does not work in the relative humidity range that you need for 6% lumber.
From the original questioner:
I am drying the wood to be used for interior walls in a house I am building. Why won't a residential dehumidifier work under my circumstances? I assume you mean that it won't bring the humidity down low enough?
From contributor L:
Residential dehumidifiers are not designed to work in rooms with temperatures much over 100-110F. They are also not designed to work in rooms below about 70F. You can be below that at night and the unit will freeze up, or above it during the day and the unit won't make the dew point. Also, they are not designed to condense water much below 45-50% RH when the room is 80-90F. At higher temperatures they may not work at all. In order to get wood to 6%, you need to get the air to 35% and they will not do that. Also, the acids in the wood will pretty rapidly eat up the bare aluminum coil.
To get the wood to 6% MC, you need to get 25% RH. I agree with the comments about a residential unit, but thinking about it as "use it once and throw it away" without worrying about pollution, material wasting, etc., the idea of a residential unit is a possibility.
With your "tent" idea, why not just go 100% solar?
Gene Wengert, forum technical advisor
Is the lumber structural or decorative? Fir only gets dried to around 18% before being shipped for use as 2x stock in walls. That's still pretty wet, as far as I'm concerned. I've had guys that I cut for air dry a stack, then finish it off in their attics. They've never had problems when they built furniture after drying the lumber like that. The attics are vented and in the summer, they get mighty warm.
From the original questioner:
100% solar sounds very good. We have lots of steady sun here in Kansas (but high humidity, too). The wood will be purely decorative, used in non-structural walls.
Can you give me some guidance for a solar tent? Lots of water will condense, so a good fan is required, obviously. Should it be tight or loose and just the size of the wood stack or larger? We're humid in the high 80s (RH) in the morning, dropping to mid-50s in the afternoon and the nights lately have been quite cool (high 80s down to 60s lately--plus some wild swings). Do I need a thermometer and a moisture gauge? How much air needs to blow through how much space? How does one deal with the condensation caused by the temperature drops at night?
I have a question about residential dehumidifiers. I take it that it is the aluminum evaporator coils that the wood acids eat. That being the case, when the unit dies, would it be beneficial to replace the evaporator coils with a copper core condenser/evaporator?
From the original questioner:
I think residential dehumidifiers fail in high heat too--when the heat of the surrounding area gets as hot as the machine, it won't work anymore.
From contributor L:
You could replace the coil with one coated with something like Heresite or use a stainless steel one. Copper won't last any longer than aluminum. Even if you solve that corrosion problem, the coil/compressor ratios are all wrong for this application.
In Fine Woodworking's book "Wood and How to Dry It", on page 86 is a design of a tent over a pile of lumber. There are also designs of how to use fans with this tent. Get the book at your local bookstore or woodworking tool store and research the design. There are many more in this book (one by Gene, also).
I think that your tent idea will work, as it is the same as a solar kiln. In this summer heat my solar kiln has been drying oak from 13% to 7% in about 15 days. If you are not on concrete you may need to put plastic on the ground. A tent big enough to walk around the pile should be a good size, as you need fans to move the air in the daytime. In the summertime a dehumidifier would be a waste of time since you don't need it. I have used a dehumidifier in my solar kiln in the winter and have used one in the summer with temperatures up to 140. The dehumidifier came from Walmart about 2 years ago and is still going strong. If your wood has been air-dried for 2 months, there should be very little corrosive material coming from the wood. You can buy a digital thermometer, with 1 remote thermometer, and a humidity gauge for about $15 at Walmart and that works well. Radio Shack has the same thing for $30.
