Wood and Water


Wood is a crucial component of any home, meaning it's a good idea to have a basic knowledge of its characteristics. Wood is an organic material that is comprised of thousands of tubular cells, filled with a mixture of tannins, waxes, starches, water and more. This unique structure is what contributes to strength of wood and also creates the grains in the wood. Specific characteristics of the wood, such as color and strength, will vary depending on the species of tree. In nature, wood's duty is to provide structural support for the tree and carry water from the ground to the leaves.

Because one of it's main jobs is to transport water, trees contain large amounts of water. The exact amount will vary depending on the species, as well as the temperature and relative humidity of the outdoor environment. The quantity of moisture that wood has naturally is described as a percentage of the wood's dry weight. This percentage is known as the initial moisture content of the wood.

Even after being cut down, wood used in home building will still hold some amount of moisture. Wood contains two types of moisture- free water and bound water. After a tree is cut down and beginning to dry, the free water, located in the hollow areas of the wood, will begin to evaporate. On the other hand, bound water, which is connected to the cell walls, requires more energy to dry out. If wood is left to air dry, eventually the free water will dry out and only the bound water will remain. Once this occurs, the wood is at it's Fiber Saturation Point (FSP). FSP does not happen evenly and can occur on the outer portions of the wood before the inner portions.

Due to its large moisture content, wood needs to go through some form of drying process before it can be used. When a tree is first cut, it is considered green and often carries more water than dry wood. The moisture content (MC) of green wood, or the amount of water contained, is often 200%. Gradually, the cut wood will lose moisture, until it becomes balanced with the amount of moisture in the air. Once wood has reached this level, it is considered to be at the Equilibrium Moisture Content (EMC). The exact timing of this drying process, known as seasoning, will vary from months to years based on several factors such as the type of wood, air temperature, and relative humidity. Keep in mind, that wood will first reach the fiber saturation point, then after further drying, it will reach EMC.

Even after wood has reached it's EMC, there will still be small shifts over time as conditions change. After it is seasoned, the moisture content of wood will typically range from 5% - 15%. For instance, wood used for flooring is usually dried to approximately 10% moisture content to mimic the indoor environment, it will be placed in. This will help prevent future shrinking or warping in the wood.

If time is an issue, the drying process can be quickened using kiln drying. In kiln drying, the wood is placed in a kiln to quickly dry it out. In some situations, kiln drying can reduce drying time to days. Not only is this useful for meeting deadlines, but drying the wood in a kiln can remove any organisms that may be living in the tree. Furthermore, quick drying is helpful at reducing the risk of mold and fungi, since they are less like to form if moisture content is below 25%. Thus, the quicker you can dry out the wood, the lower the chance of problems.

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Sources:
Technical Committee of the Log Homes Council, Building Systems Councils National Association of Home Builders. Preservation and Maintenance of Log Structures. N.p.: Technical Committee of the Log Homes Council, Building Systems Councils National Association of Home Builders, 2003. Log Homes Council, 2003. Web. 20 May 2017.
Taylor, Adam M., Stephen L. Quarles, and Karen M. Vail. Wood Protection for Log Home Owners. The University of Tennessee. University of Tennessee Institute of Agriculture, n.d. Web. 20 May 2017.