air dried vs. kiln dried

[Gary Hoemann]

I have only used kiln dried lumber in my projects, but now have a source for air dried at very good prices. What difference will I see in air dried lumber? What moisture content is acceptable?
Thanks for the help

[Todd Burch]

Hi Gary.

With some woods, there is a big difference between air dried and kiln dried. With others, like cedar, little difference exists.

Acceptable moisture content varies across the country. When wood is kiln dried, driers shoot for between 6-8% moisture content for furniture grade lumber. Construction lumber is dried to between 15-19%. In your vicinity, there will be what's called an "EMC", or Equilibrium Moisture Content, and that will be the moisture content that wood "settles at", based on the temperature where you live and the typical relative humidity in your locale. In my area of the country, southeast Texas, the EMC is about 12%.

So, if your air dried lumber has reached EMC for your area, it's ready to go.

Now, on to other differences.

Certain woods bend better that have been air dried and not kiln dried.

Other woods will have big color differences betwen kiln dried and air dried. Walnut is a noteable wood is this area. The built-in that I just finished was made with both air dried and kiln dried walnut (kiln dried from plywood and some of the 12' long pieces I needed). That's why I applied stain - to even out of the colors. Also, when walnut is steamed in a kiln, a lot of the color of the heartwood bleeds over into the sapwood, making the boards more appealling, as the sap wood is not as noticeable.

For woods like pine, kiln drying will set the pitch. Pitch will run every time the wood gets hotter than it has previously been. In other words, if pine is dried to 100°F in a kiln (which is not very hot), and you put this pine in your attic and the temp reachs 101° or hotter, you could have sap dripping out of a pitch pocket.

Lastly, (from my limited knowledgebanks...), kiln dried woods are typically a bit harder on the surface than air dried woods. This "case hardening" effect is something that driers try to minimize with steaming (to minimize the stresses in the wood) at the end of a drying cycle, but a lot of times it is not removed completely.

Anyway, that's an introduction to the subject. The point being for you I think is to find out what your local EMC is, and use that as a reference point. I think that the University of Virginia might have a web site to determine your local EMC based on either zip code of longitude/latitude/altitude... Or, you could call Forest Product Labs in Wisconsin and find out, or, go over to woodweb.com's Sawing and Drying forum and ask.

Todd.

[Rob Russell]

In some ways, air-dryed lumber can be nicer than kiln-dryed. Some woods, like walnut, change color a little during the kilning process.

Things to look for/be aware of with air-dryed lumber:

<ol><li>A rule of thumb on drying time is a year per inch of thickness. That is way overkill in some areas.</li>
<li>Your area may be damp enough, so you need to take the lumber inside and let it sit there to finish the drying process and get down to acceptable moisture levels</li>
<li>Lumber's moisture content is going to change with the seasons. If you stack 6%MC kiln dry lumber outside in Alabama, it's going to suck up moisture until it's in equilibrium with th eenvironment. Kiln drying just gets the green wood dried out faster than air drying it.</li>
<li>Depending on the species, find out if the wood was covered during the initial dry period. When wood is first cut, you don't want it to dry too quickly. That can cause problems like surface checking. Some species, like oak, are especially prone to problems when dried too quickly in the early stages. </li></ol>

The US govt (Forest Service) has some good info on air drying wood. Download this pdf <a href="http://www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr117.pdf">Air Drying Lumber</a> and read it. It's designed for someone who wants to setup an air drying operation, but there is a lot of useful info in it.

Some folks will only use air dryed lumber. Finally - get yourself a moisture meter.

Rob

[Lee Schierer]

Air dried lumber is going to be more variable than kiln dried. The weather controls the drying rate, as does the method of stacking and stickering. It should be restacked periodically. the stack needs to be weighted so the boards on top are held in place.

Kiln drying is pretty tricky to get it right a lot of things can go wrong if they don't do the process right. Case hardening is one. My local source does a great job with his kiln. He then mills all sides to dimension lumber. I rarely ( maybe twice in 15 years) have a board move in any way once I get it home. It is always 7% when I pick it up.

My suggestion is to purchase some and if you like the results go back, if not, find another source.

[Jim Izat]

Hey Gary,

My rule of thumb has always been buy air dried if I can, buy kiln dried if I have to. I've used a lot of board feet of both, and the only significant difference between the two the stands out in my memory is color. As has been said, kiln dried wood is really only dry if you buy it and use it relatively quickly and/or store it in an artificially dry environment. Leave it around long enough and it's going to be just about the same MC as air dried is. I don't know why but I've always believed that gentle slow air drying minimizes the stresses in the wood, but I have no empirical evidence to back that up.

Jim Izat

One other thing. I've read somewhere recently that there are some tropical species that won't successfully air dry, and as such must be kiln dried to avoid as much degrade as possible. I don't have the money to actually buy such species but would love to someday.