ASHRAE Journal - September 2009 - 78

A Air-Handling Unit Leaky Air-Handling Unit and Supply Ducts Air-Handling Unit Supply Return Supply Supply Return Supply Depressurized Conditioned Space Inducing Infiltration Colored shading depicts the building’s thermal barrier and pressure boundary. The thermal barrier and pressure boundary enclose the conditioned space. Colored shading depicts the building’s thermal barrier and pressure boundary. The thermal barrier and pressure boundary enclose the conditioned space. B Cold Interior Hot, Humid Attic Figure 5: Moving the Boundary. Locate the ducts inside by moving the air barrier and thermal barrier to the underside of the roof deck. Ducts are now inside. Duct leakage does not lead to major problems. Photo 5: Unvented Roof. Spray foam insulation applied to the underside of the roof deck bringing the attic and the ductwork inside the conditioned space. Infiltration of Hot, Humid Attic Air Missing Firestopping/ Draftstopping Cold Interior Condensation/Mold C Supply Air-Handling Unit Return Supply Bedroom Hall Bedroom Colored shading depicts the building’s thermal barrier and pressure boundary. The thermal barrier and pressure boundary enclose the conditioned space. Figure 4a (top): Going Negative. Leaky duct distribution system in a vented attic is a recipe for problems. Figure 4b (center): Leakage Path. Unfortunately, this is a common experience. Figure 4c (bottom): Door Closure. Closing doors magnifies the effect of pressures. All we need is a dependable return path, and this particular problem goes away. 78 ASHRAE Journal can exceed 150°F (66°C). The extreme temperatures just ruin equipment and ductwork. Finally, and most importantly, make this air-distribution system leaky (Figure 4a). The resulting negative pressure in the conditioned space draws humid air from the attic into the belly of the beast (Figure 4b). We can enhance this negative pressure by not providing return paths to the air handlers and closing doors (Figure 4c). These negative pressure fields draw attic air not just into interior framing but also down exterior walls, humidifying surfaces along the flow path, while simultaneously picking up available contaminants along the way. These negative pressure fields also draw air from under the slab into the occupied space through slab penetrations (I particularly like the bathtub plumbing trap opening) and from the garage (the “Kevorkian Option”). Wow, how to fix this mess? Once again, simple approaches work well. Construct the attic as a conditioned space, locating the air-distribution system within the conditioned space and reducing the major adverse effects of system leakage (Figure 5). The technology of choice is spray foam applied directly to the underside of the roof deck (Photo 5). This approach also handles the rain penetration through attic vent problem. It also addresses the durability problem of ashrae.org September 2009

ASHRAE Journal - September 2009

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