Why moisture loss is more serious than gain, plus FAQs
September 23, 2021 | 2:05 pm CDT
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Q: Please explain why a little loss of moisture is more severe than a bigger gain of moisture? We seldom see moisture gain problems even when we ship to more humid locations.

A: I am going to use three figures that will help answer the question. Before the figures however, we all know how important it is to have the correct moisture content (MC) of incoming lumber. We all know that loss of MC means shrinkage, and gain of MC means swelling. Further the shrinkage or swelling can be uneven due to the variation of basic properties of wood. The technical term for what we are discussing is the “hysteresis effect.”

About 110 years ago, wood researchers at the U.S. Forest Products Lab took spruce shavings from a hand plane and slowly dried them at different humidities. They measured the relative humidity (RH) and they measured MC. From this data, they developed the common chart we use today relating relative humidity to MC. This relationship is shown in Figure 1, top curve “LOSS.” It turns out their results are a good estimate for almost all North American commercial species of lumber, thin or thick. Further, this relationship is a good estimate for temperatures between 40 F to 130 F.

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FIGURE 1

The only shortcoming in the standard RH vs. MC data is this is only true for wood drying for the first time. That is, after drying, if wood gains moisture, there is a different curve (Figure 1, bottom curve “GAIN”).

Let’s use a practical example to illustrate what is going on. we have a piece of oak dried to 9.5% MC (equivalent to 5%. RH on Figure 1 “LOSS” curve). This oak is then exposed to 30% RH inside a customer’s home, so it follows the upper curve leftward and downward (red arrow in Figure 2). The end result at 30% RH is 6.0% MC, or a 3-1/2% MC loss which could be as much as a 1% shrinkage. This is a large amount of shrinkage for most products and often causes defects such as warp, open glue joints, end cracking, etc.

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FIGURE 2

Now, take this 6% MC piece of oak and expose it to 50% RH. It will gain moisture, so we have to go from the top “LOSS” curve to the bottom “GAIN” curve. The way we do this is shown by the red line in Figure 3. We are going horizontally from 6% MC, 30% RH point to 50% RH on the “GAIN” curve. Moisture at 50% RH will be 7% MC, a change of only 1% MC and less than 1/3 expected swelling.

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FIGURE 3

Stated directly: Any loss of moisture in solid wood between the kiln and the customer’s home will have three times more shrinkage than swelling associated with a gain of moisture. More shrinkage means more problems.

When we then go from 50% RH back to 30% RH with this oak, we follow the same horizontal line from 50% RH on the bottom curve to 30% RH on the top curve. Again, about 1% MC change and 1/3% shrinkage compared to when it first dried from 50% to 30% RH. The wood is more stable with losses and gains than it was when first drying. No wonder we seldom have moisture problems in finished goods after the first wintertime drying.

So, I hope that you understand now why we want to start with wood that is quite dry -- perhaps 6.8% MC average with a small variation around this MC value, 90% of the pieces of oak lumber within 1/2% MC of this 6.8% MC average. With this MC level, any drying and subsequent shrinkage will be very small and likely not an issue. Any moisture gain, even if the humidity goes up above 50% MC will be very small with little subsequent swelling. Further, with the slow drying in the second wintertime with low humidities, we will have very little shrinkage.

Q: We have an architect who wants some fancy white oak moulding pieces. I am concerned about maintaining flatness after they leave our manufacturing facility. Comments please?

A: I would have some concerns. First, because of the heavy routing done when making moulding, you are exposing both the shell and the core. If the core has a different MC than the shell, we could have problems with moisture changes and wood movement. So, kiln drying must assure a uniform final MC in all the lumber. This means an extra day or two in the kiln. 

Second, we have thick and thin spots in moulding, so some thin regions can change MC more easily than the thick regions. Uneven moisture changes increase the risk of warp. Warp with a strong wood like white oak cannot be well-controlled with glue or screws. During construction, moisture conditions in a building can vary with the lack of air heating and cooling, dry walling, painting, open windows, etc., so we must appreciate this.
White oak moves quite a bit when the humidity changes, compared to other species. Without a finish, expect larger variations in MC, which means variations in shrinkage and swelling.

State something like you will target dry lumber to 6.8% MC with essentially no shell to core MC difference, and guarantee pieces will be flat within 0.050” per foot of length when they leave your facility, but you can’t guarantee flatness if the moisture changes, as wood moves when the MC changes. Remember to document the actual MC and flatness. Make sure you charge enough to cover the special effort you will have to make.

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Q: We are making some furniture where this insect infection adds a lot of character (pictured above). Is the worm still alive? Does it weaken the wood? What else?

A: This insect is called the Colombian Timber Beetle and is most common in soft maple. Often there are two or three adjacent holes in one streak. The insect likes only wet wood, so kiln drying over 130 F will kill the insects and eggs, plus then the MC is way too low after drying to 7% MC for any activity. The streaks are the result of the tree’s attempt to limit damage. Trees do not have antibodies, like we do to fight an outside pathogen, so trees isolate the affected area. The gray color is heartwood with a few fungi that also need water to be damaging. Wood strength is minimally affected, but it is affected, nevertheless. Consequently, using this wood for chair legs or other potentially high strength or high shock resistance parts would not be prudent, especially when there are more than one or two streaks in a piece of wood.

Sometimes, we see similar streaks that are black rather than grey or brown. These black streaks have more significant strength loss potential. Also, if you ever find a piece that is noticeably lighter in weight, these also are much weaker than normal. Of course, with soft maple, we have both red and silver maple. Silver is about 2/3 the strength of red. So, wormholes in red for a person using both red and silver should not be a concern.

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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.