Wood (drying) in the shop

[Steve Demuth]

My shop is not air conditioned, and heated in the winter only when in use (so mostly for a few hours two days a week). I've got some kiln-dried Northern Red Oak that's been in the shop for 3 or 4 years. Air-dried wood around here settles at about 12-13% moisture. So today I was checking this Red Oak, and it's still metering at about 6% after 4 years in essentially ambient conditions.

I find this rather mysterious. Surely a few weekend hours with the temperature in the shop jacked up to 65 or so degrees wouldn't wring that much water out of wood, would it?

Any wood experts out there who can enlighten me on what's going on with this lumber?

[Bryan Lisowski]

If it was kiln dried it would probably be around 8%. I wouldn't worry to much about wood movement at this point being it has been in your shop for 3-4 years.

[Steve Demuth]

Well, it clearly isn't going shrink at this point. The question is how high the moisture content will go and how much expansion I have to plan for when using it.

[Bryan Lisowski]

It was kiln dried, I would highly doubt that it will increase much at all unless you leave it in a swimming pool or out in rain and snow with no air movement. The boards have probably reached equalibrium moisture content after the time in your shop. I have checked boards in my unheated and no A/C and there has never been a change (increase in MC) that would concern me.

[John TenEyck]

Temperature is far less important to the MC of the wood than RH. So what's going on is that the RH in your shop has been pretty low. Buy a $5 RH meter and prove it to yourself. Next Summer when the RH goes up to 60% on your shop your wood is going to have a MC of 10% or thereabouts.

John

[Steve Demuth]

Yeah, I get that. Just didn't expect it to be THAT dry.

[Brian Tymchak]

Not an expert, but this is what I've learned from reading. Kiln dried wood has been heated to a point that forces bound water out of the cells in the wood. This moisture is more or less permanently lost. Free water is water between cells and is what is lost when the wood is air dried. This water can be re-absorbed from the air and will fluctuate with changes in humidity. 6% is about as low as wood gets. Winter is our lowest humidity of the year, so I could see getting a low reading like that off your lumber now.

[Kevin Jenness]

If my moisture meter gave a 6% reading in an unheated shop in NJ I would suspect the meter is off, as I would expect to see the lumber at 10+% range and in equilibrium with around 60% relative humidity, roughly speaking. You can check the meter's calibration by doing an oven dry test (http://forest.wisc.edu/sites/default...cations/89.PDF

It is a good idea to have a hygrometer in the shop so you can anticipate how the lumber acclimated to it will behave in service. If the humidity where a finished piece resides is significantly different from the shop where lumber is stored, problems often result.

Whether air or kiln dried, wood will attain a balance (equilibrium moisture content) with the ambient conditions. There is an effect called hysteresis which modifies the uptake of moisture in wood dried below conditions seen in service, but the previous sentence is essentially correct. Kiln drying does not render wood immune to regaining moisture.

Winter is not necessarily a "dry" season. It seems so in our climate because we heat our living spaces, which makes the indoor RH lower than outdoors. An unheated enclosed space will be much closer to the outdoor RH.

[Scott T Smith]

Temperature is far less important to the MC of the wood than RH. So what's going on is that the RH in your shop has been pretty low. Buy a $5 RH meter and prove it to yourself. Next Summer when the RH goes up to 60% on your shop your wood is going to have a MC of 10% or thereabouts.

John

What John said.

Any environment where the RH% stays in the 30% - 40% range - regardless of the temperature - will keep KD lumber in the 6% - 8% range. Additionally, KD lumber that has been dry stacked will not regain much MC% except in the perimeter boards.

Typically RH% drops in the winter time, so it's logical that your KD lumber would measure a lower MC% now too. As far as the air dried lumber in the shop measuring a higher MC%, if there is no air flow across it it's not going to dry out much further.

[Steve Demuth]

Kevin,

Great analysis. Thanks. As it turns out, I had neglected the manual temperature correction required by my meter. The shop right now is at about 30F, and circa 50% RH. The corrected meter reading for 30F is 8.5%, so I think my meter is fine, but my original post was a bit off the mark.

Your point about winter not being necessarily dry is why I was surprised at my original reading, and why I referred to only heating the shop intermittently. My house is dry because of the heat (gas or wood), not because it's winter per se. I would expect the shop to be much closer to ambient RH given that it's only heated 10-12 hrs/week.

[John K Jordan]

Whether air or kiln dried, wood will attain a balance (equilibrium moisture content) with the ambient conditions. There is an effect called hysteresis which modifies the uptake of moisture in wood dried below conditions seen in service, but the previous sentence is essentially correct. Kiln drying does not render wood immune to regaining moisture.
Winter is not necessarily a "dry" season. It seems so in our climate because we heat our living spaces, which makes the indoor RH lower than outdoors. An unheated enclosed space will be much closer to the outdoor RH.

Steve, I agree with all of this. There is much information from the experts on the web if you want to research it. I have a sawmill for lumber and also process a lot of thick wood blanks for woodturning. I also get kiln dried wood at times. All of it eventually reaches the "outside" moisture content in my previous unheated shop. All of it now eventually reaches the indoor moisture content in my current shop with heat and air.

I highly recommend a book by R. Bruce Hoadley, "Understanding Wood" for an education. (Available on Amazon) Anyone who works with wood can benefit greatly, flat wood, carving, woodturning.

This article in the Wood Database is also a good introduction on wood moisture. http://www.wood-database.com/wood-ar...-and-moisture/

First, I would try another meter if possible, or compare your meter to one at your lumber dealer or if possible, Woodcraft might even help. There is another method to test the moisture content is to use the oven-dry method, much more accurate than any meter. It is mentioned in the Wood Database article or you can look it up for more detail. It is a destructive test, requiring a removing a small sample preferably from the center of the wood. You also need an accurate scale.

Also, you left out some details that might be important. Was the wood tightly stacked, perhaps surfaced? If so, the center boards will lose and gain moisture slowly. Is it thick? Is it wrapped with something? Is the shop in the basement of the house, a connected room like a garage, or a separate building?

JKJ

[Jim Andrew]

I had a cheapie meter from MLCS, and it would read wood at 6%, and I would plane it, and then it would warp. Bought a better meter and found the same wood would read about 12%.