Q: I have some kiln-dried 5/4 red oak lumber that is stored in an unheated shed. When I bring this wood into the plant (in the winter in Michigan), I notice that it begins to check within a few hours. How much heat do I have to add to the storage facility to stop this checking that, I think, is due to rapid heating of frozen wood?
A: The oak is checking because the surface is drying very quickly. So, the wood surface must be higher in moisture content than the air EMC. (Recall that EMC is a measure of the humidity in air, expressed in terms of the MC that wood will attain in that air. If the wood's MC is equal to the air's EMC, then no moisture change will occur and therefore no checking, warp, etc.)
So, first question is "What is the EMC of the air in your plant?" It would be common in the wintertime to have a plant at 25 percent RH (unless it is humidified) which is about 5 percent EMC. However, outside, the humidity averages about 65 percent RH, or 12 percent EMC. So, I can imagine that the lumber, when you bring it into your plant has nearly 12 percent MC on the surface, while the plant air is trying to dry the surface to 5 percent MC. This quick change in RH (not temperature) is the problem. You should heat your shed (heating air lowers the RH) about 25 F above the morning low temperature, year-round. This will provide 6 to 7 percent EMC conditions for storage. You might also check the RH in your plant to make sure that you have 35 to 40 percent RH. I suspect that you are a little too dry.
One note of caution: Do not go over 40 percent RH. Often, the customers' home or office is 30 to 35 percent RH, so if you manufacture wood products at a higher RH, the wood will dry out when it reaches the customer. Such drying might cause cracks, checks and warp at the customer's place. Rather, keep your plant dry and at least any problems will show up in the plant rather than in the field.
Temperature alone is not an issue with wood. Heat does not cause wood to swell as happens with metals, plastics and other materials. Incidentally, the water in wood, when less than 28 percent MC, does not freeze.
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