May 2003
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Thermograph of heat flow around a cooling supply duct in a hot attic.
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A key element of many Building America projects has been to reduce duct losses or gains using affordable methods. To this end, the Consortium for Advanced Residential Buildings (CARB) has been researching the energy benefits and implementation issues associated with burying ducts under low cost loose fill attic insulation.
Through improved duct sealing practices developed in recent years, many HVAC contractors are now capable of reducing duct leakage to close to 5% percent of air-handler flow. With duct leakage now under control in many cases, conductive energy losses have become a more significant component of thermal distribution inefficiency than ever before. Placing ducts under loose-fill attic insulation reduces conductive heat gains in the summer and heat loss during the winter at no additional cost. The greatest benefit associated with increased duct R-values is a reduction in peak cooling demand when attic temperatures are hottest. This peak load reduction can even result in cooling equipment downsizing of ½ ton and thus a net decrease in first cost. Because a tight duct system buried under enough attic insulation can approach the energy performance of a duct system located in conditioned space at a fraction of the incremental cost, it is an attractive alternative for cost conscious builders.
But how much attic insulation is enough? To begin to answer this question and to obtain a more quantitative understanding of the construction variables that affect the performance of buried duct systems, CARB conducted a heat transfer modeling analyses using ALGORā software. As a result of this work, an equivalent R-value that conventional hung ducts must be wrapped with to achieve the same thermal performance as buried ducts was determined for a variety of buried duct configurations. CARB looked at small ducts and large ducts, run over or between truss chords in attics with different levels of blown-in fiberglass and cellulose.
As a result of this study, design guidelines for predicting the performance of buried ducts were developed. These guidelines assign an equivalent R-value to a duct based upon its classification as either "deeply buried," "fully buried," or "partially buried." The advantage of these designations is that they are both easy to understand and verify. Because field validation of modeling results is still required, the equivalent R-values chosen are somewhat conservative. A duct system equivalent R-value is then calculated based upon the different equivalent R-values of component trunks and branches.
Applying this method to different duct layouts demonstrates the effect that a more compact duct system design has on increasing duct system equivalent R-value. With a well-engineered duct system layout and R-38 of attic insulation, our modeling-based guidelines indicate that equivalent system R-values of R-15 are achievable. Coincidentally, there is typically no further equipment downsizing potential at system R-values greater than R-15.
Recently in response to this work, the California Energy Commission (CEC) has become interested in including a buried duct credit in their state energy code. At their request, CARB has developed language concerning the design and field verification of buried duct systems that is in the process of being adopted into the 2005 California Energy Code. (For more information, see: (PDF 151 KB) Download Acrobat Reader.
In addition to this ongoing work with the CEC, CARB is planning field-testing of buried duct systems this summer to validate/refine modeling results. This approach is not currently recommended for use in hot-humid climates.
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