Q. We are a medium-sized shop and find ourselves with a little bit of extra KD lumber that we want to keep safely without a change in moisture. How can we do this?
A. There are several ways to accomplish the storage of lumber. All involve keeping the lumber stored at an average relative humidity that is in equilibrium with the moisture content (MC) that you want.
As a rough rule of thumb, 30 percent RH is 6 percent MC; 37 percent RH is 7 percent MC; 44 percent RH is 8 percent MC; and 50 percent RH is 9 percent MC.
You can control the humidity with a complex system using electric dehumidifiers, or can make it fairly simple.
Simplicity is achieved when you realize that whenever we have a closed room, if we heat the air in the room, the RH will drop. For example, foggy air at 55 degrees F will drop to around 30 percent RH if the air is heated 35 degrees F or up to 90 degrees F.
So, if we have a small room for storage, we put a humidistat in the room that is wired to turn on the heat whenever the rooms RH is too high. The key to success of this simple system is that the lumber must be already dry. If it is a little high in MC, then the little room will behave more like a kiln as the low humidity will dry the lumber. In this case, we would need some vents to exhaust this excess moisture.
Another simple system is to have lumber at the correct final MC put into a tight shipping container. Tight means no leaks and tight-fitting doors. Consider that if this container is loaded with 3,000 feet of oak, that would weigh about 7,500 pounds, it will take a 1 percent weight increase of 75 pounds of water to increase the moisture by 1 percent MC.
When we load this container, even if it is humid outside, the air in the container only will have a pound or two of water vapor. so, when we close the door, the moisture in the air will not be enough to change the moisture of the lumber by even 0.1 percent MC.
Of course, the more often we open the door when it is humid outside, the more chance that the tiny moisture changes will accumulate; so, open the door only in the afternoon when the RH is much lower than in the early morning. Note that if the roof of the container is painted a dark color, the little extra solar heat will drop the humidity inside for a few hours and can offset short periods of an open door.
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