Humidity, temperature, and wood moisture content

Water vapor is always present in the air. The amount of water vapor air can hold depends upon its temperature: warm air can hold more than cold air. The maximum amount of water vapor that air of a given temperature can hold is called its absolute humidity at saturation. Except when foggy or rainy, air seldom contains the maximum amount of water vapor that it could. The amount of water vapor actually held in air of a given temperature is termed the absolute humidity.

Importantly, changes in wood moisture content (MC) are keyed to changes in relative humidity (RH), not absolute humidity. Relative humidity is the ratio of the actual amount of water vapor contained in air of a given temperature to the maximum amount of water vapor that air at that same temperature could hold, expressed as a percent. In other words, relative humidity is the absolute humidity divided by the absolute humidity at saturation of air at the same temperature.

Wood is a hygroscopic material. Always containing water, it constantly exchanges water vapor with the air, picking it up when atmospheric relative humidity is high, and giving it off when relative humidity is low. Since wood swells as it adsorbs water, and shrinks as it releases water, both its moisture content and its dimensions are controlled by the relative humidity of the surrounding air. Wood moisture content is equal to the weight of water contained in the wood divided by the ovendry weight of the wood, expressed as a percent.

Though air temperature and relative humidity can change radically in a short time, the moisture content of unfinished wood changes slowly. Moisture content changes in finished wood happen even more slowly because water vapor must first diffuse through the coating. Because of the time lag between changes in atmospheric conditions and changes in wood moisture content, short-lived fluctuations in relative humidity usually have no appreciable effect on wood moisture content. But with prolonged exposure -weeks to months- wood will eventually stabilize at an equilibrium moisture content dictated by the average ambient relative humidity.

Since warm air can hold more water vapor than cold air, the relative humidity of air with a certain absolute humidity can be changed by simply changing its temperature. If in winter, for example, outside air at 20F and 65% RH is drawn inside and warmed to 70F without humidification, its relative humidity drops to about 10%. In summer, outside air at 70F and 60% RH that flows into a basement at 60F will end up at 82% RH.

Despite wide day-to-day fluctuations, average outdoor relative humidity actually changes little from season to season. As a result, the moisture content of wood used or stored outdoors, but protected from direct wetting, varies little through the seasons. It's inside homes however, where the relative humidity of outdoor air drawn inside is drastically altered by heating it and cooling it without humidification or dehumidification, that wide seasonal swings in relative humidity, and hence, wood moisture content and dimensions, occur.

Short of installing a humidifier/dehumidifier to control relative humidity indoors, the best way to minimize changes in wood moisture content and dimensions is to build with wood conditioned to the average equilibrium moisture content that it will see in service, and finish wood with low-permeability coatings.

Stephen Smulski, Ph.D., is President of Wood Science Specialists Inc., a consulting firm in Shutesbury, Massachusetts that specializes in solving performance problems with wood products in all types of wood-frame construction.

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