Q Is it OK to use a joist bay for a return-air plenum in an HVAC system?
Editor’s note: Since this article first appeared, the 2015 IECC no longer allows framing bays to be used for hvac ducts. This article focuses on the indoor air quality concerns of panned bays; the issues involved intersect with the energy issues. While, many states have yet to adopt the 2015 IECC, there exist a wide range of reasons not to use panned framing bays to move conditioned air in a home.
A Jeff May, an indoor air–quality specialist from Tyngsborough, Mass., responds: The type of plenum that you refer to is known in the industry as a “panned bay.” It’s made by simply attaching sheet metal to the bottoms of two or more adjacent joists to create a duct for returning air to the HVAC system. While this type of return duct is convenient and inexpensive to install, I’ve seen a number of problems associated with it. In short, it should never be done.
For one, panned bays are often leaky. In some cases, the leaks are due to penetrations for wiring, piping, or ceiling strapping, and in others, to inadequate air-sealing at connections between the bay and the ducts or between the joists and the sheet-metal pan. Because the air pressure in a return duct is lower than the pressure in the rest of the house, ambient air can be drawn into the system through any gaps or openings. Many of these panned-bay returns are located in basement ceilings, and if the basement is moldy and smelly, then air that’s drawn in through breaches in the duct can introduce mold spores and bad odors into the supply air that will be distributed throughout the house. I’ve seen many instances—in both new and old homes—where sheet metal was attached to the bottoms of joists that already had mold growing on them, which contaminated the HVAC system before it was even turned on. This situation is unhealthy, especially for individuals who suffer from mold allergies.
Pulling unwanted basement air into the return ducts also results in system imbalances, which could pressurize the upper floors and cause house air to be forced into wall cavities. In cold weather, moisture from this air could condense inside cold (exterior) wall cavities, supporting mold growth.
Another problem has to do with the location of panned returns. If these ducts are installed in a cooler space, such as a crawlspace, temperatures inside the ducts may drop below the dew point. (In some parts of the country, dew points can reach as high as 70°F in the summer.) When that happens, moisture in the air may condense on the inside metal surfaces of the ductwork, allowing mold to grow in the accumulated dust.
In systems that have moldy or allergenic dust present, periodic duct cleaning is essential if any of the occupants have allergies to these contaminants. But panned-bay returns can be impossible to thoroughly clean because of obstacles such as pipes, wires, and cross-bracing.
If you must install a return-air duct in a basement joist bay because of limited headroom, you can line the bay with ductboard (with the foil facing inside the duct, where air will flow over it) or with a closed-cell foam, such as Armaflex, that doesn’t have a fibrous inside surface that could trap dust and mold spores. Sheet metal is another option. But even foil-faced insulation board can be used to line a joist bay, as long as it’s securely attached. Just be sure to use sealant or foil tape to make all the joints in the duct airtight.
Finally, filtration is an essential part of every air-handling system. ASHRAE recommends that an air conditioning system be installed with a filter no less than MERV-8 (middle-of-the-road efficiency); MERV-11 is better for occupants with allergies. An efficient pleated filter (not less than 2 inches deep) should be installed on the air handler rather than on the grille for the return air. The access to the filter should never be open. Install coarser (MERV-3) filter material as a pre-filter behind every return grille to help capture much of the airborne dust and debris that would otherwise accumulate in the duct and support microbial growth. The pre-filter also extends the life of the more expensive media filter.