Q. We have trouble controlling the wood’s moisture content, which means relative humidity, in our storage building and in our shop. Can you talk about how heating air lowers the humidity?

A. Indeed, heating air lowers its relative humidity. To answer your question, I will use an example. If your temperatures or MCs are different, you or I can recalculate. 

Storage building. Let’s assume that you want your lumber to stay at 7.0 percent MC; this means the air must be 7.0 percent EMC (equilibrium moisture content). To calculate the amount of heat required, we need to make sure that the lumber averages 7.0 percent MC when you put it in the building...if it is wetter, then your storage building is really a drier.

We also need to assume that there is only a small amount of fresh air coming into the building. Finally, we need to make sure that there is no source of moisture in the building. 

One important fact that lets us do these calculations is that the outside air in most of the U.S. just before sunrise is the lowest temperature and 95 to 100 percent RH.

Based on these criteria, we can figure out the amount of heating above the morning’s low temperature needed to achieve 7.0 percent EMC throughout the entire day and night.

Morning’s Low (F)          Heating (F)  Daily Building Temperature (F)

0                            21                21

20                          25                45

40                          27                67

60                          28                88

Shop.  Let’s assume that you shop is 70F in the winter and 75F in the summer (average). Your dust system recycles some air, but there is always fresh air coming into the plant. As you know, the relative humidity (RH) drops if the outside air is heated.  Here are some values when heating air to 70F.

Morning’s Low (F)          Humidity at 70 F (%)       EMC at 70 F (%)

0                            5                            2

20                          14                          4

40                          34                          7

60                          67                          12

I hope you can appreciate why we encourage adding moisture to the air in the plant when it is cold outside.  Also, I hope you can see that we cannot maintain 7 percent EMC in the summertime unless we heat the shop (employees would complain) or run a dehumidifier (or air conditioner which also dehumidifies).

A final word of caution. This low humidity in the winter happens in homes and offices. So many homes and offices are humidified (including humidifiers, as well as moisture from bathroom showers, plants, cooking, snowy boots, etc.). You need to be very careful that you do not humidify your shop to a level more humid than the customer’s home or office. If you do, you can expect complaints when your wood products shrink in their environment.

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.