Wood Moisture Content

Wood contains a lot of moisture. Even when the wood is processed into a wooden floor, it still contains a considerable amount of moisture. In addition, wood constantly absorbs moisture from its environment and subsequently releases moisture to its environment. Wood moisture content is one of the most important properties of a piece of wood.

Water and Wood

On the outside, this is only a little bit visible or not visible at all in the wood itself. Water occurs in wood in two different ways; namely in the cell cavities as free water and within the cell walls as bound water. When fresh wood dries in a dry ambient air, the free water first disappears from the cell cavities. When all the free water has disappeared from the wood and water only occurs in the wood in the bound state, this is referred to as the fiber saturation point. Only after all the free water has dried from the wood does the bound water start to disappear from the wood. However, the bound water from the wood is never completely released into the air, unless the wood is dried in an oven at a temperature higher than 100 ° C.

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The Dry Weight of Wood

The wood moisture content is the mass of water present, expressed as a percentage of the mass of the absolute dry wood (dry weight of the wood). Dry weight is understood to be the weight obtained after drying the wood in an oven at a temperature of 103 ° C to 105 ° C until the weight remains constant. With heavy wood types, the wood moisture content of the living tree is lower than the weight of light wood types. After all, with heavy woods, the cell walls are thicker, so that less space is left for the storage of water.

State and Temperature of Wood

The moisture content of wood is related to the condition of the air surrounding the wood and the temperature. The amount of moisture in the air is measured in relative humidity (RH). The relative humidity thus indicates how much water vapor is in the air. Air can only contain a limited amount of moisture; how much depends on the temperature. Relative humidity indicates how much water vapor the air contains at the prevailing temperature, so how humid it is. A value of 100% means that the air has a maximum amount of water vapor. The air is then saturated.

At a relative humidity of 50%, the air at the prevailing temperature contains half the maximum possible amount of water vapor. When wood is exposed to air with a constant relative humidity for a sufficiently long period, it will acquire a moisture content that is in equilibrium with this relative humidity. This is called the equilibrium moisture content of the wood. The equilibrium moisture content of the wood is not the same for all wood species at the same relative humidity and temperature of the air.