Doing It Right the First Time

Guidelines for Designing and Installing Tight Duct Systems

Karl Stum is an engineer with Portland Energy Conservation Incorporated in Portland, Oregon.

Limited leakage from supply air ducts or into return air ducts has little impact on heating or cooling energy if the leaks are to and from conditioned space. But duct leakage to outside the thermal envelope increases energy costs and reduces comfort.

Cavities as Return Plenums

Building cavities used as return air plenums are probably the biggest duct leakage culprits. The main types of building cavity return plenums are:

• Raised floor air handler platforms. When the air handler is on, the return cavity under the platform is under negative pressure. Air will be drawn in from the return grille as designed, but in addition, air will be drawn down the stud cavities from the attic, past any firestop blocking and into the air handler. Outside air easily slips between the wall top plates and the sheetrock and comes in even greater amounts through wiring and plumbing penetrations in the wall top plates as well as from intersecting walls and adjacent dropped ceilings (see Figure 1).

• Wall cavities. Interior wall cavities below insulated attics, above crawlspaces or that intersect exterior walls have leaking potential to and from these exterior spaces.

• Panned floor joists. 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. This is much less of a problem if a conventional bulkhead is installed. The problem is severe in cases where the joists hang out past exterior walls (see Figure 2).

• Open floor truss cavities. The entire "hollow" floor cavity of open webbed floor trusses can be used as the return air plenum. This entire floor space will be under negative pressure which will cause air to be drawn into the house via all the perimeter cracks.

• Dropped ceilings. Creating a return duct below the ceiling by framing and sheetrocking without the use of metal ducting allows air to be drawn into the house from outside via floor joist cavities and their rim or band areas which are connected or open to the dropped ceiling area.

Breaches can result in significant air leakage, since both supply and return ducts are under typical pressures of 40-80 pascals (Pa). All of the concerns for attic duct systems apply to crawlspace systems, except high temperatures that speed the breakdown of duct tape may not be present.

• Metal ducts. Breaches can be located at the joints where both round and square metal duct lengths connect to each other and where they attach to fittings and plenums. However, little leakage comes from the longitudinal shop-bent joints (snap-lock) in round and square ducts and from the factory swivel joints in metal elbows (see Figure 3).

• Flex ducts. Leaks in flex ducts are found at connections of the flex to junction boxes and fittings, and to leaks in the junction boxes or fittings themselves. Duct board junction boxes put together with foil-backed duct tape come apart over time, and thus are a potential source of leakage. The metal collars that fit into these boxes leak between the collar and the duct board cut-out (see Figure 4). Metal junction boxes and fittings used with flex duct have fabrication joints that also leak.

• Duct tape. Relying on duct tape as a structural joint fastener--rather than the required sheet metal screws for metal duct and the nylon or metal band for flex duct--can allow ducts to come completely undone. Duct tape can "melt" from the attic heat and release. Even when a joint doesn't come apart, duct tape can dry out over time and lose its seal. Many practitioners refuse to use it at all although experts disagree over the appropriateness of foil-backed tape in attic applications.

• Damaged ducts. Leaks can be created in an otherwise good installation by workers stepping on, cutting, drilling, or dislodging ducts.

• Register boots. When a supply register is partially closed, supply air can be forced out of the boot, between the boot and the sheetrock cut-out and up into the attic. Ceiling return boots have a similar problem (see Figure 5).

Air handling units in the garage or attic. Air handler cabinet doors are not normally airtight. In addition, the joints of the supply and return plenums attached to the cabinet are also not airtight, resulting in air leakage to and from the garage or attic.

Air handling unit and ducts in mechanical rooms. Return air ducts in mechanical rooms that have outside combustion air ducts will be drawing in air through any joints in the return ducts that are not sealed. Part of that air mix will have originated from the outside.

Ducts under concrete slabs. Most earth is permeable, and leaks in these ducts can waste energy and pressurize the space under the slab, potentially causing an increase of radon entering the dwelling through cracks in the slab.

Design--Tight is Right

The first task in planning an efficient duct system is to decide whether it will be located in or outside of conditioned space. If ducts are considered to be in conditioned space, it is absolutely critical that the space is truly indoors--totally sealed from unconditioned areas. Putting all ducts within conditioned space requires some careful and sometimes creative planning because ducts and air handlers in conditioned space can be unsightly and noisy. Single-level homes are the most difficult challenge, while multiple-story homes more easily accommodate ducts wholly within the conditioned space.

Installation Guidelines

Below insulated ceilings. The best method for handling ducts installed below insulated ceilings is to first apply strips of sheetrock to the framing above where the ducts will go and to the side on any adjacent walls. This sheetrock should extend past the side and bottom of the duct so it can receive the framing furring members over it. All joints and penetrations through the sheetrock should be caulked before the duct system is installed (see Figure 8). The duct is now considered to be within conditioned space, and the duct joints should be sealed.

A less desirable method is to install the duct work to the bare framing and seal all the duct joints as if they were in unconditioned space. Metal or flex ducts below uninsulated ceilings require little sealing. Ducts in open-webbed floor trusses should be 100% metal duct. If flex duct is used, it should be insulated--even if it is in conditioned space. This prevents the inner lining from being cut or torn by truss-connecting plates and workers.

If the air handler is in the attic, cabinet doors should be sealed with removable foil tape or gaskets and plenum-to-cabinet connections should be sealed with mastic. Pipe penetrations can be sealed with mastic or cork, or another suitable tape.

Metal ducts in attics. Rectangular ducts or plenums in the attic must have their transverse C-clip joints sealed with mastic. The longitudinal snap-lock joints need not be sealed. Connection slip joints of metal round ducts in the attic should be securely fastened with screws and then sealed with mastic. Metal junction boxes should have all joints sealed with mastic. Dovetailed connections of collars or pipes to junction boxes or plenums must also be sealed with mastic. Duct boots should be securely fastened to the framing on at least two sides and caulked to the sheetrock where they penetrate the ceiling. The swivel joints of elbows normally need not be sealed, but if one is already at the fitting applying mastic, it's a good idea to apply mastic to the swivel joints. It is important that metal ducts be adequately secured so they cannot be pushed out of place.

Flex ducts in attics. Flex ducts must be insulated in attics to R-4 as a minimum, but R-8 is usually preferred. Metal collars extending out of metal or duct board plenums or junction boxes must be sealed to the box or duct with mastic or duct caulk (see Figure 4). The flex duct connection to the collar or other sleeves is made by slipping the inner liner of duct over the sleeve and attaching a nylon or metal band. Then a layer of mastic is applied over the joint extending about two inches onto the collar and onto the duct liner. Then fiberglass mesh is laid over the joint and another layer of mastic is applied over the mesh. The insulation and outer covering (vapor barrier) is pulled over the joint and secured to the collar with another band. Special mylar tape has been recommended in factory assemblies instead of mastic and bands, but the track record for this application is unknown.

Duct board junction boxes have been reported to come apart over time, but the exact reason for the failures is unknown. It is safer to use metal boxes, but they must be sealed with mastic and insulated on site. It is very important that flex ducts be adequately supported (every 10 ft maximum), so they are not pushed out of place, resulting in a broken seal. It is also important that workers be reminded not to move or accidentally cut or damage the ducts. Cloth-backed duct tape is not recommended for attic duct sealing and foil-backed tape is questionable (see Figures 4 and 5).

Ducts in crawlspaces. The same recommendations apply to crawlspaces as to attics for both metal and flex duct. The only differences are that the register boot in a crawlspace is sealed to the flooring rather than the sheetrock.

Ducts in vented mechanical rooms. Return ducts in mechanical rooms that have outside combustion air ducts opening into the room must be sealed with mastic or duct caulk.

Ducts in concrete slabs. If bare galvanized ducts are used, sealing all joints with mastic is recommended. The procedures for completely encasing metal ducting joints with concrete will probably result in little leakage. When using reinforced fiberglass ducts, the common metal connectors, butyl gaskets, and PVC tape wrap should provide an adequate seal.

Using Building Cavities as Return Ducts

Some practitioners recommend that no building cavities be used as return ducts. However, practical experience and some recent studies in Minnesota show that some cavity configurations result in little leakage to the outside, even when not sealed. The following recommendations should result in acceptably tight return duct systems.

Air handler platforms. Framers must be told early not to put the plywood top on the platform before the walls underneath are sheetrocked. After the platform is framed, before the plywood top is applied, all the stud wall surfaces adjacent to and under the platform should be sheetrocked. This sheetrock (or wafer board) should extend from and be sealed with caulk to the underside of the platform perimeter framing and to the floor. Joints in the sheetrock should be similarly sealed. If the framers did not use the recommended ledger type header to support the platform floor joists, the sheetrock must be notched around the joists in order to extend up to the underside of the plywood floor.

The platform top is then applied and the HVAC contractor cuts the desired hole in the top of the platform and in the floor below or sidewall for the return intake. If the return inlet is just straight through the wall, the best method is to cut any interfering studs off 8 in. or so and install a header in the wall. Then install a 4.5 in. section of rectangular duct the size of the return grill through the wall and seal it to the sheetrock under the platform. Another alternative is to "hard duct" the return air under the platform with a large metal "L"-box and eliminate the need to sheetrock under the platform.

If the return is intended to be drawn from a high inlet wall grille, down the wall cavity and into the space under the platform, 3/8 in. x 3/16 in. common foam tape weatherstrip should be installed on the face edge of the wall top plate and down the edge of the side studs to the floor on both sides of the wall which will seal the stud cavity from adjacent spaces, once the sheetrock is applied (see Figure 6).

Wall cavities. Besides the wall cavities associated with air handler platforms, another common wall cavity return is where the return ducts from a floor below open up into a wall above. For walls below attics, from which air will otherwise be drawn, foam tape should be applied across the face edge of any header blocks at the top of the grille opening and down the side studs' face edges to the floor on both sides of the wall. The blocking or header between the studs at the top of the grille should be caulked to the studs (see Figure 7).

Panned floor joists. The panning of floor joists which have conditioned space above and below requires no sealing if the bulkheads between the floor joists fit tightly. Metal, plywood, or sheetrock panning or bulkheads that are above a crawlspace or garage should be sealed to the joists using standard caulk or duct caulk, or else mastic (see Figure 2).

Floor trusses. Using the floor truss cavity as the return plenum requires all cracks and holes from the space through the rim or band area to the outside or up or down into exterior walls or floors to be sealed. If the general contractor is not willing to do this sealing, the HVAC contractor should not use the cavity as the return plenum. Rather, the return should be hard, flex, or metal duct.

Dropped ceilings. The large variety of dropped ceilings used as return air plenums make specific recommendations difficult. The main points that must be followed are that the dropped ceilings must be sheetrocked above first, if they are below unconditioned space. Sheetrocking to the sides on adjacent walls is also critical and all sheetrock joints must be sealed. The spaces must be built so no air can be drawn into the cavity from adjacent walls and floors that may communicate with the outdoors (see Figure 8).

Keeping an Eye on Tightness

It is critical for the general contractor to inspect the duct system before the insulation and sheetrock is installed. Most problems are easy to see and repair at this point. The duct system should be checked for leaks with diagnostic equipment (see "Leak Detectors: Experts Explain the Techniques," p. 26). Testing may not be required for ducts inside conditioned space, if proper procedures are followed. But when ducts are outside conditioned space, a trained technician should be hired to test and pinpoint any leakage. After a half dozen successful installations, if the same contractor (and crew) is used on all jobs, random testing on a fraction of the jobs may suffice. Tests should be conducted on all significantly different duct configurations and when new crews install them. In climates with significant heating or cooling loads, total leakage to the outside should be less than 4%-7% of the nominal flow rate of the system when it is under 50 Pa of pressure.

This article is based on a paper published in the proceedings of the 1993 Energy Efficient Building Association (EEBA) Conference, Building Solutions Conference and EEBA, 1000 Campus Drive, Wausau, WI 54401-1899. Tel:(715)675-6331, Fax: (715)675-9776. n

Figure 1. Raised air handler platform leakage.

Figure 2. Panned floor joist, section.

Figure 3. Metal attic ducts.

Figure 4. Duct board junction box, section.

Figure 5. Ceiling return air, section.

Table 1. Recommended Fasteners and Sealants for Ducts
Duct joint or connection                          Inside conditioned space                      Outside conditioned space

_________________________________________________________________________________________________________________________________________
Flex duct to collars, sleeves, or fittings        Band plus cloth aluminum tape 		All-weather band
_________________________________________________________________________________________________________________________________________
Metal collar to metal or duct-board               Duct caulk or mastic unless extremely         Duct caulk or mastic
  duct or junction box                              well built            
_________________________________________________________________________________________________________________________________________
Metal round pipe consecutive lengths              Screws plus mastic, or cloth or mylar tape    Screws plus mastic
  and fittings        
_________________________________________________________________________________________________________________________________________
Metal round pipe to plenums (dovetailed)          Mastic                                        Mastic        
_________________________________________________________________________________________________________________________________________
Longitudinal snap-lock joints in square           None                                          None
  or round duct        
_________________________________________________________________________________________________________________________________________
Poor fitting plenum, corners and joints           Mastic                                        Mastic        
_________________________________________________________________________________________________________________________________________
Metal elbow swivel joints                         None                                          Mastic optional        
_________________________________________________________________________________________________________________________________________
Plenums to air handler                            Mastic                                        Mastic        
_________________________________________________________________________________________________________________________________________
Air handler cabinet doors                         None                                          Aluminum tape or gasket        
_________________________________________________________________________________________________________________________________________
Duct boot to sheetrock ceiling or flooring        None                                          Secure tight, standard, or duct caulk        
_________________________________________________________________________________________________________________________________________
Metal return panning to joists                    None                                          Duct caulk or mastic        
_________________________________________________________________________________________________________________________________________
Sheetrock return panning to joists                None                                          Foam tape or standard caulk        
_________________________________________________________________________________________________________________________________________
Sheetrock to studs above wall return register     Foam tape unless below an insulated ceiling   n/a 
_________________________________________________________________________________________________________________________________________
Sheetrock to framing under air handler            Standard caulk                                n/a
  raised platform        
_________________________________________________________________________________________________________________________________________
Band joist areas of open floor truss return       Standard caulk or foam tape, as applicable    Standard caulk or foam tape as applicable        
_________________________________________________________________________________________________________________________________________
Miscellaneous joints between framing              Standard caulk and foam tape, as applicable   n/a
  members and sheetrock in building
  return cavities        
_________________________________________________________________________________________________________________________________________
Definitions:

Cloth tape           High quality, dense cloth, thick adhesive, Underwriter Laboratory-rated

Aluminum tape        High quality aluminum backed with butyl adhesive (15 mm minimum)
Duct caulk           Silicone or other Underwriter Laboratory-rated duct caulk

Standard caulk       Construction adhesive, such as siliconized acrylic latex caulk

Foam tape            Door weatherstripping tape, 3/8" x 3/16" thick, sticky 
on one side

Mastic               Underwriter Laboratory-rated duct mastic

Reinforced mastic    Layer of mastic, layer of fiberglass mesh, layer of mastic

Figure 6. Sealing raised air handler platform, section.

Figure 7. Wall cavity, return air.

Figure 8. Dropped ceilings.

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