Relative humidity, temperature and wood
May 31, 2014 | 7:00 pm CDT

The relationship between relative humidity when cold air from outside is brought into a heated plant, office or home and wood is rather straightforward.

I firmly believe that woodworkers should be able to answer and understand this relationship if they want to make the best quality products that will not shrink, swell or warp in use.

As everyone knows, air has a temperature and a relative humidity. For woodworkers, air also has an equilibrium moisture content, EMC, which represents the moisture content, MC, that wood will try to achieve at a given temperature and RH.

As an example, at 70F and 30 percent RH, the air is at 6 percent EMC, meaning that wood will be trying to dry to 6 percent MC. Note that the EMC is related to RH; temperature alone with constant RH does not change the EMC substantially.

When air is heated, the temperature rises and RH drops. RH is the ratio between the amount of water vapor in the air compared to the maximum amount of water the air can hold at a given temperature. (So, 50 percent RH means that the air has 50 percent of the maximum water vapor that it can hold before the air turns foggy.)

When air is heated, it will still have the same pounds of water vapor per cubic foot (unless you add or subtract water). This value is sometimes called the absolute humidity. However, warmer air can hold more water, so the RH drops because the amount of water vapor is a lower percentage of the maximum. When the RH drops, the EMC drops. Therefore, the wood will dry in an attempt to reach this lower value.

The reverse situation is true if the air is cooled; that is, the RH and EMC increase, causing the wood to gain moisture.

Wood changes size when the MC changes. As a rough guideline, it takes about a 45 MC change to result in a 1 percent size change across the grain.

Some species of wood, such as teak, are less sensitive to moisture. Some are less sensitive when going perpendicular to the annual growth rings than when going parallel to the rings; stated another way, quartersawn is more stable than flatsawn by about 50 percent.

The real world

In a plant, oftentimes there is no source of moisture added to the air when the air is heated to a comfortable level. So it is easy to calculate the exact change in RH and EMC when air is heated.

If the plant has humidifiers, then calculations would be difficult, therefore, actual conditions must be measured. The calculated values below will be too dry.

Now, consider a home. Moisture is added when a humidifier is used and when people take showers, when someone cooks spaghetti on the stove, when house plants release moisture, and so on. Hence, the values below are too dry.

Finally, consider an office. As most offices do not have humidification and as outside sources of moisture are not common or are small, then the calculations below are accurate for offices.

Specific examples

Let's consider six specific examples. (I used a psychrometric chart to do the calculations.)

1. Outside air at 32F is brought into the building and is heated to 65F. The outside RH is 99 percent RH or 28 percent EMC, typical for many early wintertime mornings. The heated air will be at 27 percent RH and about 5.8 percent EMC.

2. As above, but the air is heated to 70F. The heated air will be at 34 percent RH and 5 percent EMC.

3. As above, but the outside air is 42F. The heated air at 70F will be 26 percent RH and 7 percent EMC.

Question for the reader: If you have lumber that was dried to 8 percent MC (give or take), what is going to happen to this lumber it is freezing outside when it is brought into your plant that is heated to 65F or 70F?

What is going to happen to the MC and the size of small parts cut from this lumber before the lumber can change MC?

4. The air outside is 65F on a humid (95 percent RH) summer morning and the plant is heated to 85F by the afternoon. The plant is at 50 percent RH and 9 percent EMC.

5. Air outside in much of North America averages 65 percent RH, both in summertime and wintertime. This is 12 percent EMC, so the lumber that is stored in an unheated warehouse or cabinets that are stored in an unheated home which is under construction will try to achieve 12 percent MC.

6. Most heated homes (considering moisture added by living) will average about 33 percent RH or 6.5 percent EMC in the wintertime. For a storage building or manufacturing facility to achieve 6.5 percent EMC, heating the outside air about 32F above the morning's low temperature (we assume that the RH is close to 100 percent every morning, evidenced by dew on the grass and fog), will drop the air to 33 percent RH and 6.5 percent EMC.

Question for the reader: If you store lumber (originally kiln-dried to 7 percent MC) in an unheated building, what is likely to happen in a few weeks or so? If you want to store this lumber at 7 percent EMC, how much heat must be added? If it is freezing outside and the storage building is heated to 70F, what will happen to the lumber?

Practical conclusions

Lumber should be kiln-dried to 6.5-7 percent MC, especially in the wintertime, unless the products are to be used in a humid location (Miami, Seattle, New Orleans, etc.).

Lumber should be stored in a heated warehouse; the heat should be about 30F above the morning's low temperature to keep it at 7 percent MC. (Measure the RH to be certain or to fine-tune the amount of heating.)

A plant should be heated no more than 30F above the morning's low; if more heat is required, some humidification is prudent.

Wrapping products, after manufacturing but before installation, in plastic so that the moisture cannot change even though the outside environment is temporarily out of range (such as 12 percent EMC), is a smart move.

Be aware of any unusual moisture conditions (such as a swimming pool) in a home or an office.

Have something to say? Share your thoughts with us in the comments below.

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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.