Building Cavities Used as Supply or Return Ducts
by Nick Gromicko and Ben Gromicko
Nearly all building codes restrict the use of cavity spaces as supply ducts. However, it has been common practice to use cavity spaces as return-air pathways. Building cavities used as return-air plenums is one of the leading causes of duct leakage in homes today. Inspectors can learn how air leakage from ductwork may cause home energy loss, increase utility bills, lower comfort levels, and make the HVAC system less efficient.
Still commonly used is the panned floor joist. Using floor joists as return ducts by panning can cause leakage because negative pressure in the cavity will draw air from the outside into the cavity through the construction joints of the rim area at the end of the joist cavity.
The illustration above shows a floor joist cavity used as a return-air duct by nailing material, such as gypsum board, sheet metal, foil insulation or OSB, to the bottom of the floor joists. There are manufacturers that advertise “insulating” panning sheet products that aid in this practice; however, using panned floor joists as an HVAC air pathway is highly discouraged because air leakage will be very difficult, if not impossible, to prevent.
Some builders create pan joists by attaching a solid panning sheet material to the bottom of a floor joist to create a return-air pathway. Using panned joists is not the best practice because the return-air pathways cannot be air-sealed properly.
Cavities (or interstitial spaces) within walls are also sometimes used as supply- or return-air pathways. These cavities often create a connection of inside air with outside air from an attic or crawlspace. It is very difficult to make such cavity spaces airtight. When cavity spaces are used as return-air pathways or supply-air ducts, a few issues will arise.
Because cavity spaces are leaky, building pressure imbalances across the building envelope will occur, driving air infiltration into the building. A cavity space used as a return-air pathway will pull pollutants into the building from unknown sources. Another issue with using cavity spaces as return-air pathways is fire safety. Building materials, such as wood products, do not meet the flame- and smoke-spread criteria as do approved duct materials. Using cavities as return or supply ducts is not a fire hazard in itself, but it will encourage a fire to spread throughout the building. In humid climates, a cavity space used as a return-air pathway will pull humid air into the cavity space, possibly encouraging mold growth or the deterioration of building materials.
Other common framing cavities used as return-air pathways or plenums are air-handler platforms, open-floor truss cavities, and dropped ceilings. Open-floor trusses used as return-air plenums can draw air from any place connected to that floor. Air-handler platforms used as return-air plenums can draw air from vented attics and crawlspaces through other connected framing cavities. While none of these spaces makes an acceptable air pathway on its own, some building cavities, such as floor joists, can make acceptable duct chases to contain an insulated, air-sealed, metal, or flex supply or return duct.
How to Use Building Cavities as Duct Chases for Supply and Return Pathways
- The builder must plan the duct layout at the design stage. Floor joist cavities, dropped-ceiling soffits, or other building cavities that will be used as duct chases should be indicated. Required duct sizes using ACCA Manual D (ACCA 2009) must be calculated. The cavity spaces must be free of obstructions and large enough to hold the duct plus insulation.
Floor joist cavities can make acceptable duct chases for insulated, air-sealed metal, flex, or fiberboard ducts. See the illustration by the U.S. Department of Energy below.
- Only approved duct materials, such as galvanized steel, aluminum, fiberglass duct board, and flexible duct, that meet local code smoke- and flame-spread criteria must be used.
- All supply- and return-duct connections should be sealed with mastic or approved tape.
- Because ductwork in cavity spaces is likely to be inaccessible, the duct system for airtightness should be tested with a duct-blaster test before installing the drywall.
Duct Distribution Quality Installation
Building cavities used as supply or return ducts should be avoided because of the difficulty of properly air sealing and insulating them.
If building cavities are used, insulation should be installed without misalignments, compressions, gaps, or voids in all cavities used for ducts. If non-rigid insulation is used, a rigid air barrier or other supporting material should be installed to hold insulation in place. All seams, gaps and holes of the air barrier should be sealed with caulk or foam.
According to the U.S. Department of Energy's ENERGY STAR program, if building cavities are used as supply and return ducts, then:
Duct Installation Tips
- Supply ducts in an unconditioned attic must have insulation equal to or greater than R-8.
- Supply ducts in an unconditioned attic must have insulation equal to or greater than R-6.
- All other supply ducts and all return ducts in unconditioned spaces must have insulation equal to or greater than R-6.
- Total rater-measured duct leakage must be equal to or less than 8 CFM25 per 100 square feet of conditioned area.
- Rater-measured duct leakage to the exterior must be equal to or less than 4 CFM25 per 100 square feet of conditioned floor area.
- Duct leakage shall be determined and documented by a rater using RESNET-approved testing protocol only after all components of the system have been installed (e.g., air handler and register grilles). Leakage limits shall be assessed on a per-system (rather than per-home) basis.
- For homes that have 1,200 square feet or less of conditioned floor area, measured duct leakage to the outdoors shall be equal to or less than 5 CFM25 per 100 square feet of conditioned floor area. Testing of duct leakage to the outside can be waived if all ducts and air-handler equipment are located within the home’s air and thermal barriers, and envelope leakage has been tested to be less than or equal to half of the Prescriptive Path infiltration limit for the Climate Zone where the home is to be built. Alternatively, testing of duct leakage to the outside can be waived if total duct leakage is equal to or less than 4 CFM25 per 100 square feet of conditioned floor area, or equal to or less than 5 CFM25 per 100 square feet of conditioned floor area for homes that have less than 1,200 square feet of conditioned floor area.
ENERGY STAR requires that all ducts in exterior walls must be within the air barrier as well as the thermal boundary. It is important for the framer and HVAC contractor to coordinate on the location of a return duct. This allows for proper spacing of the floor or roof structure for installation of the return. If installing supply ducts within the walls, verify that the duct is capable of outputting the necessary air flow. Typically, only double-wall assemblies will have enough depth to allow for proper insulation and duct size. If installing return ducts using the floor or ceiling structure, ENERGY STAR recommends sealing both the exterior and the interior of all return boxes to prevent air leakage.
Section 403.2.3 Building cavities (Mandatory). Building framing
cavities cannot be used as supply ducts. Section 403.2.1 Insulation
(Prescriptive). Supply ducts in attics are insulated to a minimum of
R-8. All other ducts in unconditioned spaces or outside the building
envelope are insulated to at least R-6.
Section M1601.1.1 Above-ground duct systems. Stud wall cavities and
spaces between solid floor joists cannot be used as supply-air plenums.
Section R403.2.3 Building cavities (Mandatory). Building framing
cavities cannot be used as supply ducts or plenums. Section R403.2.1
Insulation (Prescriptive). Supply ducts in attics are insulated to a
minimum of R-8. All other ducts in unconditioned spaces or outside the
building envelope are insulated to at least R-6.
Section M1601.1.1 Above-ground duct systems. Stud-wall cavities and
spaces between solid floor joists cannot be used as supply-air plenums.
Stud-wall cavities in building envelope exterior walls cannot be used as
Here's a joist cavity being used as a supply duct.
Here's a joist cavity with a disconnected duct. It has dropped down from the floor.
Here's the interior of an insulated duct.
Here's the interior of a joist cavity being used as a supply duct.
Here's a joist cavity being used as the main return duct. This is also the location of the air filter.
This is a panned floor joist cavity being used as supply duct.
Drainpipes should not pass through ductwork.
This ceiling register was part of a return duct that used the floor joist cavity above.
This is a panned floor joist cavity being used as a return duct.
Here are two joist cavities above the central I-beam being used as part of a main supply duct to the second floor.
Here's a floor joist cavity being used as a return duct.
Here's a floor joist cavity being used as a return duct. The rest of the duct was never installed and connected to the HVAC system.
Here's a floor joist cavity being used as a supply duct.
Minimizing air leakage from ductwork can help reduce home energy
loss, lower utility bills, increase comfort levels, and make the HVAC
system operate more efficiently. Recognized and acceptable duct materials should be used for all HVAC airways. Acceptable duct materials include galvanized
steel, aluminum, fiberglass duct board, and flexible duct. The duct
layout should be considered in the initial framing design stage. Building
cavity space alone should not be used as a supply- or return-air pathway. For the cavity to
serve as a supply- or return-air pathway, it must contain a sealed,
insulated duct made of approved duct materials. A duct-blaster test
can be used to detect duct leakage and to confirm proper air flow at each duct