Air-Drying Wood in a Grain Bin

      A discussion of air-drying sheds light on issues relating to target moisture content. July 28, 2008

Question
I'm had a big black walnut milled last summer and then stored it in a grain bin. My new Christmas moisture meter said that it was dry, which I found hard to believe. I did an oven test and came up with 13.5% moisture. Should I dead stack it, or leave it alone until it gets somewhat dryer?

Forum Responses
(Sawing and Drying Forum)
From contributor S:
Leave it alone, unless you really need the space.



From Gene Wengert, forum technical advisor:
If you need the space, it is ok to stack it without stickers. However, as you probably know, 13% MC is too high for interior usage.


From the original questioner:
I don't need the space. At what moisture can I dead stack it, and at what moisture can I use it?


From contributor S:
If you don't need the space, why dead stack it at all? If it’s stored in a "grain bin you probably have seasonal changes in temperature and humidity. So letting the air circulate will be a good thing.

The argument over using air dried versus kiln dried lumber in modern woodworking has been raging for years and will continue. But do you really want to put a lot of work into a furniture project and then have problems due to moisture content of the wood?

Build a small solar lumber kiln and get it down to 6-8%. Just remember once the lumber is kiln dried, you need a dry place to store it or it will begin picking up moisture again.



From contributor D:
13.5% is great for outdoor use. I'm assuming you wish to use the walnut indoor, needs to be lower. I bring my AD wood into the shop and put back on stick, usually blow a low speed fan on it, while I'm in the shop. It takes about two weeks or so of treated shop air to get the lumber to 8-9%. At this MC or lower you can dead stack the lumber, as I do, or start using.


From the original questioner:
The grain bin is 21 foot in diameter, 18 foot high and made of galvanized metal. I'm sure it gets to 130 degrees in there in the summer. It has a perforated floor and has a plenum below with a fan to suck the moisture out. In this case I just used a 3/4 horse fan.

I've tried to read everything I can about drying lumber and it seems that an awful lot of people want me to have it sit for a year and then put it in a kiln. Which I don't quite understand (and believe me I'm not trying to be snotty here, I just can't figure this out). If I put the stuff in a kiln and theoretically dry it to 0%, won't it revert back up to whatever the average humidity is in my climate? Likewise if I only air dry it, won't it eventually get down to that point? What am I missing?



From Gene Wengert, forum technical advisor:
The lumber will dry to its equilibrium condition in the bin, in a kiln, or in storage after kiln drying. Also, it will dry to the equilibrium value in a home or office. Almost all homes and offices average about 7% EMC (38% RH; a bit drier in wintertime and a bit wetter in summertime). As shrinkage is a bit more of an issue than swelling and due to the hysteresis effect, we target about 6.5% MC for hardwoods used indoors to prevent any moisture problems. Anything wetter than 8.0% MC is risky.

I do not like drying wood in a shop as often the shop is not as dry as a home. Further, it can provide entry for insects. Also, the moisture from the lumber can raise the humidity possibly enough to create some rust issues or mold in the shop.

You can store as long as you like. Your only risk in the bin is that there could be an insect in the wood and so you could get a bit of damage. However, if you can get the wood up to 130 F, that will kill any insects.

The average outside RH in almost all of the US is 65% RH, which results in 12% MC in lumber. Hence the outside conditions are called 12% EMC (equilibrium MC). With the heat in your bin, you would be lowering the RH, so you should see a lower MC value, perhaps 10% or lower. Therefore, I wonder if your MC measurements are correct. Just a thought.



From contributor B:
Moisture content issues aside, what are the issues with wood degradations if dead stacked at 12% MC after reaching a temperature high enough to kill the bugs?


From Gene Wengert, forum technical advisor:
None, if you keep it dry and do not expose it to re-infestation by insects.



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