Q. What humidity should we keep in our manufacturing plant? Is this 24/7 or can we have other values at night or during weekends?
Answer. The ideal humidity for a plant is the humidity that the customer has when the furniture, flooring, cabinets, etc. are delivered. Of course, the humidity will vary from customer to customer, so we need to use some sort of average or safe value.
So, three concepts help us arrive at an RH value for our plant.
First, is the concept that drying a bit more is worse than a small moisture regain (especially because there is about a 1 percent MC dead zone when higher humidity causes no important change in the wood).
Second is the concept based on the idea that many customers will have wintertime conditions that will be about 30 percent RH average in their home or office (much drier and printers or copy machines do not work so well; lips get chapped; and sinus and throat dryness problems) with many homes using humidifiers to avoid the dry conditions. Summertime could be 50 percent RH. (Exceptions would be more humid in coastal areas, such as Florida., and drier in southwest locations.)
Third, a good finish on wood will slow (no finish seals the wood from moisture changes) the moisture changes, so short term exceptionally dry or wet conditions are seldom an issue.
These concepts tell us that 35 to 38 percent RH would be reasonable for a wintertime manufacturing plant condition, which means 7.0 percent MC in the wood. In the summer, we might go 5 percent RH higher, which is good for the summer, but will result in wintertime shrinkage.
Note that we sometimes hear about people who seem to get rid of all their moisture content shrinkage issue, open joint issues, and so on by using a substantially higher RH. Indeed, if the lumber is a bit too wet, this higher RH will help avoid manufacturing problems, but it will push those problems to the consumer that has the normal, dry RH conditions.
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