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The equilibrium moisture content is the point when wood is not losing or gaining water. A piece of wood's moisture content is in constant flux due to external conditions, such as temperature and humidity. The moisture content of wood is measured by subtracting the wood's mass in its completely dry state from its normal mass, and dividing the total by the dry state mass.
A wood's equilibrium content changes depending on the surroundings in which the wood is found. If the humidity of the surrounding area is high, then there are more water particles present to soak into the wood. The equilibrium constant will be high when the humidity is high. Conversely, it will be low when the humidity of the external environment is low.
When placed in an oven with a temperature of 220 degrees Fahrenheit (105 degrees Celsius) or above, the equilibrium constant of wood will fall to zero. This happens because the oven prevents any water from forming, thus preventing any water from being absorbed by the wood. In this sort of an environment, water vapor occurs quickly at first and then slows as the drying process continues. Loss of water vapor stops completely when the equilibrium moisture content returns to normal.
There is a predictable correlation between relative humidity and the equilibrium moisture content in wood. For example, when the relative humidity is at 25%, the moisture content will be at 5%. Similarly, the moisture content will be at 14% when the relative humidity is at 75%.
The Hailwood-Horrobin equation calculates relationships between temperature, relative humidity, and the moisture content. In the equation, the constant used is found using the temperature of the environment in degrees Fahrenheit. An chart of equilibrium moisture content can be created by substituting values into the equation. The equation, which is fairly complex, can be found online.
Inside of homes, wood usually contains a moisture content of 6%. This number is based on the fact that most homes usually have a relative indoor humidity of 30%. Wood that is placed outside but protected from rain typically has a higher moisture content; 14% to 18%. In greenhouse environments, the relative humidity is at 90% and the moisture content will be at about 20%. No matter the environment, the equilibrium content does not rise above 30%.
It is important to note that the equilibrium moisture content is critical to wood framing. To test the strength of the wood being used for these purposes, the wood is developed in an environment of 60% humidity and a moisture content of 12%. Joists, rafters, studs and other structures must be able to withstand these conditions in order to be reliable and durable in construction.