Building Cavities Not Used as Supply or Return Ducts
No climate specific information applies.
Nearly all 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 are probably the biggest duct leakage culprits we have in the HVAC industry today.
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.
Figure 1 shows a floor joist cavity used as a return air duct by nailing a sheet good, such as gypsum board, sheet metal, foil insulation, or OSB to the bottom of the floor joists. There are manufacturers advertising “insulating” sheet good products to 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.
Figure 1 - Some builders create pan joists by attaching a solid sheet good to the bottom of a floor joist to create a return air pathway. Pan joists should not be used as return air pathways because they cannot be air sealed properly.
Cavities (or interstitial spaces) within walls are also sometimes used as supply or return air pathways. These cavities often connect 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 building infiltration. A cavity space used as a return air pathway will pull pollutants into the building from unknown sources. Another issue (less talked about) 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 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 or rotting 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.
While none of these spaces make acceptable air pathways on their 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:
- Plan duct layout at the design stage. Indicate floor joist cavities, dropped ceiling soffits, or other building cavities that will be used as duct chases. Calculate required duct sizes using ACCA Manual D (ACCA 2009). Ensure that the cavity spaces are free of obstructions and large enough to hold the duct plus insulation.
- Use only approved duct materials such as galvanized steel, aluminum, fiberglass duct board, and flexible duct that meet local code smoke and flame spread criteria.
- Make sure that all supply and return duct connections are sealed with mastic or approved tape.
- Because ductwork in cavity spaces is likely to be inaccessible, test the duct system for airtightness with a duct blaster test before installing the drywall.
- At a minimum, line the air handler platform with duct board and mastic seal the corners.
Use recognized and acceptable duct materials for all HVAC airways. For residential construction, acceptable duct materials include galvanized steel, aluminum, fiberglass duct board, and flexible duct. Consider duct layout in the initial framing design stage. Do not use a building cavity space alone 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. Use a duct blaster test to detect duct leakage and to confirm proper air flow at each duct supply outlet.
Duct Distribution Quality Installation
Building cavities not used as supply or return ducts unless they meet the items listed in the notes below:
- Avoid using building cavities as ducts due to the difficulty of properly air sealing and insulating them.
If building cavities are used:
- Install insulation without misalignments, compressions, gaps, or voids in all cavities used for ducts.
- If non-rigid insulation is used, install a rigid air barrier or other supporting material to hold insulation in place.
- Seal all seams, gaps, and holes of the air barrier with caulk or foam.
If building cavities are used as supply and return ducts, the following is required by ENERGY STAR:
- Supply ducts in an unconditioned attic have insulation >= R-8 (prescriptive path).
- Supply ducts in an unconditioned attic have insulation >= R-6 (performance path).
- All other supply ducts and all return ducts in unconditioned space have insulation >= R-6.
- Total rater-measured duct leakage <= 8 CFM25 per 100 ft 2 of conditioned area.
- Rater-measured duct leakage to outdoors <= 4 CFM25 per 100 ft 2 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 ft 2 of conditioned floor area, measured duct leakage to the outdoors shall be <= 5 CFM25 per 100 ft 2 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 <= 4 CFM25 per 100 ft 2 of conditioned floor area, or <= 5 CFM25 per 100 ft 2 of conditioned floor area for homes that have less than 1,200 ft 2 of conditioned floor area.
Duct Installation Tips
- 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.
HVAC System Quality Checklist, Duct Quality Installation. Building cavities not used as supply or return ducts unless: 1) supply ducts in unconditioned attic have insulation >= R-8 (prescriptive path) or >= R-6 (performance path); 2) all other supply ducts and all return ducts in unconditioned space have insulation >= R-6; 3) total rater-measured duct leakage is <= 8 CFM25 per 100 ft2 of conditioned area; or 4) rater-measured duct leakage to outdoors <= 4 CFM25 per 100 ft2 of conditioned floor area.
Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.
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 air plenums.*
*Due to copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.