wood moisture in moist environment
I live in western NC where humidity is genrally over 70% year roundand frequently 85% or better. Like many People in this region I have no central HVAC in my shop or home. 7-8 months out of the year windows and doors are open and in the winter we heat with wood and wood only (might throw a pot of H2O on the stove to increase humidity). This means my interior spaces for materials and finished pieces are the same as outside most of the year and in the winter those spaces are fairly dry. If you follow the written advice I can find find on the subject it would suggest I can’t have air dry wood or even have furniture for that matter without bringing in HVAC or a kiln. Very brief run down on the advice I’ve found: Stack and sticker to %MC appropriate for climate in your area, finish drying indoors, use wood when at approximately 10%MC , wood gains and loses Moistrure relative to environment it is in. I use wood straight off the airdry stack (which is the same MC if I restack in my shop except for the short winter heating season) sometimes off the damp concrete floor under mycarport (16-20%MC) and go to work. Nothings cracked or distorted irreperably yet; including pieces that I’ve sent north. The pieces I retain live in my home which is downright arid in the winter. I have taken no more than the most basic precautions regarding wood movement in the design and construction of my pieces (large and small) yet they survive in an environment that changhes to the extremes. The books, articles, etc (dozens if not hundreds) I have read generally all seem to suggest my pieces should distort or crack. Why do “the experts” all suggest I must go through excessive processes beyond air drying outside in order to have a useable product. For example continuing to air dry indoors to aclimate to the wood to the presumably lower RH and achieve a lower MC. THis all seems to be based on the assumption that we all live and work in HVAC controlled homes and shops. At least in my area that simply isn’t the case so out of heating season the RH inside differs little from outside. Another issue this brings up for my is woodworking of the past. While it may be possible that most people have 24/7 HVAC now certainly 100,200 years ago that was not the case. Yet our most treasured antiques and woodworking techniques come from an era where interior moisture changed significantly relative to the weather.
One more conundrum:
According to “the experts” would I not need to find out the RH of a particular customers home in order to build out of wood acclimated to their interior environment? After all around here the differnce in RH in an HVAC home versus an open window home can be the difference between wood air dried exclusively outdoors (16-20%)and wood restacked indoors and brought down to say 10-12%MC.
Yeah I’m long winded and ca’t type for **** but what r your thoughts on the sunject.
Replies
I have a little experience on this subject. I have found that in Georgia (which is every bit as humid as in your area), wood dried to 10% will work well for furniture. There is more change in humidity and equilibrium moisture content than in many other parts of the country. At 10%, if the humidity drops, then you can stand that level of shrinkage. If the humidity rises, the wood may move to 12% moisture content, and you can tolerate that.
If you built at 7%, you could have problems with joint failure due to swelling at higher humidity without HVAC. If you build at 12%, my experience has been that you get too much shrinkage at lower humidity and can get some cracking. I take care to engineer movement into my pieces to allow for an 8% to 12% moisture swing over the season. Given that, I have found 10% to be the sweet spot.
In my shop (no HVAC), air dried lumber will equilibrate down to 10% when stored inside the shop. So, I keep an inventory of air-dried wood of the species that I commonly use in reserve. As the reserve drops, I replenish from the inventory stored outside in the shed. The stuff from the shed comes into the shop at about 12 - 14% moisture, so it has to dry several more % to get to the target of 10%.
In general I think you'll find the "experts" are not wrong. On the other hamd your experience suggests your practices are suitable for your area.
It's certainly not the case that air dried wood is always unsuitable to build furniture without further conditioning. If, as you say, in your area RH hovers somewhere between 70% and about 85% then air dried wood will reach a moisture content of about 14% to 16%, if it's dried in the open. Put the same wood in a building open at either end and it will likely dry out a little more than this to about 13% or 15% MC because the building protects it a little better than when the wood is fully exposed to the weather. When a stack of wood is protected in this manner it's true that RH is not much different to external conditions, but a building with a roof , two long walls and open at either end encourages at least a very gentle breeze to pass through. A breeze encourages drying not because the air is necessarily dryer than the surrounding RH, but because a breeze brings in fresh air that can carry away moisture expelled from the wood. Stationary air gets wetter and wetter until it reaches saturation point and cannot absorb more water from the wood.
I'm not sure I explained that particularly well, but it's a bit like the difference between hanging a wet towel out to dry on still day with a hint of drizzle in the air and hanging it out on a similarly drizzly but breezy day. The towel dries better in the second case than the first.
There is also temperature to consider. The higher the temperature at a given RH, eg, 79%RH, the lower is the eventual MC of the wood: for example 70% RH and temperature of 60ºF = MC of 14%, and 70%RH and temperature of 77ºF = about 13% MC. I suspect that with the high summer temperatures you experience in the Carolinas probably results in air dried wood ending up a little drier than you expect, ie, around 13 or 14% MC.
I also think it likely that you fortuitously live in a part of the world where using air dried wood combined with the conditions you experience in your house match reasonably well. Granted, some of your furniture ends up in houses that do have climate control, but a combination of air dried wood that is at the upper end of the MC levels suitable for building furniture for indoor use, and good joinery techniques, lets the pieces you build adapt to their new surroundings satisfactorily.
Consider the scenario in which I work. Here in Great Britain you will never see air dried wood straight off the stack that is below 18% MC. It just doesn't happen because our climate won't allow it, and 20% MC is about the norm. Typical RH conditions in UK houses range from about 60%RH in the summer to about 40% in winter. That equates to a moisture content of wood ranging from possibly as little as 7.5%MC in winter to about 11.5% to 13% MC in the summer. In this country it is just asking for trouble if you build furniture out of air dried wood at 18 or 20% MC. We therefore do need to do a bit of further conditioning before we build: the sort of conditioning in fact that you've read in articles and books.
I hope this goes a little way to providing you with a helpful response. I have actually created a text on this subject that whitters on for more than 24,000 words, but I guess that's just a teensy bit too long and windy for a response in a woodworking forum, ha, ha. Slainte.
richardjonesfurniture.com
Edited 9/8/2009 7:48 am by SgianDubh
thanks for the input my friends.
Edited 9/7/2009 9:29 pm ET by OtherPeoplesTrash
Richard,
Thanks for the information, very informative. Looking forward to the book.
OH, kin I hang my clothes out to dry when it's -20°F?
:-)
Regards, Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
OH, kin I hang my clothes out to dry when it's -20°F?
Won't take long to dry at all, but if it's your scivvies you might want to warm them up a bit before you put them on. Otherwise, the wife will be wondering... nah, I'd better not say it. :)
♫ If you’re OCD and you know it wash your hands ♫
"kin I hang my clothes out to dry when it's -20°F?"
I'm sure you can but, most importantly, do you want to, and they won't dry until they thaw. You, however, will dehydrate very rapidly in those cold conditions and you'll need to take a lot of water on board to replace the water you lose.
When it's -20ºF outside it might be easier and perhaps more sensible to hang your clothes on a rack in front of the roaring log fire you would be sure to have going in the house. And whilst your kecks are drying you could perhaps frolic about naked doing something a bit more fun than freezing your nuts off outside in the cold, ha, ha. Slainte.
richardjonesfurniture.com
Edited 9/8/2009 12:21 pm by SgianDubh
The benefits of air drying lumber to equilibrium moisture content are significant. The wood dries slow, which minimizes cracks and warp. The wood is stickered flat well above the ground and strapped down tight (or weighted)--this is crucial. The wood must have an adequate roof to protect against rain and direct sun while still permitting maximum airflow. But once your stacks have reached EMC, it seems to me that you are at an advantage in not having an overly climate controlled house or shop. You can bring the wood in and have it freak out not very much before you use it. The problem, if there is any may be in moving a finished piece to to someone with a tight, controlled house when conditions are much different outside. But room for wood movement built into any serious piece, and a good finish will let it adapt more gradually to changing conditions. Wood moves. My opinion is that once you're at EMC inside and out (you need a good meter and probably a hammer probe), sure bring the boards into the shop for a while, but it becomes a woodworking situation, not a lumber drying situation. Use the rough mill/rest/final mill technique and don't build in wood movement problems.
Brian
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