Despite code restrictions on supply ducting in building cavities, they are regularly utilized for return ducting. However, this practice compromises your building envelope and leads to leaks. When return-air plenums are installed, it results in energy loss, an increase in energy bills and a less efficient HVAC system.
According to Energy Star: “Supply-side leakage to the outside can cause a negative pressure difference in the building with reference to outside. Return-side leakage, on the other hand, can cause a positive pressure difference in the building with reference to the outside. On average, such leakage can cause a 10% to 20% increase in heating and cooling energy use, along with a 20% to 50% decrease in heating and cooling equipment efficiency.”
Panned floor joist are a commonly utilized as return ducts by nailing gypsum board, foil insulation, OSB flooring panels or sheet metal to the joists. It is impossible to create an airtight seal with this method and, together with the negative pressure that is created inside the cavity, air leakage occurs through joints of the rim at the end of the joist cavity. Some manufacturers produce insulating panning sheet products also encourage air leakage.
Another method is to attach a solid panning sheet material to the bottom of a floor joist, but these can also not be sealed completely and leakage will occur.
Flooring joists aren’t the only cavities that are used for ducting, wall cavities are also used for air return or supply ducting, but these are equally difficult to insulate.
Just like the panned floor joists, wall cavities will leak air and a negative pressure inside the ducting will pull air in from outside and, with it, pollutants and humidity. When humidity condenses inside the wall cavity, it can lead to mold and mildew which could cause allergies and sick building syndrome and cause deterioration of the building materials.
Using wall cavities as return air spaces reduces the efficacy of your fire prevention strategies. These ducts don’t pose a fire hazard, but will spread flame and smoke in the event of a fire.
Other Framing Cavities
Aside from wall and floor cavities, other framing cavities are also used as plenums or return-air ducts. These include open-floor truss cavities, dropped ceilings and air-handler platforms. All of these cavities also lead to air leakage and the problems associated with that.
While the cavities themselves don’t make good pathways for air, they are a good place to house insulated, metal or flex return or supply ducting. This places your ducting in a conditioned space and reduces the strain on your HVAC system.
Using Building Cavities as Duct Chases
The design professional must plan the ducting at the design stage, indicating which building cavities will house ducting. Ducting sizes must also be indicated and can be calculated using the ACCA Manual D (ACCA 2009) and the spaces must be designed to hold both ducting and insulation.
The insulation in building cavities should be installed without gaps or compression. Where non-rigid insulation is used, insulation can be held in place with a rigid air barrier or other supports. All gaps and seams should be sealed with foam or caulk.
Only ducting made from approved materials and with the smoke and flame-spread criteria stipulated in your local building code can be utilized. Ductwork connections must be properly sealed to prevent air leakage with mastic or tape. Since ductwork in cavities can be very costly to access once it’s sealed in, test that all the joints are properly sealed with a duct-blaster test.
- If cavities are used to house supply or return ducting they should meet the following Energy Star criteria:
- Ducting in unconditioned attics must have a minimum of R-8 insulation.
- All other supply or return ducts must have a minimum of R-6 insulation.
- Duct leakage can be a maximum of 8 CFM25 per 100 square feet of conditioned area and 4 CFM25 per 100 square feet of conditioned floor area to the exterior of the building.
- Homes with 1,200 square feet or less of conditioned floor area can have a maximum of 5 CFM25 per 100 square feet of conditioned floor area.