Ambient Humidity and Wood Moisture Content

A problem of wood warping in a dry basement storage area leads to a discussion of temperature, relative humidity, and wood moisture. July 10, 2007

I've read DHL and learned a lot, but I'm still unclear about some basic things. I know that when I'm working wood in my basement with kiln dried stuff (quality stuff purchased from a hardwood floor outfit), I need to bag my boards when finished daily due to the ECM changing the dimensions. Mostly warp. I'm wondering, since my cheapo hydrometer only goes down to 20 percent, is my goal to get the basement MC down to 6-8 percent so there is no change in MC? I'm thinking a DH should do the trick, right? I can't seem to locate any hydrometers on the internet that go down to 6 percent for measuring purposes. What's wrong with my logic? I'm missing something important.

Forum Responses
(Sawing and Drying Forum)
From contributor W:
If your hydrometer is reading 20% rh, then your wood should be in the 5% mc range. Your hygrometer is measuring RH, not the MC of the wood. There is a chart in the drying archives that gives RH to MC.

From the original questioner:
Thanks. I can't seem to come up with that chart.

From contributor W:
58-64 RH -11%
52-58 RH -10%MC

46-52 RH - 9% MC
39 -46 RH - 8% MC
32 -39 RH - 7% MC
25 -32 RH - 6% MC
19-25 RH - 5% MC

From the original questioner:
I found the chart I'm looking for in DHL Pg. 16. Leaves me with an EMC of 4.5%, right? I think that's too low. I want to stop the warp. How do I correct the EMC? I mark "pins off the tails" and it's difficult (to say the least) without a true board for a reference.

From contributor R:
RH alone is not enough to determine the EMC of a piece of wood in a given environment. You must monitor both temperature and RH to determine the EMC of an area. Adjusting either will change the EMC. In your case it may be easier to raise the temp to produce the EMC conditions you desire.

From contributor W:
What is the MC of your wood? By saying kiln dry, it could be anything from 4%- 20%. I try to keep my shop at 36% RH and my lumber at 7% MC and have no problems unless I have a wet board.

From the original questioner:
The lumber was dried to 6% MC originally. It has been equalizing for over a year to whatever the basement's humidity/temp is. Summer and winter swing doesn't seem to be a big swing except for the extremes. (Like now, low digits outside.) Unfortunately, I've only obtained the cheesy RH/temp unit recently, so I can't comment as to the difference in RH, but as I say, temp is maybe 10 degrees difference save the extremes. This winter it's pretty stable at 55 F. Since introducing more humidity isn't wise for my cast iron, the only solution seems to be to introduce higher temps, but to what degree? Will this boost the RH as well? Thanks for posting the previous chart on zeroing in on RH/MC factors, but can I expect increasing temps to eventually get where I'm trying to go?

From contributor D:
If you raise the temperature, the humidity goes down. You said you were in a cold area but if you check the weather report, it will probably say it is close to 100% humidity outside, but when the air is heated to the house temperature, it will be 20-30% RH.

From contributor W:
To my understanding if you want to keep the wood at 6-8 % MC, you will need to keep the RH around 39% RH. I heat my shop to 70 degrees F 8 hours a day and let it go to 55 degrees at night and the RH will only fluctuate a few points. But I have to keep adding moisture to the air to maintain the RH. Where I live I can air dry lumber to 9% MC so it gets real dry.

From the original questioner:
Wow, was I off on this! It does make sense that heating the basement better would only cause the humidity to go down, only furthering dryer wood. Contributor W, you've stated that you add humidity to your shop. How do you do this in a controlled manner? Humidifier? I never used one. I assume you can set it, right? Keep in mind that my cheapo humid/temp sensor only goes as low as 20%. The basement might be even lower than this.

From contributor W:
I use an old electric crock pot and run it while in the shop as needed. Real low tech but it works. I use a $25 Radio Shack thermo-hygrometer.

From Professor Gene Wengert, forum technical advisor:
The EMC and RH values posted at the beginning of this thread by contributor W are true for a wide temperature range, 30 F to 100 F, so temperature is not an issue if you know the RH. As mentioned, heat does lower the RH, but the RH to EMC is fairly constant at any temperature in a shop.

From contributor J:
You can change the humidity by heating or cooling the air. This refers to the outside air to inside air. As contributor D said, heating the outside air of 100% RH to about 70F can get the inside RH down to 20%. And in the summer a 70% RH outside cooled to say 50F in your basement can put the RH in the basement at 100%. But if you measure the RH where lumber is, the values contributor W gave are close enough, like Gene said.

From the original questioner:
Okay, I think what I'm going to look into is a humidifier (a small one). The reason for this is I do now store my lumber in a sidewall of my basement on racks. I can plastic this section off very easily with regard to increasing the RH. Should I include a fan for circulating properly or would these home center humidifiers suffice? Since it doesn't get any colder than 50 degrees and no warmer than say 70ish in the summer, according to Doc my temps shouldn't be of much concern. I will refocus when the temps/humidity changes during the warmer months. Much of my woodworking is done during the winter months so it would be a huge help to aid in controlling most of this situation. Anybody think this is the wrong course of action?

From contributor W:
Why the special room? Control the RH in the whole shop and you don't have to keep shifting back and forth. You need to control the RH for your projects also and it takes very little moisture to change the RH in the room.

From contributor K:
I have not been following this very closely, but a lot of what you have told makes one of my eyes twitch. First, a lot of basements tend to be moist, at least some of the time. I think you stated that your lumber has been acclimating for a year, and maybe off in a side area. Is this like with a dirt floor? I personally don't think acclimating lumber to one's shop is a great idea, unless your shop atmosphere matches the RH of the atmosphere of its final destination.

I could not tell from your first post if you had planed the lumber recently, or if it had been surfaced a year ago. Whatever the case, if you are talking about cupping of a board, this is the warp across the face of a flat sawn board, when you look at the end of the board. If it is cupped away from the heart since it was planed, it is drying. If it is cupping toward the heart/pith, then it is taking moisture back on.

The point I am trying to make is that if your wood was already milled, and is cupped toward the heart now, it has more to do with your lumber sitting down there during the warmer months than it does with what you are experiencing now.

From the original questioner:
The lumber was surfaced last spring, so that's why the warp, I think. The final destination of my project would probably be in the 8-10% range. I don't think I've ever seen a board that cups with the edges toward the pith even if it was taking on moisture. Both walls and floor of my basement are concrete and I agree that most have a high humidity problem, but drainage around here is phenomenal, almost gravel, so that may contribute to my "dry in winter" situation. I originally wanted to find out from you folks what my best course of action was for working my lumber in the winter because that's when I spend more time on that. Seems that others introduce humidity and monitor it in their shops, so that seems to be best for me as well.