Furniture for High Humidity Conditions
Advice on controlling moisture content during construction for furniture destined for a relatively uncontrolled environment. November 15, 2010
Question
I'm in upper New York State, working on a project in south Texas. It includes a couple of soft maple turnings that are 18" diameter. They will be delivered finished as legs on a massive table. The job site is climate controlled, but I still have concerns. What MC should I look for in the 8/4 I glue up for the blanks? For color and grain, I was planning to alternate orientation of the planks so that edges show to the outside as much as possible, rather than just gluing up layers. What else can I do to prevent moisture related problems? Should I put a rider in the contract that non-structural checking isn't an issue?
Forum Responses
Architectural Woodworking Forum
From Gene Wengert, forum technical advisor:
Convert the RH at the site into an EMC. Then dry, manufacture and store the pieces at this EMC and MC. Note that a round piece is extremely sensitive to MC changes, so you need to be perfect, using a top quality MC meter (over $200) to verify the MC at various stages. If there is not MC change, then there can be no shrinkage and resultant cracking.
From contributor F:
Great guidance Gene. Another consideration as well as material drying and careful handling is a design element which might minimize issues. A very thick walled, staved glue up will expand and contract without being visible. I have done several big round bases similar to your description and they never checked or cracked. They were 16/4 by 25" long by 12 segments.
From Gene Wengert, forum technical advisor:
Design is indeed very important, as Contributor F indicates. There are other design features too. One feature that is not important is alternating the pieces end for end and top to bottom. Getting the correct MC is so much more important, yet I see people worried about orientation and yet they do not have a moisture meter. You may wish to indicate to the buyer that the piece is designed to perform at --% RH. Deviations of 5% RH from this value will potentially cause cracking and warping, which you cannot be held responsible for.
From the original questioner:
Actually, this is not a perfect turning so staves are not an option. It is a worm shaped spiral, carved perhaps 3" deep at the deepest. Gene, any specific suggestions about reliable brands or models of moisture meter? I'll admit my pinless $110 model is not reliable. Also, how do you convert RH to MC?
From Gene Wengert, forum technical advisor:
Below is an article in the WOODWEB Knowledge Base about EMC and RH ratios.
EMC/RH ratios
From contributor L:
I talked with the MEP engineer for the site. He tells me that the MC will vary from 45-60%, as the building will be only slightly controlled through part of the week. By the table you cite, this means an EMC of 9-10%. My lumber supplier tells me they dry wood to 6-8%, though it picks up some moisture as they store it. I guess this means getting the wood, building a tent, putting in a humidifier and checking, checking, checking until it's about right. I have the time to do this on this job, though it's a pain, but I wonder if this is what everyone goes through.
From Gene Wengert, forum technical advisor:
When the building is heated and the a/c is on for comfortable living, then the EMC in the building in most cases will be 6.0% EMC to 8.5% EMC. As shrinkage is more of a problem than swelling, we target a low MC for wood, such as 7.0% MC. The biggest risk is when the wood first experiences its in-use conditions, as they often are far from what the wood was manufactured at, so there is a rapid MC change. The annual change is smaller per week or per month, so the wood can more easily absorb the changes in size.
Note that not everyone does worry about MC and try to control it, and it is those who do not that contact me, as a consultant, or their lawyers do, and it is how I earn my professional consulting money. Also, some designs and some pieces are more sensitive to MC change than other items. Also, some customers just seem to tolerate some defects without much complaining.
Regarding the 45% to 60% RH, this is quite high and fairly unusual. If he reconfirms these values and agrees in writing that this is true, then you can design for it, but again, insist if it is drier, you cannot be held responsible for the normal behavior of wood in a drier-than-expected condition.
