With the finalization of the 2018 International Energy Efficiency Code (IECC) performance path, home builders now have option to bury ducts in fibrous insulation in vented attics in all climate zones. While the practice has been used by builders in many areas of the country for years, it was less prevalent in hot/ humid climates because of concerns about condensation. Following extensive field research to identify the appropriate R-value and duct tightness to prevent condensation in those climates, builders can now use the practice with confidence nationwide, giving them an additional compliance option for delivering energy efficient, high performance homes.
According to data from the Department of Energy’s (DOE) Building America Program, buried ducts in vented attics can save energy in homes. DOE’s data shows deeply buried ducts are 20 percent to 30 percent more efficient than unvented attic systems with closed cell spray foam. Moreover, the practice is a less expensive alternative (see chart).
For slab-on-grade houses and two-story houses with open floor plans, installing ducts inside conditioned spaces can be a challenge, as ducts within floors, chases or dropped ceilings can have considerable cost implications for builders and bulkheads concealing ductwork may not be acceptable to buyers. Building unvented attics for housing HVAC systems is a viable alternative, but it’s costly and involves special installation procedures. Installing ducts conventionally above the attic insulation is a more straightforward energy approach, but comes with a significant energy penalty. A buried duct system strikes the balance between ease of installation and reduced heating/cooling loads and may allow for lower capacity and lower costs for HVAC equipment.
General Criteria for Buried Ducts under the 2018 IECC
The recently-approved provisions of the code allow three options for HVAC ducts buried within ceiling insulation in vented attics: the general option, the deeply buried option and an option for buried ducts considered inside a conditioned space.
The general option allows supply and/or return ducts to be fully or partially surrounded by ceiling insulation. For the general option, the code doesn’t specify a minimum amount of insulation above the duct as long as the sum of the insulation value above and below the duct is at least R-19 total, excluding the duct R-value. There is also no minimum level which ducts must be buried into the insulation.
For deeply buried ducts when using a simulated energy performance analysis, the code permits a duct insulation value of R-25 be used in the energy model for sections of ducts that comply with the general criteria. In addition there is a requirement that the ducts be located directly on the ceiling or within 5.5 inches of the ceiling and be surrounded by ceiling insulation of at least R-30. Ducts must also be covered in at least 3.5 inches of ceiling insulation.
Finally, when using a simulated energy performance analysis, deeply buried ducts may be considered as ducts inside a conditioned space, and must comply with the general buried duct criteria and have an air handler that is located inside the conditioned space (not in the attic). Further, the duct leakage must be within prescribed limits and the R-value of the insulation above the duct must be at least the proposed ceiling insulation R-value minus the R-value of the duct insulation.
Although adoption of the 2018 IECC may be years away in some jurisdictions, as builders become increasingly familiar with this option, they may want to use it immediately, rather than waiting until their states adopt the new code. The IRC does not preclude use of products and practices which aren’t specifically referenced in prior versions of the code. Because some builders may be unfamiliar with the practices of burying ducts in vented attics, Home Innovation Research Labs has developed a TechSpec to provide an overview of the key attributes and implementation details for transitioning from a traditional duct design to a buried duct system. The TechSpec includes detailed tables and illustrations that explain specific select climate zones.
