The moisture content of a piece of wood and equilibrium with its environment is determined by the relative humidity of the air surrounding the piece of wood. This is true whether the wood is lumber and the manufacturing process is an unfinished component in transit or storage or is finished with the coatings, such as varnish and placed in use.
Temperature does not affect the moisture content appreciatively, and therefore does not make wood shrink or swell appreciatively either. This lack of temperature sensitivity is one advantage of wood when compared to other building materials such as aluminums or plastics.
Before proceeding further, let's look at the critical relationship between relative humidity and moisture content. For a given average relative humidity in the air, there is an average moisture content that wood would equilibrate to.
For example, if the relative humidity in the air is 30%, wood stored at this condition will eventually achieve 6% moisture content. Hours, days, weeks, or even months may be required for this final equilibrium value to be reached.
The time depends on many factors, including size of the piece of wood, thicker will take longer; the grain orientation, and grain will be shorter; the ambient air temperature, hotter will be shorter; and the coating applied if any, coated wood will take longer.
The moisture in the air is referred to as the Equilibrium Moisture Content or EMC.
For our example of 30% relative humidity, the air is 6% EMC.
There are 4-key values of relative humidity and corresponding EMC that a wood manufacturer or user should be very familiar with, and in fact, would be wise to commit to memory.
The first two, 6% to 9% EMC, correspond to 30% relative humidity to 50% relative humidity, respectively. They are very important, as they are the typical interior values for heated and air-conditioned offices and homes in most of North America. These values apply to lumber, green or kiln-dried, as well as rough parts, manufactured components, cabinets, furniture and millwork.
In cold weather climates, interior conditions may be even drier than 6% EMC. In manufacturing facilities with dust-exhaust systems, and where air is heated for comfort, the in-plant conditions are also often drier than 6% EMC, unless the air is humidified. On the other hand, in humid summer months, especially without air-conditioning, conditions in homes, offices, and manufacturing facilities may sometimes exceed 9% EMC.
For exterior exposures protected from the rain in most of the U.S., including warehouses, unheated sheds, and even unheated homes that are under construction, the air averages 12% EMC equivalent to 65% relative humidity.
For coastal areas, Miami, New Orleans, Seattle, and island climates, Jamaica and Japan, the EMC is higher. Interior locations can be up to 12% EMC, exterior locations up to 16% EMC.
Uncoated wood will change its moisture content faster than well wooded that is coated. But, note that the coating only slows the change in moisture content. Often, wood products with a coating, will change moisture content quite slowly, meaning, that brief extremes of very dry or very humid conditions will not be as pronounced as within uncoated product.
And one other basic concept, kiln-drying does not provide a wood product that will not change moisture. All wood changes its moisture content when the humidity changes. Kiln-drying is just a rapid and quality technique to bring the moisture content of lumber down to a desired value. If kiln-dried lumber is properly dried, but the parts and components made from this lumber are not properly handled and are allowed to regain moisture. Then, subsequent shrinking in a dry manufacturing- plant environment, or in a customer's dry home or office can be expected.
We will discuss measurement of EMC and moisture content in a moment. But, before we do that, let's consider some of the reasons for our critical concern about moisture content.