Q: We live in a fairly humid environment in southern Florida. As a result, even though we target 6 to 7 percent MC for the lumber we use, the wood gains moisture in storage and during manufacture. Then when we ship it north, it dries and we have some problems - mainly splits, but a little warp at times too. Can you suggest some possible approaches to correcting this problem of moisture gain?
A: Certainly, the only way to address this problem is to store the lumber, as well as the work in process, in a controlled-humidity environment. For ideal, trouble-free conditions, 37 percent RH is your target. The temperature is not critical.
Storing lumber is easily done in a fairly tight warehouse. In most cases, making a small room enclosed in plastic (basically sealed tight to avoid moisture entry) is the reasonable way to store goods in process. Lumber could also be stored this way. Although the wood can spend two or three days out of the room, the work needs to go back in the room for an equal amount of time to return to the correct MC.
You have two options for controlling the RH. First, you can add heat to the air. Adding about 15 to 20 F to humid outside air will typically achieve the desired 37 percent RH. This is probably the most practical approach for lumber storage. In fact, if you have a black roof and use a fan to blow the heat down from the attic area, you will probably have enough heat. If the RH is too high, however, you will have to add additional heat.
In the plant, the best option is probably to use an electric dehumidifier that is commonly used to dehumidify basements. Put this unit in the plastic room. As there is no major source of moisture entering the room, the DH unit will easily achieve the correct RH conditions.
You do need to monitor the RH and MC. I suggest a $25 electronic RH sensor from a store such as Radio Shack. Also, so you can check the surface and shell MCs, you should use a pin-type moisture meter. (Special note: The core MC should always be at the MC it was when it left the kiln. Core MCs do not change very much or very rapidly, even when the lumber is stored at more humid conditions than desired.)
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