Moisture and Wood Movement In Kiln-Dried Versus Air-Dried Lumber

In principle, wood's response to moisture changes is the same, no matter how the wood was dried. In practice, there may be a slight difference. July 23, 2014

I have seen several assertions lately that kiln dried lumber is less affected by varying levels of relative humidity in use than air dried lumber. I have not observed this myself, and would like to know if there is any published research on this subject, or if anyone would like to offer their expert opinion.

Forum Responses
(Cabinetmaking Forum)
From contributor D:
It has been my experience that air dried wood tends to be affected more by seasonal movements and warpage. Depending on how you will use the wood and what species you are talking about and perhaps the humidity in your local has a bit to do with it. Also as I mentioned it depends on what you want to do. If you want to steam bend or something like that then air dried may be better. What do you want to do with the wood?

From Carl Hagstrom, Systems Administrator at WOODWEB
It's my understanding that there is no difference in MC movement characteristics between air dried lumber and kiln dried lumber. MC related movement is affected by the relative humidity, the species, grain orientation, and temperature. If you take two pieces of lumber with the identical characteristics mentioned above, with the only difference being that one is air dried, and the other is kiln dried (both at the same MC), they will both behave the same regarding in-service moisture related movement.

From contributor R:
The MC of air dried will vary depending on your location in the USA. Up here in the upper Midwest the best you can hope for air drying is 12% MC. If you bring the wood in to your shop and let it finish drying down to even 8% before using the lumber, in reality you had to finish drying the wood with heat and lower humidity. It almost sounds like kiln drying?

From Gene Wengert, forum technical advisor:
There is no practical (but maybe we could see it in the lab with extremely accurate and precise measurement equipment) movement difference between two pieces of wood that were dried differently, so long as they were dried under 160 F. If we dry over 170 F, some permanent changes do occur; the hotter, the more. That is why veneer, plywood and particle products have a slightly different EMC relationship than solid wood. However, the key to any comparison of wood movement is to have the two pieces both start at the same initial MC and at the same ambient humidity. If they do when the MC changes the two pieces will change the same.

Note that sometimes in a kiln, the wood is over-dried, especially on the outside. This very dry wood on the outside, even if allowed to come back up in MC after drying and will behave differently - machining, movement, drying stress, etc. One other factor is the temperature used in kiln drying. Over 160 F and we begin to see small differences in movement. Another factor when drying at high temperatures is machinability. If air dried or kiln-dried under 160 F wood machines with higher quality than wood kiln-dried at temperatures over 160 F.

From contributor D:
I don't know any professional cabinetmakers that would use air dried wood to build a set of doors or a kitchen when KD is readily available at least out west. The KD process has what they call a kiln schedule and for certain species at a certain point they can add moisture back into the wood to help get the wood to the perfect MC in a controlled environment where the air dry process is certainly not as controlled of a process . I have used air dried woods but not for a clientís kitchen? Not all wood gets treated the same even before kiln drying, such is the case with eastern white oak. They may air dry the wood for several years before kiln drying.

From the original questioner
I work as a cabinetmaker and am familiar with the practice of drying wood and appropriate MC levels for interior woodwork. We have a small dehumidification kiln at my workplace, and I have a solar kiln at home. I am not proposing to use wood that has been air dried down to outdoor EMC for cabinetwork, though it certainly would be more appropriate than KD lumber for steam bending. What I was trying to get at was the relative stability and reaction to humidity changes of two similar pieces, one kiln dried and one air dried down to the same moisture content.

Geneís comments accord with my observations, but I have seen statements on this site like Contributor Dís and wondered just what they were based on. Certainly wood air dried down to 12-15% MC is going to move a lot more in an interior environment than KD wood at 6-8%. Commercial kiln drying is a well-established technology, but I am sure most of us have seen the effects of poorly controlled kiln drying in purchased lumber. I would venture to say that carefully air dried wood of most species would behave at least as well as well kiln dried wood once they were at the same MC, given that the kiln drying happens so much more rapidly and can induce more stress in the wood. A somewhat analogous situation would be jacking an old barn all in one day rather than over a period of weeks.

From Gene Wengert, forum technical advisor:
Like the original questioner, I had thought that the comparison was between wood that was air dried and then kiln dried to the final MC versus wood that was kiln-dried without any air drying. He has some good points in his most recent posting, but a properly operated large commercial drying operation will provide as good material as a slow DH unit. As he states, not all commercial operations do the best job, partly because they are drying for a general audience with a variety of precise specifications so they have to compromise at times. I do agree that when it is your own wood, you do tend to take better care of it during drying, so drying your own wood will often give you higher quality. His approach is indeed usually the best way.

From Contributor J:
Personally, I have used wood in my cabinetmaking business that was air dried only, but only when I was the one in charge of the air drying process. We often harvest our own logs, saw them, air dry outside and then bring them into the shop for final drying. Usually, this means a time period of three-five years before final use, as our volume of usage is low with this wood (we also purchase and use kiln dried for many projects). The key is checking moisture content, same as when I use kiln dried. We've run into issues with the kiln dried lumber if it's storage allows it to gain back MC. Overall, I'd say we have had very minimal problems using air dried wood for projects, you just have to use some common sense. All of my antiques were built using air dried wood!

From contributor I:
I had a job in the American SW desert and was greatly concerned with movement and humidity. I researched it on this forum and elsewhere, and found that for the building conditions, 9% MC was best. I ordered wood custom dried to 9% and worked at controlling the shop humidity. Lo and behold I found that the wood was stable through most seasons at 9% in my shop and home, in fact in my area in general. I then realized I was seeing wood movement from wood gaining moisture, not losing it.