Thermal and moisture performance of buried ducts

Simulation, laboratory, and field studies in a hot, humid climate were conducted to investigate the thermal and hygrothermal performance of ducts buried in blown-in insulation in vented attics. Ducts that leak in vented attic can carry a large energy penalty. The U.S. Department of Energy’s Zero Energy Ready Home National Program Requirements ask for ducts to be located within the homes’ thermal and air barrier boundary. An alternative to placing the ducts in conditioned space is to bury the ducts inside the insulation. The surface temperature of the duct insulation that includes a vapor barrier may fall below the dew point of the attic air and cause condensation. This paper includes details of a study performed to evaluate the condensation potential with different duct R-values, duct size, ceiling R-values, and air leakage characteristics. Both flexible ducts and square ducts with duct board were investigated. The measured relative humidity around the ducts suggests that current encapsulation recommendations may be overly conservative and that ducts buried in the insulation with proper duct R-value for each climate do not experience condensation that could cause moisture damage. The research for this paper focused on Climate Zones 5A (Cleveland, OH) and 3A (Charleston, SC).