Implementing an interior duct system involves bringing the forced air system, including the air handler, within the dwelling’s air and thermal barrier and integrating the new detail into the design and construction process. There are many benefits to bringing the ducts and HVAC equipment within conditioned space – the HVAC equipment is protected from the extreme temperatures of unconditioned attics and crawlspaces, the furnace and air handler are easier to access for maintenance, interior installation requires far fewer holes through the building envelope, and any air losses through duct or cabinet leaks will go to conditioned space rather than being lost to unconditioned space. Installing the ducts in a dropped ceiling chase in a central hallway (with the air handler in a utility closet) is one option for locating ducts in conditioned space. For other options, see the guides Unvented Attic Insulation, Unvented Crawlspaces and Conditioned Basements, Raised Ceilings, Ducts Buried in Attic Insulation, Ducts Buried in Attic Insulation and Encapsulated, and Ducts in Interstitial Floor Framing. Another option is to install a heating and cooling system that does not require ducts, such as ductless heat pumps and radiant floor heating.

In a dropped ceiling or “fur-down” duct chase installation, a critical detail is the integrity of the air barrier between the top of the chase and the unconditioned attic or floor cavity above.

The builder must make a clear and consistent commitment to building an interior duct system. Then this commitment must be communicated to all of the relevant trades. All involved parties (trades, designers, architects, etc.) must be dedicated to this task. Four critical trades are the home designer, the HVAC designers and installers, the drywall installers, and the framing carpenters. The best opportunity to minimize costs while ensuring an aesthetically pleasing outcome is during the preliminary schematic design phase of home construction.

• HVAC designers must first determine the needed size of the duct work based on the ACCA Manual D calculation.
• The size and location of the chase can now be planned by the house designer.
• The method and technique of constructing the chase must be clearly planned, documented, and communicated to the drywall contractors, field crews, and carpenters.

U.S. Department of Energy Building America Partners Tommy Williams Homes and Florida H.E.R.O. have developed and implemented an innovative approach to the dropped ceiling or fur-down chase method that addresses the typical failure point of fur-down chases: the integrity of the air sealing between the chase and the attic above. In the common approach to a fur down chase (Figure 1), the air barrier is adhered to the underside of the truss bottom chords and to the face of the studs in the adjacent hallway walls (between the top plates). The intersection of the air barrier material with these framing members creates a complex set of air sealing details, as shown in the circle in Figure 1. The innovative approach described in this guide eliminates the majority of these leakage paths.

By designing the chase’s path to avoid intersecting load-bearing walls, air sealing of the chase’s top plane, or ceiling, is fairly easy. For chases that will use a non-load-bearing interior wall for one or both sides of the chase, the walls can be constructed to leave a gap of ¾” between the top plate of the wall and the bottom chord of the attic trusses. The ¾” space allows for the installation of drywall over the top plates of the walls to form a continuous ceiling or top of the duct chase. If the chase walls will not be part of an interior wall, the drywall for the ceiling of the chase is installed first. The chase’s ceiling drywall should be extended beyond the borders of the chase over the top plates of the walls and anticipated framing of the chase sides (Figure 2).

This drywall will be the primary air barrier separating the chase from unconditioned attic space above. It is continuous over the wall framing, thereby eliminating the complex intersections and air sealing details between the ceiling of the chase and the tops of the walls.

The HVAC ducts are hung after the chase ceiling drywall is installed. Then, the framing for the sides and bottom of the chase are installed. These will be drywalled when the interior walls of the home are drywalled.

With information derived from the HVAC designer’s Manual J and D calculations, the designer can finalize plans that consider the size requirements and path of the distribution system. This may require an iterative approach as the vertical dimensions of the required duct sizes, the chase, and the ceiling are refined. Preplanning can help facilitate a compact duct design and reduce the amount of ducting needed.

Ducts brought into the conditioned space should include both the return and supply ducts to provide maximum efficiency for the effort. To reduce the size and amount of chase required and to avoid duct sizing miss-matches between the supplies and returns, one or more central main return registers can be installed in central locations in the home with passive return air pathways consisting of transoms over the interior doors of occupiable rooms (bedrooms). The transoms must be sized to allow adequate return air passage to the main body of the house with the goal of maintaining nearly neutral pressure with respect to the home (<2.5 Pascals) when the mechanical system is operating and the bedroom doors are closed. (See the guides Transfer Grilles and Jump Ducts for more information.) The passive transom returns should be sized at least 1 ½ times larger than the room’s cumulative supply duct areas. The supply ducts should be sized according to ACCA Manual D. By eliminating all ducted returns, labor and material costs are reduced. In addition, the system’s overall operating static pressure can be reduced, allowing for less wear on the air handler’s fan motor. Partner builders implementing transom returns in hundreds of Building America homes have not received any noise or light-related complaints. Pressure mapping during the final commissioning should be performed to confirm that the return air system is working as designed.

How to Build a Dropped Ceiling or Fur-Down Chase

Step 1: Plan ahead and document the location, style, and path of the interior duct chase; show the path on the floor plan with dimensions (Figure 3). Provide a detail of any additional framing necessary to complete the chase after the mechanical rough in.

Step 2: Install sheetrock at the top of the chase path as indicated in the design plan (Figure 3).

Step 3: Fabricate and install the supply ductwork in the chase (Figure 4). Fabricate the duct system. Construct and air seal as much of the duct system as possible on the ground where it is easier to align the sections and reach the joints to seal them with mastic. The main supply trunk lines and supply ducts are then hung from the drywall using strapping secured to the framing. Care must be taken to avoid damaging the chase’s ceiling. Rigid ducts are used whenever possible because they are easier to manage than flex ducts during the lift from floor to ceiling.

Step 4: Install the framing for the sides and bottom of the chase (Figure 5). Care must be used to not penetrate the chase’s ceiling while completing the framing of the chase.

Step 5: Install drywall on the sides and bottom of the chase when the rest of the house is drywalled, finishing the chase’s construction (Figure 6).

Step 6: Install return transoms (Figure 7) or other passive return paths to a central return register (jump ducts, etc.).