In 1999, renowned Canadian consultants Donald Hazleden and Paul Morris presented a seminal paper that summarized the 4 D’s (deflection, drainage, drying, and durable materials) of successful wall design. This Case Study demonstrates that until 50 years ago, successfully weather-resistive wall assemblies at wood-framed homes could be designed with an emphasis on deflection, outward drying during winter months, and reasonably durable building materials. Then, energy efficiency mandates arising from the “1973 oil crisis” and ensuing “1979 second oil shock” served to rapidly eliminate the outward drying option from a designer’s toolkit. Concurrently, because rainwater deflection (wide eaves and overhangs) became a far less common design choice, weather-exposed exterior walls have tended to experience greater levels of rainfall. As a result of these historical transformations, modern building envelope designers have, by necessity, become increasingly reliant upon their last remaining 4D design option – secondary drainage behind the exterior cladding.
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