Q. I am a bit confused about wood shrinkage and final MC. Why do you and others suggest drying to the lower moisture content level, such as 6.5 percent MC? Won’t the swelling be an issue?
A. As probably all of the readers know, wood shrinks when it loses moisture and swells when it gains moisture. However, this is not 100 percent true. Let’s start at the beginning so we can understand this total issue.
First, when drying lumber, if we dried it at 50 percent RH, every U.S. species will achieve 9 percent final MC, if we wait long enough. If we dry lumber at 30 percent relative humidity, the lumber will reach 6 percent MC. (The hotter we dry wood, the faster it dries.)
In most homes and offices in the U.S. and Canada, the interior environment is about 50 percent RH average in the summertime and 30 percent RH (or a bit drier without a humidifier) in the wintertime.
When wood that has been losing moisture is exposed to a higher humidity so that the wood now gains moisture, there is a delay. That is, the moisture will increase, but the wood will not swell immediately. Further, although a piece that is wet and drying at 50 percent RH will achieve 9 percent MC, a piece that is dry and is now gaining moisture at 50 percent RH will only reach about 8.5 percent MC. With the lower moisture when gaining moisture, there will be less swelling when going from 30 percent RH to 50 percent RH than shrinkage when going from 50 percent RH to 30 percent RH. (The technical term is hysteresis effect.)
What this means can be seen by looking at an example. Assume we have a flatsawn piece of lumber, 2 inches wide. Assume that like many hardwoods, this piece shrinks or swells 1/4 percent with a 1 percent MC change.
Example 1: If the wood is at 50 percent RH and 9 percent MC, and then is exposed to 30 percent RH in the manufacturing shop or customer’s home or office, the piece will dry to 6 percent MC. This 3 percent MC loss will result in 3/4 percent shrinkage in width across the grain, or, for a 2-inch-wide piece, a shrinkage of 1/64 of an inch. This is not much shrinkage, except when edge gluing, the maximum gap allowed is 0.006 inch before the joint weakens.
Now, when this dry piece is exposed to 50 percent RH again, it will be gaining moisture up to 8.5 percent (not 9.0 percent MC). The swelling will not begin immediately, but will begin when the piece reaches 6.25 percent to 6.5 percent MC and continue up to 8.5 percent MC. So, the moisture change is less, meaning that the swelling is about 1/80 inch to 1/100 inch instead of 1/64 inch.
Of course, if the wood now loses moisture down to 30 percent RH, there will be a lag and associated lesser amount of shrinkage. In fact, this stabilization effect is why almost all shrinkage and swelling issues with wood cabinets, furniture and flooring (including splits in glued panels and cracks between flooring strips) occur during the first year; that is, the changes in subsequent years, unless conditions are unusual, will be substantially moderated.
Bottom line: By drying the lumber a bit below the average expected moisture content in use, we may get a little shrinkage and only a little swelling as the humidity varies from wintertime to summertime, rather than a larger shrinkage change if the wood is not dried down to 6.5 percent MC.
We can add to this technical discussion, that when wood shrinks too much, we tend to see cracks; cracks are an unacceptable defect. On the other hand, when wood swells a bit, we may have a few crushed fibers, but the surfaces should still be without any voids or defects that need repair. So, shrinkage is a higher risk event than swelling.
In either case, when the moisture changes, we may get some warping, especially if there is some slope of grain. Smaller moisture changes mean less warping.
Finally, we do know that over-dried wood is much more brittle when machining, so we do need to avoid over-drying as well.
Advice: Every load if lumber should be properly equalized. That is, when the driest piece of lumber reaches 5.0 percent or 5.5 percent MC, then the kiln is set to a humidity that will give 5.0 percent or 5.5 percent EMC. This means that the driest piece will stop drying and no piece can dry under this EMC value, but the wetter pieces will continue to dry. Equalizing is continued until the wettest piece of lumber reaches 6.5 percent to 7.0 percent MC. Failure to equalize properly means over-dried wood (bad machining and possible excessive swelling) and / or wet wood that will shrink in use.
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