Indoor Wood Shrinkage and Expansion

A learned discussion of the way building interior humidity causes dimensional change in wood. December 1, 2005

We all know that lumber to be used for interior purposes needs to be kiln dried to moisture contents acceptable for that purpose, say 6-9%. If lumber dried to these levels is stored at conditions of higher equilibrium moisture content and gains moisture, at what point does the lumber begin to expand? For that matter, when lumber is being dried, at what moisture contents does the lumber begin to shrink? I have been told that kiln dried lumber begins to "grow" once it reaches 11% moisture content.

Forum Responses
(Solid Wood Machining Forum)
From contributor D:
Wood is hygroscopic. It responds to changes in humidity as the humidity/environment changes. It does take some time (hours to days) to penetrate to the core of a piece of wood, dependent of course on the dimensions. A piece of veneer will respond to a breath. Finishing will slow down this transfer of moisture, but will not eliminate it.

Your wood that is 6% now will start to swell as soon as the humidity is increased. I would suggest getting Bruce Hoadley's book – “Understanding Wood”, and find out about EMC and FSP, and you can see what effects this movement/transfer will have on what species and where in the U.S. Also, you can do a search here on WOODWEB and find a wealth of information on movement and EMC.

From contributor F:
I thought I'd throw this in because I had it backwards when I first started this trade. Seasonal wood movement indoors equals wood increasing in dimension across the grain in season of summer, and decreasing in winter.

I find it useful to keep this in mind when in the midst of building a project where I am extra concerned about wood movement. This of course is not as important as using material while it is at its proper moisture content.

From contributor A:

The rule of swelling in summer and shrinking in winter might work where you live, but it can be the opposite in other places. For example, in the northwest, winters are humid and summers are dry, so wood exposed to those conditions would swell in winter and shrink in summer.

Wood dimensional change in service is a response to relative humidity changes in the surroundings. Relative humidity indoors can fluctuate due to seasonal changes (which are different in different places), but also in response to heating and air conditioning systems, etc.

From contributor K:
Another little factor is that shrinkage does not occur until the moisture content drops below the fiber saturation point, which can vary relative to density. It seems that I have seen the average is around 35%, and since it will typically be dried to 6-8 %, each 1 % of moisture content change will be about equal to 4 % of the total movement which was lost during drying.

From contributor F:
I live in the Pacific Northwest and I’ll guarantee you that my back door is loose in winter and rubs the jamb in summer. I don’t know all the technical facts, but it’s something about the heat in summer bringing the moisture in the air to a saturation point. In winter, although it is very damp outside, the wood inside the house gets drier than it was in summer when the air reaches the saturation point due to the higher temperatures.

From contributor D:
I live in the Midwest. My primary business is doors, and for many years I have noticed that exterior doors are loose in the summer, and swell in the winter - contrary to what one might guess. The opposite is true for interior doors.

I have to assume that exterior doors being in two different worlds at one time adjust according to the dominant humidity. Like Contributor F , we try to take into consideration during fabrication where we are in the humidity cycle and whether we should tighten or loosen things based upon these seemingly unpredictable and mysterious factors.

From contributor A:
I would stress that it is not a simple matter of the how wet or dry it is outside that influences the swelling and shrinkage of wood inside - or partially inside, as in an exterior door. The key is the relative humidity of the air that the wood is exposed to (note that temperature alone has very little effect on the dimensions of wood).

Relative humidity is a function of the amount of water vapor in the air and temperature. So, I certainly can believe that Contributor F's door swells and shrinks seasonally. However, it won't be the same for everyone everywhere, given differences in a) the conditions of the air outside and b) what you do to the air when it comes inside (heating, air conditioning, dehumidifying, etc.) I wouldn't say that wood swelling is magic, but, there are a number of factors to consider.

From Gene Wengert, forum technical advisor:
I need to correct the information that Contributor K provided above. His information about swelling is true, in that a wood cell does not begin to shrink until it reaches about 28% MC (or 30% MC). This is called the fiber saturation point. Shrinkage continues until 0% MC is achieved. But this is for an individual cell. When drying lumber, the lumber begins to shrink right away, as the outer fibers soon drop under the FSP, even though the average MC of the lumber is 70% MC (or other very high value).

As a rule of thumb for dried lumber, 4% MC change is 1% size change. This change in MC (and therefore size change) will occur whenever the moisture in the air (usually given as an EMC value rather than RH) is not equal to the MC of the wood. (Example, at 6% EMC or 30% RH, wood that is 8% MC will immediately try to achieve 6% MC. The surface will dry within hours to 6% MC at room temperature. The core may take many months to have much change.)

A special event happens when you are at a dry MC and the conditions are more humid (that is, the EMC is greater than the MC). There will be a slight lag both in MC gain and in swelling. It is called the hysteresis effect. So, a piece of wood dried to 8% MC and then exposed to 6% EMC will dry 2% more (eventually) and shrink about 1/2%. But a piece dried to 8% MC and then exposed to 10% MC may only gain 1% MC and therefore will swell very little.

From contributor K:
Do you mean 1 % of the overall dimension, or 1 % of the total shrinkage?

From Gene Wengert, forum technical advisor:
Once the wood is dried to a value under 20% MC, the 4% MC for 1% shrinkage is 1% of the total size. It is a rough rule of thumb and certainly shrinkage can be less than this.

From contributor S:
To the original questioner: If you prepare a solid wood panel that is of a typical size for a frame and panel door, for example 16" wide by 24" high and keep track of the dimensional changes to this panel on a weekly basis you will have a pretty good idea how wood will behave in your area. I will start out at 16" and log in the data directly on the panel for a year or two. Also you can monitor the moisture content as well.

From Gene Wengert, forum technical advisor:
Contributor S has a good idea, but you also need to monitor the RH. If your panel is in one RH, but in-use it will be in another RH, then the test results will be flawed. So, make sure the panel is in an in-use environment. Of course, the interior environment varies from a bathroom to kitchen to office. It depends on whether the room has a humidifier or dehumidifier or and a/c unit. We find that most RHs run around 30% RH to 45% RH (6% to 8% EMC). When they are lower, we will likely have complaints about cracking and warp. If higher, the risk is less, but warp may occur.