Understanding MC levels
October 15, 2009 | 7:00 pm CDT

The best moisture level for hardwood lumber used for cabinets and other interior wood products is 6.5 percent MC in almost all cases. This level (equivalent to 35 percent RH) is the moisture that wood will attain in use in most parts of North America in the wintertime. If the product is made at 6.5 percent MC and then equalizes in use to 6.5 percent MC, there will be no shrinkage. It takes a moisture change for wood to shrink or swell; no MC change means no size change (also, no warping).

In the summertime, the moisture might go to 9 percent MC, causing some swelling, but usually a small amount of swelling is tolerated by most wood products much easier than compared to slight shrinkage.

Because softwoods shrink less than hardwoods and also machine better at higher MCs, the suggested average moisture level at the time of manufacturing is 9.5 percent MC. Softwoods machine and glue so much better at this slightly higher MC, compared to hardwoods.

I hope that with the following example, which is fairly technical, some of the confusion and misinformation will be removed. The implications are extremely practical and useful. I encourage you to set aside 5 minutes of uninterrupted time to read it.

Drying after processing

Let's assume we have a 2-inch wide piece of flatsawn upland (Northern or Appalachian) red oak.We will assume, based on published data, that the total shrinkage for this red oak in width (or across the grain) is 9 percent, going from wet in the tree to 0 percent MC.

This piece of lumber, after drying, will shrink or swell 0.32 percent for each 1 percent MC change. A 1 percent MC change is roughly a 5 percent RH change. If the 2-inch piece is sent through a router and the width is exactly 2 inches at 10 percent MC, then as the piece dries to 6 percent MC in use, there will be a 4 percent MC change and therefore, a total shrinkage of 0.32 x 4 = 1.28 percent.

Further, 1.28 percent of 2 inches is 0.0256 inches, roughly 1/32 of an inch. Therefore, the size at 6 percent MC is 1.9744 inches. For a 20-inch wide door made of 10 pieces edge-glued together, this 4 percent MC change would result in over 1/4-inch size change.

Measurement resolution

If the size is measured in 1/16-inch divisions, then this piece will still measure 2 inches even though it shrank 0.0256 inches. It will not have shrunk enough to reach 1-15/16 inch. (A measurement of 1-15/16 inch is actually any reading between 1-29/32 to 1-31/32 inch; decimally-speaking 1.90625 to 1.96875 inch.) What I am trying to illustrate is that if a standard is written with all the measurements mentioned in 1/16 inch increments, the piece of oak that shrank from 10 percent MC to 6 percent MC is still within the standard requiring 2-inches in width. It would be better to have a standard based on 1/64 inch or decimal measurements.

Regaining moisture

Let us assume that the piece mentioned above is 1.9744 inches at 6 percent MC and is allowed to regain moisture. If the humidity around the piece is increased slightly to 7 percent EMC, will the piece reach 7 percent MC? The answer is no.

When approaching the equilibrium conditions from a drying situation, the piece will reach 7 percent MC if the condition is 7 percent EMC. But there is a lag when approaching this condition from a drier situation...the lag is about 1/2 percent MC. So, a piece that is dry and is now regaining toward 7 percent EMC, the piece will reach only 6.5 percent MC. This lag effect is called the hysteresis effect. The estimated increase in size would be based on the actual MC change...6 to 6.5 percent MC, in this case. If the piece at 6.5 percent MC is now exposed to 6 percent EMC, it will have a 1/2 percent MC lag as it goes from regain to drying; in this case, it will essentially have no moisture change. Basically, as the EMC goes from 6 to 7 percent EMC, the piece will remain stable in size.

This illustration explains why a new piece of furniture, cabinetry or flooring that might have been manufactured at a slightly higher moisture content will have shrinkage problems during the first winter or drying-out situation, but then perform without problems in future annual humidity change cycles.

I strongly encourage that initial drying of lumber intended for manufacturing into other wood products that will be used inside a home or office avoid higher moisture contents and shoot for an average of 6.5 to 7.0 percent MC, which is just a bit on the dry side. If we can get the initial product very close to its lowest MC expected in use (6 percent MC in North America), then shrinking and swelling in-use will be a much smaller issue than if the lumber is wetter or drier.

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.