Best Practices: Attaching Flex Duct to a Duct Board Trunk
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In the Controlling Condensation on Duct Systems article, we discuss how duct systems can experience condensation problems if not located within conditioned space or are not insulated when they are located in unconditioned spaces. While locating duct systems in spaces that receive conditioned air is ideal for controlling condensation, it’s not always possible due to design and time constraints. Some options are to fabricate duct systems from fiber glass duct board, or to wrap sheet metal ducts with an insulating duct wrap.
The connections between flex duct and duct board is one detail that can be tricky, so care needs to be taken to get the job done right. In order to control condensation on flex duct connections, the nominal thickness of the insulation layer must be maintained over the connection collar. This is difficult, since in the attempt to adequately seal the vapor retarder to the duct, the thickness is frequently reduced. IBACOS has developed a best practice for this detail. After sealing the collar to the duct board trunk or splitter box, attach the inner liner as usual. Then apply two or three layers of fiberglass pipe insulation around the collar prior to extending the duct insulation. This creates a short higher density zone that is tolerant of the mechanical pressures of applying sealants or duct tapes to seal the vapor retarder cover to the duct board. This solution works equally well when using a zip tie as the termination means.
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Step 1
Apply mastic sealant to the metal collar to ensure an airtight seal between the collar and the duct board. |
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Step 2
Install metal collar into duct board hole and bend metal tabs to lock in place.
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Step 3
Apply UL-181 approved foil-backed tape around the metal collar’s flange. Use a plastic applicator to apply firm pressure to ensure a thorough bond between the tape and the duct board. NOTE: A good bond is indicated when the pattern of the duct board surface is visible through the tape.
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Step 4
Clean collar. Tape the metal collar to inner liner of flex duct with foil-backed UL-181 approved tape.
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Step 5
Apply 3-inch wide pipe insulation wrap around the metal collar. The insulation should be wrapped around the collar two or three times to increase the R-value at this area.
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Step 6
Slide the insulation and vapor retarder of the flex duct over the collar and the insulation wrap so it butts up against the duct board.
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Step 7
Tape the outer lining of the flex duct to the duct board to attach the vapor retarder and insulation of the flex duct to the surface of the duct board.
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Step 7 Alternate Method
An alternate method would be to fasten the outer lining with a heavy duty tie wrap. This method is only an option when used in conjunction with a metal collar that has a retention ridge.
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A Note About Duct Tape
In testing by Lawrence Berkley National Laboratory, cloth-backed duct tape was found to not be a good sealant in ducts that operate much above ambient temperature. For more information, read the LBNL report, available at: http://epb.lbl.gov/Publications/lbl-41434.pdf .
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