I've never paid any attention to moisture content in wood or relative humidity in the air. I buy wood and build cabinets. The more I'm learning about the relationship of relative humidity and moisture content in wood, and the subsequent changes in the wood after it's installed in a house with a lower relative humidity than my shop, the more I worry about my shop's lack of climate control.
But from what I've read, it's not the moisture content that causes the problems as much as the change in moisture content. I live in western WA state. The relative humidity in my shop is about 60-70%. It's cold here. The moisture content of the spalted maple I'm building with right now is 12-14%. Obviously higher than the projected 6-8% the perfect world hopes for. I left a piece at the client's house for several weeks and checked it tonight and it was also at 14%. Do any of you see a problem using western maple at 14% MC if the client's house seems to have the same conditions? I'm starting to think that ignorance was bliss.
From contributor J:
Your understanding that it's large changes in MC which cause problems is basically correct. Keep in mind, though, that indoor humidity levels fluctuate quite a bit over the seasons in most parts of the country, and it's generally a good idea to have the wood's MC at the time of manufacture similar to what it will be at the driest part of the year because a bit of swelling causes fewer problems than a similar amount of shrinkage does. If you or your customer is far enough west to enjoy the rainy coastal winter, then this may be the wettest part of the year. If you build at 12% and the MC drops to 9% in the dry season, then you may have some shrinkage issues. Similarly, if you sell cabinets to someone who keeps the windows closed and the space comfortably heated and air conditioned for much of the year, their RH will probably be much lower and is likely to cause serious problems with cabinets built from damp wood in what is essentially an unheated shed.
It's great to remember that recommendations to keep your shop at 35% RH and the wood at 6-8% MC don't make sense in every situation. It sounds as if you live in one of the moister parts of the country. If the work you do is relatively rustic then a few gaps and cupped boards might just add to the charm. But if you're trying to do tight, refined work that will continue to look the way it does when you built it, then it sounds like you're asking for trouble with your current setup.
"The relative humidity in my shop is about 60-70%."
This sounds too high a value, particularly if you heat during the winter season. In buildings that are heated in the winter, typical RH ranges are 35% (winter) to 60%. Are you sure your hygrometer is accurate?
"I left a piece at the client's house for several weeks and checked it tonight and it was also at 14%."
This value also seems suspect. My reading at WOODWEB over the years indicates that wood left in an unheated protected area will reach a *maximum* MC of 12%. Are you sure that your moisture meter readings are correct?
Over the years, Gene Wengert has responded to this type of question hundreds of times. I'd first suggest reading the article:
Humidity, Temperature, Wood Moisture Content, and Wood Movement
written by Gene. It will bring you up to speed on the essentials.
I have a brand new Ligno moisture meter and every piece of wood in my shop showed 12-14, from the wall framing (been there a long time) to my lumber to the plywood that's sat there for a bit.
But I think the maple I have was air dried. Changed hands before I got it, so not entirely sure. But before I got it, it was stacked for a year in a very warm, dry, controlled shop. I did just build a dresser out of it (finally) a few months ago and brought it in my house and it seems to be fine so far. This is the dresser that sold the client on this material. I thought it was regular western maple till I started planing it... Then I had to look it up to figure out what it was.
If you are using a dial RH gauge (often brass mounting bezel), it will often have large errors. I suggest a $30 digital device from Radio Shack.
Your moisture meter also seems to be in error. The 14% MC is just not right. Your client's house could not be this humid as they would have mold in the bathroom, damp sheets, and water dripping from the windows, etc. Further, with wood that was in your warm shop for some weeks, it is next to impossible to get 14% MC readings, as your shop would be closer to 7% EMC.
You are correct that the moisture change causes problems, and not the moisture level itself... within limits. Above 12% MC (+ or -), wood machines poorly, glues differently, and may have other undesirable properties at times. Likewise, under 6% MC, wood properties change so that gluing is poor, machining is poor, etc., unless special precautions are taken. But between 6 to 8% MC for hardwoods and 9 to 11% MC for softwoods, it is the change and not the level that we worry about.
That sounds like every day here. Certainly today. I have that very Radio Shack device you're referring to. RH in the shop at 9am this morning was 77% @ 45 degrees. I started a fire in the stove and got a secondary electric oil heater going and two hours later I'm at 65% RH and about 54 degrees.
I certainly don't have the shop of my dreams, but I look forward to a more controlled space. Just hard when starting a shop in a new location (home based) to afford the time and cost to insulate, heat constantly, etc. I read most of your essays yesterday on moisture and humidity, Gene, and I must say it was eye opening.
If the beautiful dresser reads the same and is stable, then I would think as long as the work stays in your area, it would be stable. Was the dresser's wood any different?
Note that a pin type moisture meter measures the wettest spot along the length of the pin. So, if the lumber is 10% MC, but the surface was exposed briefly to 75% RH, then the meter will give a reading of 15% MC. For this reason, we often prefer to use insulated needles.