Temperature change and its effect on wood

Q: Does temperature alone make wood change size if the RH is constant?

First, I am only talking about wood, and not the metal hardware or other metal or plastic components.

Second, I think we would both agree that temperature does affect certain wood adhesives, softening some of them when they get warm, but most in-use conditions would not get warm enough to see this effect with modern woodworking adhesives.

Third, wood, unlike most materials, moves a great deal when the moisture content changes...approximate 1/2 to 1 percent size change with a 1 percent MC change. This MC change can happen when the relative humidity changes about 5 percent RH.

When air is heated, the RH will drop. If air at 100 percent RH and 50F is heated about 25F warmer, without the addition or subtraction of humidity with a dehumidifier or humidifier, the humidity will automatically drop to around 30 percent RH. At 100 percent RH, the wood will be around 28 percent MC if we wait long enough, and at 30 percent RH, 6 percent MC. So, heating air will cause shrinkage in wood because the humidity drops, not because of the heat. That is, if we heat the air 25F hotter and add moisture to keep the RH at 100 percent RH, there will be no size change.

The outside conditions in most of the U.S. average 65 percent RH, summer and winter, using average daily RH values from NOAA. This is 12 percent EMC.

An unheated warehouse will be just a bit drier than the outside conditions, so we will expect about 60 percent RH and 11 percent EMC in much of the U.S. in a warehouse. However, if there is heat in the warehouse from solar heating of the roof or other sources, the warehouse will be slightly drier. (Calculations of the change in RH can be easily made using a constant absolute humidity.)

We know that temperature alone, when the RH is held at a constant value, does not change the size or shape of wood. The key here is that the RH is constant. One reference for this fact is The Wood Handbook, chapter 3.

In a home, we might find cooler temperatures in a basement or garage. This will mean higher RH and higher EMC.

In a home in the summertime, we might have the windows opening which would allow the 65 percent RH into the home, so the average home will be running at 50 percent RH and 9 percent EMC, increasing the MC in furniture in the summer. This increase is due to the higher RH and not the temperature.

Likewise, in the wintertime, the outside air brought into the home when an exhaust fan is run, doors are opened, etc., will bring in humid 65 percent RH air, but then the heating of this air to 70 F will lower the RH substantially. Due to moisture from showers, moisture from humidifiers in furnaces, moisture from cooking and moisture from house plants, we seldom will see a house with under 25 to 30 percent RH in the winter, but this will be 6 percent EMC. This is drier than in the summertime.

A cool warehouse or cold warehouse in the wintertime will be closer to the average outside 65 percent RH or 12 percent EMC. So, when very dry furniture is stored in the cool warehouse, it will gain moisture, but the gain is because of the RH change. When brought into a warm house, the furniture will dry and shrink due to the low RH and not the warm temperatures. That is, if we brought cold furniture into a warm house in the wintertime and somehow kept the house at about 65 percent RH, we would not see any shrinking or swelling of the wood because the RH or the MC would not change. (At the high RH in a house, we would see condensation on the windows for sure. Also, cold furniture might get some temporary condensation, but that will go away and evaporate as soon as the wood warms and dries.)

So, the bottom line is that temperature alone does not make wood change size if the RH is constant. It is the change in RH, whether the temperature changes or not, that makes the difference.


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.