Air-drying oak

Can a suitable MC for cabinet construction be reached? February 6, 2002

Can oak boards be air-dried in a heated and air-conditioned living area and reach a suitable moisture level for cabinet construction? How long would it take?

Forum Responses
Yes, eventually you would achieve a suitable MC. How long depends on the MC now and thickness of the lumber as well as species. If you're in a hurry, have it kiln dried. If not, use the attic in the summer for small quantities.

Not if it is going to be used in most of the USA or in Canada. You cannot get the EMC low enough to do a good job.

From contributor J:
I have air-dried furniture and flooring that is fine in NH, USA. Cut, dried and assembled by me. It takes more work and time but it will be ok.

The outside relative humidity in most of the USA is equivalent to 12% MC in wood. We call that 12% EMC. This is true in NH. Inside a house, the conditions are 6% EMC.

Air-dried wood will be no lower than 12% MC, even after many years.

You cannot make something with 12% MC wood and expect it to work okay when it is brought into a house at 6% EMC. The wood will shrink and shrinkage means problems. For example, oak will shrink about 2% in size across the grain, even in New Hampshire, when going from air-dried to house-dry (12% MC to 6% MC).

Perhaps if you work slowly, the wood will dry out as you are working with it in your heated shop, so when you are done it is 6% MC. Perhaps your air drying is heated so it is not really air drying. We have reviewed the benefits of kiln drying before in this forum, including sterilization and also setting the pitch for softwoods. We have also mentioned that a few board feet can be dried to a low enough MC in an attic.

Contributor J, Are you saying that you make flooring at 12% MC and then install it in a house and when it dries to 6% MC, that it is still okay? Doesn't a 30' wide oak floor shrink about 2% or nearly 6"? How can this amount of shrinkage be considered okay? When a tabletop shrinks 2% in size, how can this be okay? Or when a 30" door shrinks 1/2", how can that be okay? I do not believe that you are using 12% MC air-dried wood.

Gene Wengert, forum technical advisor

From contributor J:
I let my wood acclimate inside before final planing or use. For example, the first floor I allowed one month. Small gaps between the boards showed after the heating season, but not enough to reveal any beyond the tongue and groove. The second was allowed to spend three months and to this day shows only miniscule movement. The third was stickered inside for two years and appears very stable. I use outside air-dried and then condition the wood indoors before use. Flooring I have produced for others will spend between 6 weeks and two years on stickers inside heated space before final planing and edge treatment. I have had the ability to do this with the given space and part time nature of my mill operation. In preparation for full time beginning in 2002 I have purchased two small Koetter kilns. My point is one can produce quality products without a kiln as long as facts about shrinkage and wood movement are not ignored.

This subject will never get the full agreement of everybody. I am surprised at some of Gene's statements. For example, that 30 foot wide floor would not be made up of one 30 foot wide board, which at 2% shrinkage would have 7-1/4” movement. Would it not typically be made up of 160 boards 2-1/4” wide? And that 7-1/4” shrinkage would be averaged over the 160 boards or about .045” or more accurately 7.2” over 159 gaps is still .045” per gap. For comparison, 3 dimes in my pocket change measured .050-.051" at the edge rims. Does door skin plywood or the doubled 3/4" plywood top of a tile-topped table move the same way? A single layer might like to, but the whole cannot.

Where is the rule that says that a board that has been kiln dried to 6% moisture is going to stay there forever? I recently visited a person that had some kiln dried wood in storage in his shop. He said that it had been at 6% in the kiln but had been out for many months and was at 12%. I said that I had some oak that had been in my storage barn attic for more than 6 months and asked what he thought the moisture would be. He was kind enough to check it for me to be sure, but he told me ahead of time that it would be 12%, the same as his. It was.

I don’t have air conditioning. My kids tend to leave the door open summer and winter. I haven’t been checking long, but the relative humidity in my house has been between 50 and 60% for the last several weeks—since I got the gauge. I really do not expect it to vary that much through the winter with basement walls and floor that let a little humidity in, tracked-in snow, adults and teenagers showering and even an outside furnace vent to let outside air in which removes condensation from the windows. If I worked with 6% dried wood and if I even knew where to get it for sure, wouldn’t its expansion be the same sort of potential problem in my 12% environment?

I do definitely agree that setting the pitch in softwoods and killing the varmints in everything is important. Drying to 6% just doesn’t seem always that appropriate. And my daughter's 160-year-old house, where the wood never saw a kiln from inside or outside is still standing, water-powered, reciprocating sawn wood and all. Isn't knowing that movement is going to happen the most important thing?

What happens to 6% MC wood that has been in your barn, shed or garage for a couple of months? Does the moisture content go back up to 12%?

It is moving in that direction, for sure.

Gene Wengert, forum technical advisor

The original question was: Can air-dried lumber be brought inside and dried down to a suitable moisture content and how long would it take? Isn't the answer to the first part "yes"? And the second part "it depends"? It will eventually achieve the same EMC of the surrounding environment, via the law of diffusion, and it depends on species, thickness, RH, etc. for the length of time required to do this. And yes, wood at 12% EMC stored in a 6% environment will move in that direction.

I read a post by an individual who air dried some cherry to 12% outside and then moved some of it into a wood heated space on stickers. He states that it is now at 6%.

Read carefully! When one person says "I use air-dried lumber" they do not mean that the only drying was air drying. They also dried the wood in a low humidity in their house, shop, etc. This served as their kiln--a low temperature kiln. Such low temperature drying is okay, but it has certain drawbacks, such as the powderpost beetle is still active, time is long, MC is not as low as might be needed, and resin is not set. Also, remember that except for cupping and incorrect MCs, drying defects originate at very high MCs. Hence, the carefulness of early drying is really the key to high quality.

Gene Wengert, forum technical advisor

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
I have air dried lumber in a open shed for around 20 years. It gets to 12% here in MN. Then I resticker it in a small room with a dehumidifier running in the summer and a wood stove in the winter. It gets down to 6% in about 2-3 weeks for the 1 inch; 2 inch takes longer.