Design Solutions - Spring 2008 - (Page 54) Figure 100-07, page 27, Quality Standards Ilustrated Wood is a hygroscopic material, and under normal conditions all wood products contain some moisture. Wood readily exchanges this molecular moisture with the water vapor in the surrounding atmosphere according to the existing relative humidity. In high humidity, wood picks up moisture and swells. In low humidity, wood releases moisture and shrinks. As normal minor fluctuations in humidity occur, the resulting dimensional response in properly designed construction will be insignificant. To avoid problems, it is recommended that relative humidity be maintained within the range of 25% to 55%. Uncontrolled extremes—below 20% or above 80% relative humidity— are likely to cause problems. Together with proper design, fabrication, and installation, humidity control is obviously the most important factor in preventing dimensional change problems. Wood is anisotropic in its shrinkage characteristics. It shrinks most in the direction of the annual rings when it loses moisture from the cell walls. Figure 100-03, from Page 24 of the AWI Quality Standards, shows the typical distortion of cuts from various parts of a log. This illustration can help you plan for the inevitable move- ment of lumber elements. Notice that true quartered material, the board on the lower left of the illustration, will usually be the most stable. You can be sure of two things with respect to solid lumber: 1. It will move, and 2. It will change color. These are two laws of physics which all of us must recognize. When we design and build with them in mind, we go a long way toward a successful project. N at u r a l C h a r a C t e r i s t i C s aNd Moisture CoNCerNs How to Select Solid Lumber SHRINKAGE DUE TO DRYING Shrinkage of … 1" x 8" x 10' Dried from Green to Oven Dry 1" thickness = 3/64" 8" width = 3/4" 10' length = 1/8" Figure 100-01, page 23, Quality Standards Ilustrated We all need to work with our customers to help them understand that variation and natural characteristics are the hallmark of fine custom woodwork. Only mass-produced woodwork looks “all the same”—often because imitation wood overlays are used, or because extensive finishing labor is applied to inexpensive substrates. The insistence of some customers on wood completely free of natural characteristics is also an environmental tragedy. It tells woodworkers to discard a large percentage of our valuable natural resources. The single biggest problem associated with woodworking, resulting in hundreds of technical support calls to AWI headquarters, is wood and building movement as a result of poorly controlled relative humidity. AWI is often asked what temperature is required to meet the Quality Standards. While temperature is a component of the equation, it is the indoor relative humidity—that is, moisture in the air at a given temperature—which is most important. The best relative humidity for most parts of North America is between 25-55%. Not only will the woodwork be happier, but also the people will be happier and healthier within that range. 54 DESIGN SOLUTIONS I Spring 2008
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