Air temp, relative humidity and MC

How much moisture can air hold, and what is the effect on drying lumber? July 4, 2000

Working with hardwoods, I understand that the wood needs to maintain an equilibrium of moisture content (MC) to reduce the amount of warpage and shrink. But is there a simple explanation of the effect temperature has on the amount of moisture in the air?

It seems to me that the wood will absorb more moisture in a hot, humid environment than a cold humid one -- is that correct? Is any literature available?

Air will hold a certain amount of moisture. The warmer the air temperature, the more moisture it will hold.

The term "grain" is used as a measure of the moisture that air will hold at a given temperature.

One grain is 1/7000 of a pound.

For example, air will hold about six grains of water at 60 degress F, 35 grains at 120 degrees.

When air has only 3 grains of moisture at 60 degrees, it is at 50 percent relative humidity (RH). When air is holding its maximum moisture at a given temperature, it is saturated. If the air cools, the moisture will condense out.

Since wood is hygroscopic, it will absorb or lose bound water, depending on the humidity. I always try to remember three relative humidity levels and their corresponding wood moisture contents:

At 25 percent RH, wood will reach an equilibrium moisture content (EMC) of 5 percent.

At 50 percent RH, wood will reach an EMC of 9 percent.

At 75 percent RH, wood will reach an EMC of 14 percent.

The temperature will affect the moisture content 1 percent for every 30-degree gain in temperature.

Also, remember that different woods will gain and lose different amounts of moisture.

If I am off on this, I hope someone will let me know.

Brian has said it all, except that the temperature is irrelevant between 30 degrees F to 130 degrees, in terms of EMC. So, in response to your questions, at 30 percent RH, the EMC is 6 percent, so the wood will achieve 6 percent MC (between 30 and 130 degrees).

It'll be faster when warmer, but the end result is the same.
Gene Wengert, forum moderator

Think of it as a battle between the air and the wood. They both want to hold as much moisture as they can.

The hotter the air, the faster it can grab those moisture molecules. If the wood has more "free hands" to grab them than the air has, it will gain moisture. If the air has little moisture and is warmer, giving it more free hands, it will grab the moisture from the wood surface.

If the wood is dry, it has more free hands and will gain moisture. When the wood is at EMC, the wood has as many free hands as the air and is grabbing as much moisture from the air as the air is grabbing from the wood.

As far as your specific question, will warm moist air cause the wood to gain more moisture than cold, moist air, Gene said it, the wood will come to the same MC in both, it will just happen faster in the warm air, if both warm and cold air has the same EMC.

If you want to be very accurate, here are the EMCs for 50 percent RH at various temperatures.

30 F = 9.6 percent EMC
80 F = 9.1
100 F = 8.7
130 F = 7.8
150 F = 7.2
210 F = 5.1

Note that the change between 30 and 100 degrees F is small.