The extensive nature of the leakage problems and the resulting structural damage during the 90’s in the Seattle area has caused a great deal of investigation and some major changes in the design approach taken for new buildings. These new design techniques have a history in theory of over 50 years in North America and a practical application of approximately 15 years. The buildings constructed using these techniques for the past several years are performing extremely well and exhibit few of the problems associated with the previous generation of buildings.
This improved design concept for the building envelope provides for a cavity between the exterior cladding and the main structure of the wall, and is known as a rainscreen. This cavity allows for free drainage of any water which may penetrate the cladding, an air space to provide rapid drying, as well as a means of moderating pressure equalization across the cavity to prevent water being drawn into the building itself. The accepted minimum width of this cavity is ½” to ¾”. This cavity can be provided in many ways, the important feature being that it should allow for free drainage and air drying potential while limiting the overall size of cavity created. We usually expect air change rates to range between 100 and 250 air changes per hour in a well designed and constructed system.
The design concept accommodates for any leakage through the exterior building components by redirecting the water to the exterior or through evaporation before damage can occur.
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