Wood Moisture Content Question

[Tom Saurer]

This may seem like a weird question, but it's been bothering me for awhile.

I've heard a lot of people say that kiln dried wood is better, because it gets an initial lower moisture content, but can't understand why.

I understand that wood is hydroscopic, it absorbs moisture from the air or will give moisture back into the air until it reaches a uniform level with regards to the relative humidity and itself.

So with that understanding a kiln dried timber sitting next to an air dried timber will eventually be the same moisture content no matter what they were dried to.

Can someone explain why the kiln dried wood be better?

[Mike Wenzloff]

Kiln-dried isn't better--but neither is air-dried better. Moisture is moisture and once both stabilze in the same environment they contain the same moisture levels.

Certain woods we in the US call exotics are more stable if properly kiln-dried. That is, they can dry straighter than air-dried. Too, with woods of a certain density, air-drying can take decades.

Certain woods have different coloration and perhaps workability, such as Walnut, where air-dried is suppose to be more favorable. My experience is air-dried in so far as color can be different, more vibrant, in Walnut. I don't personally notice a difference in workability.

Take care, Mike

[Kent Fitzgerald]

It's actually a very good question. I've heard two reasons cited for preferring kiln-dried wood. The first, and more straightforward, one is that the high heat of kiln drying destroys insects and fungi.

The second reason is what is called hysteresis effect. To quote from an FPL document:

"Wood exhibits hysteresis, which means that if wood comes to equilibrium at a given relative humidity and temperature, the EMC will be slightly higher if this equilibrium is reached by losing moisture than it would be if it reaches equilibrium by gaining moisture. In the extreme, this effect can be as much as 3% moisture content."
(http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf)

I.e., wood that is "overdried" in a kiln should equilbate to a lower MC than air dried wood.

Personally, I think that (1) equilibrating to local EMC before millling and (2) good design to allow for seasonal wood movement are far more important than initial MC. I usually use kiln dried, because that's what's commercially available, but I wouldn't balk at using good quaility air dried lumber.

[Bob Smalser]

This may seem like a weird question, but it's been bothering me for awhile.

I've heard a lot of people say that kiln dried wood is better, because it gets an initial lower moisture content, but can't understand why.

I understand that wood is hydroscopic, it absorbs moisture from the air or will give moisture back into the air until it reaches a uniform level with regards to the relative humidity and itself.

So with that understanding a kiln dried timber sitting next to an air dried timber will eventually be the same moisture content no matter what they were dried to.

Can someone explain why the kiln dried wood be better?

You are largely correct.

The advantages of airdried wood in color, texture and bendability far outweigh the minor effect of hysteresis in anything but the worst-case scenarion, which is flooring applied over radiant floor heating, where 4% EMC is desirable.

It's impossible to overcook airdried wood, while kilns overcook all the boards coming from logs lower in EMC than the average in the kiln. That's why yards try to "pond" their logs under sprinklers for as long as possible. Overcooked boards are brittle, with faded color and in the worst case, casehardened and reactive inside. In outdoor wood they are also more prone to rot because overcooking damages lignin, the glue that holds the fibers together.

I have an entire house full of furniture whose wood never saw a kiln....and with no problems at all. Few woodworkers get to use anything but kilned wood and have little basis of experience with airdried stock.

[Tom Saurer]

Thanks for the responses all. Very informative.