Published: Jan 2009
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TO PREVENT POTENTIAL MOISTURE PROBLEMS IN housing in cold climates, various rules of thumb were developed during the 1930s and 40s. In principle they all stated that a “vapor barrier” should be incorporated into exterior walls on the inside (warm in winter side) of the insulation . These rules were later refined to include a statement that the exterior of the wall should have a water vapor permeance five times greater than the interior vapor barrier . Further refinements attempted to include rules for warm and humid climates . By and large, these rules were useful, although they did not cover moisture transport mechanisms other than vapor diffusion which, it was found later, is not the greatest source of moisture transport into and through building envelopes. While these simple rules may be appropriate for extreme cold or extreme warm climates, they ignore the fact that in many locations design rules targeted at one season (summer or winter) may not be appropriate for conditions during the rest of the year. To account for diffusion mechanisms more effectively, manual, steady-state methods were developed to determine moisture movement in walls and roofs. Being steady-state, based on a single set of assumptions, such as indoor and outdoor temperatures and relative humidities; such methods are of limited value, although their application is a significant improvement over the simple adoption of the rules of thumb previously used. These methods, known as manual design tools, all have severe limitations, and the results are difficult to interpret. However, these methods are widely used by design professionals and have traditionally been used to formulate building code requirements for vapor re-tarders. The proper use and limitations of these methods are discussed in the first section of this chapter, Manual Design Tools. In an effort to provide a better understanding of the various mechanisms and interactions of moisture and heat transfer in building envelopes, transient mathematical models were developed. First thought of as strictly research tools, they soon began to be useful tools for designers. In their most simple form, they applied hourly weather data to model moisture diffusion through multilayer envelope sections. In their most sophisticated form they account not only for diffusion, but also for air movement and rainwater wetting of the exterior surface and rainwater leakage.
Research Physicist, USDA Forest Service, Madison, WI
Bomberg, Mark T.
Research Professor, Syracuse University, Syracuse, NY