JOINT FASTENING AND AIR-SEALING
Duct performance is strongly influenced by installation quality. Joints must be made according to industry stan-
dards, and all potential leak points must be sealed with approved methods.

Figure 7-41 calls out critical air-leakage points that must be sealed. Make sure surfaces are clean and dry before applying tape: Use cleaner/degreaser if dirt or oil is present. In addition to sealing duct joints, seal register boxes to the wall or floor with caulk or mastic and any penetrations in the air handler or plenums.

Metal Duct Joints
Mechanically fasten, clean, and seal all metal-to-metal connections.

• Use at least three equally spaced #8 screws to fasten round ducts that are up to 12 in. diameter, and five screws for ducts over 12 in. diameter.
• Use at least one screw per side for square or rectangular ducts.
• Crimp joints should have an overlap of at least 1 1/2 in.
• Seal openings greater than 1/16 in. with mastic and mesh or butyl adhesive tape. Mastic alone, or UL 181A-listed duct-sealing tape, can be used for gaps smaller than 1/16 in.
• Seal collar connections between ducts and boxes or plenums thoroughly with mastic or tape.

Fiberglass Ductboard Joints
Seal ductboard connections with adhesive, mastic, or UL 181A-listed pressure-sensitive or heat-activated tape, following manufacturer’s instructions.

Flex-duct joints. Flexible ducts should be joined by a metal sleeve, collar, or coupling:

• Extend fitting at least 2 in. into the inner core, leaving a 1-in. attachment area on the fitting for a tape seal.
• Mechanically attach the inner core to the fitting with a drawband over the inner core and fitting.
• If screws are used instead of a drawband, use #8 screws positioned to pin the wire coil of the inner liner to the fitting.
• Use at least three screws for duct material smaller than 12 in. diameter, and five screws for ducts larger than 12 in. diameter.
• Seal the inner core to the fitting with mastic or UL 181-compliant tape, and seal the outer vapor-barrier sleeve with a drawband and tape. Patch any holes or rips with mastic or tape.
• The inner core should be mechanically fastened to all fittings, preferably using drawbands installed directly over the inner core and beaded fitting.
• If beaded sleeves and collars are not used, then the inner core should be fastened to the fitting using #8 screws equally spaced around the diameter of the duct, and installed to capture the wire coil of the inner liner (three screws for ducts up to 12 in. diameter, and five screws for ducts over 12 in. diameter).

Insulating Ducts
Wherever possible, run all ductwork within the conditioned envelope of the building (Figure 7-42, next page). Ducts running through unconditioned space must be insulated to R-6 or better. Energy analysis software can estimate the operating cost savings from ductwork insulation.

As a rule of thumb, use the following values.

Hot climates with minimal heating requirements:
• Ducts in attics: R-4 to R-8
• Ducts in basements and crawlspaces: None to R-4

Mixed climates with moderate heating and cooling requirements:
• Ducts in attics: R-4 to R-8
• Ducts in basements and crawlspaces: R-2 to R-8

Cold climates:
• Ducts in attics: R-6 to R-11
• Ducts in basements and crawlspaces: R-2 to R-11

Duct Leakage Minimums
One reasonable standard is that duct leakage in cfm should not exceed 3% of the conditioned square footage of the building. Another standard is that leakage should not exceed 5% of the design airflow capacity of the system.

Equipment to test duct leakage is readily available, and duct-testing services are available in most markets. Testing duct leakage allows quality control of duct installation, and allows accurate estimates of the heat loss or heat gain associated with leaks.

Testing ducts takes one to two hours to accomplish. It’s best to test before drywall is installed, so that leak points are accessible for pinpoint identification and on-the-spot repair. To test, technicians connect a powerful, metered and calibrated fan to the system at a return plenum or at the air handler, and then pressurize the system and measure airflow. A fog machine can be used to trace leaks. Companies supplying duct-testing equipment can also provide training for testers and information about applicable standards for airtightness.