Equilibrium Moisture Content
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Molecules of water are constantly leaving and returning to the wood surface. If the same number of molecules of water return to as leave from the surface an equilibrium condition exists. Since the wood is neither gaining or losing water it is said to have reached an equilibrium moisture content, or EMC. |
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If, on the other hand, more molecules of water leave than return to the wood surface, evaporation or drying takes place. |
| If more molecules of water return to than leave the wood surface, wetting takes place and the wood increases in moisture content. | |
Water exposed in an open dish in a dry room gradually evaporates, and wet wood dries for the same reason.
When wood is placed in a drying atmosphere, loss of water vapor is rapid at first, slows as drying continues, then ceases when equilibrium is restored.
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The equilibrium moisture content of wood is determined largely by the water content or relative humidity of the surrounding air. If the water vapor content or relative humidity of the air is high, the equilibrium moisture content of the wood will be high. If the relative humidity of the surrounding air is low the EMC of the wood will be low. If the air contains no water, as when wood is dried in an oven at 105 degrees Celsius (220 degrees Fahrenheit) or at a temperature just above the boiling point of water, the moisture content of the wood will then be zero. The table at the right shows the approximate moisture content of wood at various levels of relative humidity. An important reality regarding the EMC or moisture content of wood, and one that will be discussed in more detail later in this lesson, is that as long as wood is below a moisture content of about 30 percent, the water within the wood will be held in the walls of the wood cells. Wood below around 30 percent moisture content has no liquid water within its cell lumens. |
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Wood furniture attains an EMC of about 6% in homes which have a relative humidity of about 30%. |
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For strength testing purposes wood is conditioned to an EMC of 12% at a relative humidity of 60%. Wood framing, such as joists, rafters, and studs in buildings commonly attain an EMC of 10-12%. |
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Wood located outdoors, but which is protected from rainwater, commonly attains an EMC of 14-18%. The EMC will be lower during prolonged dry periods. |
| Under warm humid conditions, as in a greenhouse with a relative humidity of 90%, wood will have an EMC of about 20%. This is an important point, and so will be repeated. Again, if wood is in a greenhouse condition with a very high relative humidity the EMC will only be at about 20%, sometimes as high as 25%. Therefore, the moisture within such wood will be held in the cell walls, and none will be contained within the cell lumens. Once wood has been dried to a low moisture content, liquid water will not again be found in cell lumens unless the wood is exposed to liquid water. |
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