In the past, it was common practice to run ducts inside a wall cavity of an exterior frame wall. It is sometimes done today. However, we have learned that this practice will create an energy penalty and can cause durability issues within the wall cavity.
Ducts in exterior wall cavities experience unnecessary heat gain and loss through conduction, convection, and radiation, which creates an extra load on the HVAC equipment. Heat transfer through conduction takes place when the surface of the duct is in contact with the exterior sheathing. If there is an air space between the exterior sheathing and the duct, heat gain or loss will occur as air moves around the duct (convection). At the same time, the duct loses or gains heat from the exterior siding through radiant heat transfer.
Durability issues occur when humid air moves through cavity spaces which contain ducts that are carrying cool air because the humid air in the cavity space can condense on the exterior surfaces of the cool ducts. This scenario is a common occurrence in older homes in humid climates where cooling has been added to a forced air heating system that has ducts located in the exterior framed walls.
In older homes with poorly insulated walls and single-pane windows, supply registers are often located at or on exterior walls to condition the cold walls and cold air that would leak in at the windows. In new homes with better air sealing and insulation and higher performance double-pane windows, there is less heat transfer through exterior walls and less air leakage in and around windows. Exterior supply air throws are no longer necessary to maintain comfort; shorter duct runs with interior throws are preferred for improved energy efficiency and better HVAC performance. Duct layout should be considered at the design stage and the duct system should be designed for short, compact duct runs with interior throws and appropriately selected registers to achieve comfort across each room.
Another concern with ducts in exterior walls is adequate space. Standard wall cavities are not deep enough to house flex duct and may not be deep enough to house sheet metal duct, which should be wrapped in a minimum of 1 inch (R-4) of insulation. Rectangular fiber board duct might fit but analysis should be done to confirm that the space would accommodate the duct size required to meet the designed air flow per ACCA Manual D (ANSI/ACCA 2009).
If ducts must be run in exterior wall cavity spaces, then R-6 rigid foam board insulation should be placed on the exterior side of the duct, between the exterior sheathing and the insulated duct. This rigid foam takes the place of the cavity insulation which is displaced by the duct and is in addition to any rigid foam used over or in place of the external wall sheathing. If ducts must run in exterior walls, make sure the interior and exterior air barrier are continuous and air tight.
Figure 1 - Ducts should not be located in exterior wall cavities.
How to Correctly Locate Ducts
1. Plan duct layout at the design stage. Design short, compact duct runs that do not run along exterior walls. Use floor or ceiling register locations. Be sure that the register box is air sealed to the floor or ceiling framing assembly. The register or grille should be removable so the register box can be cleaned periodically.
Figure 2 - Install supply registers in floors or ceilings to avoid routing ducts through exterior walls.
2. Or, if wall register locations are desired, run ducts in interior walls. Be sure the wall cavity space being used to house the ductwork is totally sealed from a vented attic or crawlspace.
Figure 3 - If wall registers are desired, install HVAC ducts in interior wall cavities.
3. If ductwork must run inside an exterior wall cavity (not recommended), install continuous R-6 foam insulation board along the wall between the ductwork and the siding. Fill the cavity space around the duct with insulation. Ensure that a continuous air barrier exists along the interior and exterior of the exterior wall containing the duct by gluing exterior sheathing and drywall to framing, mudding and taping drywall seams, taping rigid foam sheathing seams, and caulking or foam air sealing around any wall penetrations.
Figure 4 - If HVAC duct must be installed in an exterior wall, separate it from the exterior with at least R-6 of continuous rigid insulation.