How to Install Flex Duct Properly

We actually like flex duct. But you have to install it properly

That’s the thing about flex duct. When HVAC contractors use it the way it’s intended to be used, it’s fine. However, in many homes, flex duct is overused because it’s relatively easy to install compared to hard pipe duct.

In addition to being overused, it’s frequently installed sloppily. Over time, it sags or gets bent and inhibits precious airflow through your system.

This doesn’t have to happen. HVAC installers can use flex duct and avoid these problems. They just need to install it the right way

Speaking of, the following steps cover the right approach to take when installing flex duct:

Seal the ducts with mastic

All ducts should be sealed and located within the conditioned space (dropped ceiling, soffit, within a floor system, or in a conditioned crawlspace or attic). It would be nice if this were always true, but the fact is that this is rarely the case. When ducts are outside the conditioned space, they should be heavily insulated in addition to being sealed.

Tip: Always use UL-181 rated duct mastic and tie wraps to make the seal between the ducts and other connectors.



Cutting holes for the registers

You can begin the installation by cutting the holes in the ceiling for the registers. (We recommend using MV4 registers. They are a plastic register with built in collar.) They come with a template that is traced onto the ceiling. Then the drywall is cut out. The registers should be centered in the rafter space and kept approximately two feet from the wall. For the most common type of system, there is a common return grille that is usually installed in the ceiling of a central hallway in the home. This is called a return air filter grille because it also houses the air filter for the system. This is recommended to avoid going into the attic to replace the filter. Locate the register between the rafters in the hall and cut the drywall out for the proper size filter grille. Then, 2×4 blocks should be nailed between the rafters to make a box and a piece of plywood can be nailed over the top to fully enclose it. Wood is recommended to avoid condensation.

3. Use flex duct for straight runs

You’ll know you can use flex duct if your design contains straight runs (i.e. long, straight sections of duct). Since flex duct typically underperforms due to sagging and kinking, straight runs pretty much eliminate this.

Yes, the duct can still sag on a straight run. But it won’t sag if your contractor supports it properly.

Anything duct sections that aren’t straight? Use hard pipe! This includes bends, elbows, and anything that’s coming right off the plenum. Bent flex duct will not allow air to flow properly, but hard pipe duct will.

The only time it might be ok to use flex duct for a bend is on a very long one with a big radius. But even then, hard pipe is probably better.


Support horizontal duct runs with 1-1/2-inch-minimum-width hanger/saddle material. Support intervals should be in accordance with the manufacturer’s recommendations, or as your local code specifies, but at no greater a distance than 5 feet.

Long duct runs with sharp bends may require additional supports before and after the bends.

The maximum permissible sag is 1/2 inch per foot of length between supports. Ceiling joists offer an alternative to suspension straps for supporting ducts in attic spaces.

To prevent damage to the duct by the edge of the metal collar, the duct should have an additional support between the metal connection and the bend. This allows the duct to extend straight for a few inches before making the bend.

Some local codes may preclude any vertical use of flexible air duct. Where it is allowed, ADC guidelines say that factory-made flex air ducts may not be used for vertical risers in air duct systems serving more than two stories. Vertically installed ducts also should be stabilized by support straps every 6 feet.

Installing return air ducts

The flexible duct installation continues with the return air duct. The duct will be attached to the return air plenum using a collar. Then stretch the duct to the return air box that was previously built. Cut a hole in the top of the box and insert the collar and attach the duct.

Sidebar: Contractors Speak Out On The Merits Of Flex Duct

Bucky Holland said that he installs flexible duct “as often as possible.” The owner of Holland Heating and Air, Macon, Ga., said that 99 percent of his installations include flex duct.

“There are many reasons for that,” he said. “No. 1 is packaging. It is also quicker and easier to install.”

Holland said the fact that his business is a labor-driven company makes it a necessity to save as much time as possible. He said that the use of flex duct in many cases can reduce the labor costs of an installation.

His company uses flex duct mainly for turns and run-outs, which are tied into the main metal duct lines. “If used properly, flex duct makes a neat job,” he stated. “It is abused every day, but if it’s installed correctly, it will do as good a job or better than hard metal ductwork.”

A Knoxville, Tenn., contractor pointed to the acoustic properties of flex duct as another advantage. “When installed properly, flex duct has a sound-deadening effect without the restrictions of other installation methods,” said Mike Davis, president of City Heat & Air Conditioning. “It is also more cost effective to install than hard pipe with 2-inch ductwrap.”

Michael Hays, general manager for Custom Air Concepts, Winder, Ga., said that a substantial number of installations in the Atlanta market include flex duct. He said his company offers customers a choice of sheet metal, ductboard, or flex duct products.

“Ease of use and speed of installation are the obvious advantages of flex duct,” Hays added. “There are also fewer seams in flex duct, which means less potential for air leakage.”

He noted that systems should be designed using flex duct specs, as flex duct is more restrictive to airflow than some other systems. “But if you design it properly, you could arguably make a better system,” he said.

Hays emphasized that proper installation is essential. “It’s easy to abuse because someone may decide to connect 5-foot runs with 25-foot lengths of flex duct,” he said. “The product is fine – it’s the installation that needs to be addressed. And flex duct manufacturers do a good job of providing installation information.”

A Michigan contractor lists one more use for flex duct – moving make-up air. “We don’t use flex duct much except for piping in make-up air,” said Larry Paquette of Hutchison Mechanical Co. Inc., Macomb, Mich. “I buy a lot of 6- and 4-inch [flex duct] for make-up air. Make-up air should be insulated. Drawing in cold air causes condensate, which doesn’t happen with flex duct.”

– John R. Hall

Publication date: 10/25/2004

4. Support the Ducts

Sometimes the best route for a flexible duct run is up and over an obstruction. But without proper support, ducts can develops sags or kinks that reduce airflow [4A, 4B].

To prevent problems, use saddles or straps to support at regular intervals [4C, 4D]. Follow the manufacturer’s instructions, but in case they “get lost,” here is a list of best practices for supporting ducts: 

  • Space supports no more than 4 feet apart (a connection to a rigid duct or to equipment counts as a support point).
  • Supports should be at least 1½ inches wide.
  • In long horizontal runs with sharp bends, use extra supports before and after the bends.
  • Don’t let the ducts sag more than ½ inch per foot between supports; maximum sag shouldn’t exceed 2½ inches [4E].
  • Support straps should not compress the inner core or constrict airflow.
  • Supports should not constrict the insulation because that can cause cold spots and condensation, which can lead to mold growth. 

When ducts must be routed up and over an obstructi

When ducts must be routed up and over an obstruction, too few supports can cause kinks [4A]. The solution is to create a gradual bend using supports at frequent intervals [4B]. Supporting material should be wide enough to avoid compressing the duct.

A good rule of thumb is to provide a duct support

A good rule of thumb is to provide a duct support no less than every 4 feet, and more frequently when using large-diameter flexible duct [4C], even when ducts are partially supported by framing members [4D]. Ideally, ducts shouldn’t sag more than ½ inch per foot between supports, and maximum sag should not exceed 2½ inches [4E]. 

Illustration: courtesy U.S. Dept. of Energy

Illustration: courtesy U.S. Dept. of Energy

This article was adapted from a video series at, which is based on material from the Building America Solution Center (BASC), an online tool from the U.S. Department of Energy that collects best-practices recommendations from the country’s top building science and home building experts.

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Support your ducts

Use saddles or support straps to hold ducts up, and use enough of them to make sure they don’t cause kinks (there are those kinks again) in the flex duct.

Follow the manufacturer’s instructions, but in case they “get lost” … 

  1. Space the supports no more than 4 feet apart.
  2. Don’t let the ducts sag more than 1/2 inch between supports.
  3. Any connection to a rigid duct or to equipment is considered a support joint.
  4. Long horizontal runs with sharp bends should have extra supports before and after the bends.
  5. Supports should be at least 1-1/2 inches wide.
  6. Straps should not construct the inner core or reduce airflow.
  7. Supports should not constrict the insulation because that can cause cold spots, which can cause condensation inside the ducts, which can lead to mold growth.




Building America Solution Center (BASC) YouTube channel


— Building America Solution Center is an online tool that collects best practice recommendations from the country’s top building science and home building experts to help builders and remodelers get to net-zero ENERGY READY homes.