Flapping Ductwork

Last Updated on February 25, 2023 by admin

flapping ductwork is inexpensive and extremely adaptable, allowing it to fit into awkward and tight spaces that solid metal ducting cannot. However, if we have not installed the product properly, its efficiency and cost-saving properties will rapidly deteriorate with each heating or cooling bill. Before you begin, check with your local building codes department for specifics on the use and installation of flapping ducts.

How To Install A Flapping Ductwork – Complete Guide

Step 1:

Measure the plenums or collars where the flapping duct will be installed. With the utility knife, cut the flex duct to the desired length. Using wire cutters, snip the wire.

Step 2:

Roll back the plastic or foil jacket and the fiberglass lining 8 to 12 inches from the core.

Using a metal hose clamp, slide it over the core and back toward the rolled-up fiberglass section.

Step 3:

Apply a thin layer of mastic glue on the beaded end of the plenum or metal collar. Slip the core of the flex duct over the plenum or collar and onto the mastic glue. Wrap two rounds of duct tape in a strip over the core to cover at least 1 or 2 inches of the plenum or collar.

Step 4:

Place the metal hose clamp over the core and wrap it around the duct tape covering the core and the metal plenum or collar.

Using a screwdriver, tighten the clamp. Place a vinyl cable tie over the duct tape and tightly zip the cable with pliers. Additionally, you need to clean them. For cleaning services, you may contact professionals for Air Duct Sanitizing Cumming.

Step 5:

Unroll the fiberglass section over the core until the plenum or collar is completely covered. Unroll or pull the jacket over the fiberglass until it completely covers it. Wrap two rounds of duct tape tightly around the plenum or collar, about one inch from the edge of the jacket.

Step 6:

Pull the duct’s other, unattached end toward the plenum or collar to which it will be attached. To avoid friction loss, fully extend the duct-like accordion to avoid bunching or sagging.

If the duct is longer than 5 feet horizontally, support it with wire or vinyl cable hung from the rafters so that it does not sag more than 1/2 inch every foot. If you’re hanging the duct vertically against a wall, use metal or plastic support straps secured to the wall every six feet. Avoid bending the duct around sharp corners, but keep in mind that the duct’s central radius should not be greater than the duct’s diameter.

Step 7:

If necessary, trim any excess duct length. Attach this end of the duct in the same way that you did the first.

Common Installation Blunders

  1. Trunk supply ducts (those that serve more than one branch duct) and central return ducts that bend at more than a 45-degree angle across an obstruction or support. These ducts are frequently constricted, and those that make a vertical bend over an obstruction may become even more so over time.
  2. Trunk supply ducts and central return ducts with a core more than 20% constrained in size. Because only one boot is affected, more constriction may be reasonable for a duct serving a single boot.
  3. Duct bends that deviate from the one-duct-diameter-bend guideline.
  4. Parallel vents that exit a plenum or enter a boot.
  5. Vents with a sloppy collar or no metal sleeve at the splice.
  6. Flues that have a pierced inner core
  7. The Ducts are close to a heat-generating appliance vent.
  8. Vents that leak a “substantial” amount of conditioned air at a plenum collar, splice, or junction.
  9. Flapping Ducts that are in contact with the ground and ducts buried in insulation should be avoided if there is evidence of condensation on or near the duct. Unless the local authority with jurisdiction accepts the practice, ducts that breach a fire-rated assembly are prohibited.
  10. Rain or plumbing leaks may cause Wet insulation in ducts. You need to get them clean and insulated. Call for HVAC cleaning in Cumming immediately.

Precautions for safety:

1. Do Not Bend The Flapping Ductwork

Experts do not recommend Bending flapping ductwork across or around framing members, pipes, and other objects. Such bends can reduce the size of the duct at the bend point, thereby restricting airflow and increasing air friction. Over time, the core of the duct can crumble at the bending points, that can also  restrict airflow. One of the most common installation mistakes is bending flapping ductwork around framing members, pipes, and other objects.

In addition, installers should avoid bending flapping ductwork so that the radius at the centerline is less than one duct diameter. Such deformations also decrease airflow and increase air friction. It is yet another common installation error.

2. Avoid Flapping Ductwork Compression

Technicians need to route the flapping ductwork through spaces at least as large as the inner core diameter of the duct.

While this may seem like a no-brainer to some, compressing flapping ductwork is a common installation mistake. People commonly tend to compress the ducts. They usually do this to fit ducts into small spaces. such as the spaces between truss webs and truss braces in the floor and roof space.

The installation mistake is very common when anyone installs vents between different places  or areas, such as the garage roof or the roof space over the conditioned area. Minor duct compression is permissible; however, we should not compress the core at any cost. We need to support the Flapping ductwork horizontally at 1.5-meter intervals and vertically at no more than 1.8-meter intervals.


In conclusion, we can say that the installation of flapping ductwork is a simple process.  However,  one should carefully deal with the ducts. Flapping ducts are very fragile, and we may end up destroying them.

Apart from this, if you are interested to know about Checklist for Air Conditioning Repair, then visit Home Improvement category.