Q. My situation is I'm in a 7,200 square foot building, and I share this space with another woodworking business. We are in the Midwest, right now the outside temp. is around 20F. They keep the humidity between 45 and 50 percent. I have talked to other shops in the area & they tell me 37 percent is perfect. I would like to get your opinion.
A. As most homes are 30 to 35 percent RH in the wintertime, it is better to shoot for 33-37 percent RH in storage and manufacturing to assure that the wood will change very little in moisture, which means the wood will change very little in size, gluing will be better, and will not warp.
At 50 percent RH, wood will be at 9 percent MC. So, furniture at 9 percent MC will lose moisture and shrink when it gets into a home at 30 percent RH (6 percent MC in wood) and that drying out will often cause some expensive issues.
The reason your neighbors like 50 percent RH is that with wood that is a bit wet (has not been dried to 7 percent MC), there will be no drying and this means no shrinkage, open glue joints, and warping in manufacturing. Of course, they likely will show up in the customer's home. I think it is better to have wet lumber problems show up in manufacturing rather than when the customer has the piece. But, the best solution is to get properly dried lumber: 6.7 to 7.0 percent MC.
You should double check the RH they have, however, as 50 percent RH means lots of condensation on windows and cold walls and lots of water for humidification; gages in a brass holder for a desk are not reliable enough. Radio Shack sells a $30 digital humidity and temperature measuring instrument that will work for our needs.
The bottom line is that wood and wood products that will be manufactured and put into use in the wintertime should be dried to 6.7 to 7.0 percent MC. This is equivalent to 34 percent to 37 percent RH, so the shrinkage and warping after manufacturing will be so small, if any, that it will not be a problem. Obviously, lumber storage conditions also should be 34 percent RH to 37 percent RH. Likewise, after manufacturing, we need to store the furniture in a plastic bag or at 37 percent RH so no moisture changes occur.
Note that temperature is not a factor, other than moisture moves faster when the wood is warmer.
Have something to say? Share your thoughts with us in the comments below.