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Achieving Increased Envelope Performance by David Cook, RA Photo © BigStockPhoto/Photo Roller AMONG ITS MANY FUNCTIONS, THE EXTERIOR ENVELOPE TYPICALLY MUST CONTROL THE MOVEMENT OF HEAT, AIR, BULK WATER, AND WATER VAPOR BOTH IN AND OUT OF A BUILDING. OVER TIME, VARIOUS SYSTEMS AND COMPONENTS HAVE BEEN DEVELOPED TO CONTROL THIS MOVEMENT. In a properly functioning envelope, these systems work together to maintain the building’s temperature and allow components to dry quickly if they become wet. While these systems contribute to overall performance, they also contribute to the complexity of the envelope design and sensitivity to error. Recent editions of the model codes have increased requirements for thermal insulation, and heightened the base level of acceptable performance for air barriers and vapor retarders. Increasing energy costs have also caused building owners to demand better performance. If one of an envelope’s systems is improperly designed or installed, the increased performance of the other systems may reduce the resulting envelope’s capacity to dry. This may heighten the potential for problems to occur (Figure 1). 70 the construction specifier | april 2013 CS_April2013.indd 70 3/14/13 11:50:04 AM