Why Wood Windows and Doors?
Mankind has been fascinated with wood as a building material for thousands of years. Relatively light weight and easily shaped, wood allows us to build a wide array of products which serve both utilitarian and artistic needs. Doors were one of the first items made of wood by man; they provide both security and beauty. Each board has its own characteristics formed by the tree which it once was a part of in nature. The wide variety of different species of trees provides us with woods of different color, grain patterns, and textures. Each species has characteristics capable of changing both the décor and mood of a location and the people there in. There is no other building material better suited for custom doors. But at the same time, wood has properties that the craftsman and the homeowner need to be plan for.
Wood is nature’s renewable resource. In fact, in the USA today, we have more wood growing every year than the year before for the past century…we are not running out of wood. Although wood is stable as temperature changes, is strong for its weight, resists most acids, is easy to work and finish, wood’s reaction to changing relative humidity (RH) conditions is critically important. As the RH increases, wood absorbs moisture and with this absorption comes swelling. Conversely, as the RH decreases, moisture is lost and the wood shrinks. Experience shows that a change in humidity of roughly 10% RH can result in a 1% size change across the grain; on the other hand, wood seldom shrinks or swells lengthwise. Owners of wood decks can easily see the effects of moisture in that in dry weather, cracks develop between the deck boards, but during wet weather these cracks can close. There is no practical way to stabilize wood and protect it fully from expected humidity changes.
Changes in humidity, which occur naturally throughout the year, affect wood windows and doors, but they can be well buffered by using a moisture resistant finish, such as three coats of Coastal Shield varnish. Humidity can be absorbed by the wood fairly well if the change in humidity is slow, taking weeks or months. Rapid changes, however, can result in excessive swelling and shrinking that may be evidenced as warping or cracking. Most wood products are designed to accommodate the annual changes in humidity in a heated and air conditioned home or office. Unusual humidity conditions, including exposure to liquid water, must be avoided to avoid high risk of poor performance.
In addition to vapor resistant finishes, plywood has only 5% to 10% of the shrinkage of solid wood. This greatly reduced shrinkage is a result of having the grain at 90 degrees in adjacent layers; that is, individual veneer pieces will shrink in width but not in length, so by alternating the grain in each layer, the lack of lengthwise shrinkage in one layer restricts the across-the-grain shrinkage in the adjacent layers. Using plywood in areas that are large but must remain flat when the humidity changes, is therefore essential.
There are special concerns when wood products are stored in poor humidity controlled conditions prior to be placed in use. Wood products can be manufactured at the same moisture content that they will achieve in use, which will minimize any moisture size change issues. However, if, after manufacturing, the wood products are stored at higher moisture conditions, unless they are well wrapped in plastic, the wood will slowly gain moisture. (Wrapped products, without holes in the wrap, protected from rain or other liquid water will not change moisture content in storage for many months.) Then, when the products are installed and the heat or a/c is eventually turned on and the typical humidity is achieved, the wood will quickly dry out and potentially shrink excessively. It is best to avoid humid storage; it is critical to maintain the integrity of any plastic wraps until the finish is applied.