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4 Rules for Flexible Ducts that Remodelers Need to Know

Using flexible ducts saves time, but careless installation can cause performance-related problems that are hard to find and even harder to fix after the drywall goes up

January 16, 2016
Rules for remodelers when installing flexbile ductwork

All photos: courtesy U.S. Dept. of Energy

On most projects, HVAC flexible ductwork is located in designated chases that maximize the efficiency of the airflow. But the path for the final branch of the flexible ductwork tree—the one that runs from the trunk or plenum to the outlet register—is typically chosen by the subcontractor. Often, they find themselves competing to find space to run the ducts among all the wiring and piping already installed by the electrical and plumbing trades. 

Flexible ducts can speed installation by reducing the number of joints in a run and eliminating the need for elbows and offsets. They also easily conform to oval as well as round connectors. But the physical properties of flexible ducts that create advantages are also the source of potential problems if care isn’t taken during installation. A good first step, or course, is to make sure there is effective communication among all the parties involved, including designers, framers, and HVAC, plumbing, and electrical tradespeople when installing flexible ducts.

But it doesn’t hurt to have visual aids when discussing, monitoring, or inspecting the work. The details presented here were captured from the U.S. Department of Energy’s Building America videos and cover the fundamentals of flexible duct design and installation. A lot of what you’ll find here is common sense, but don’t let that fool you. Airflow is more complicated than it seems, and carelessness or disregard for these best practices can really come back to bite you. 

1. Seal the Ducts With Mastic

All ducts should be sealed using UL-181–rated duct mastic [1], and flexible ducts should be held in place with tie wraps at all connectors. Ideally, all ducts should be located within the conditioned space. This is typically the case for ducts that run through a dropped ceiling or soffit, or between joists, so long as the floor system sits on top of a conditioned basement or crawlspace. Attics can be problematic when insulation is located in the attic floor rather than in the rafter bays or, better yet, outside the roof sheathing.

When flexible ducts are located outside the conditioned space, they should be sealed to prevent loss of conditioned air and also insulated to prevent thermal loss or gain from the ambient air.

install flexible duct for heating and cooling

All ducts, whether rigid or flexible, should be sealed with UL-181–rated duct mastic. Duct tape has many uses, but sealing ducts isn’t one of them.

install flexible duct for heating and cooling

Illustration: Air Diffusion Council, “Flexible Duct Performance & Installation Standards,” 5th Edition

RELATED: The Body's Defenses Against Breathing Dusty Air

2. Use Only What You Need 

Friction is the enemy of airflow. Unlike rigid ducts, which are cut to length within a tolerance of 1 inch or less, it’s easy to cut a length of flexible duct that’s several feet longer than is needed to get from point A to point B [2A]. That creates slack in the duct, which reduces airflow for two reasons. First, because the air has to travel farther, it is exposed to more of the duct’s interior surface area. Second, because the duct isn’t stretched tight, the wire ribs in the duct create more friction than usual along the entire length of duct.

To maintain strong airflow, plan for short, straight runs during the design phase. Pay particular attention to the framing plan, and route ducts through floor trusses where possible [2B]. Monitor the plan during framing and be prepared to create chases to carry ducts where necessary. Also make sure that all ducts, regardless of length, are stretched tight between fittings.

flexible ductwork

flexible ducts

Avoid taking up slack with sharp bends [2A], which severely reduce airflow. Plan the framing package to include chases or trusses to accommodate ductwork [2B]. Ducts that are not stretched tight or that contain sharp bends can increase the equivalent length of the duct run by several multiples (see illustration, above).

3. Don’t Get Kinky With Your Flexible Ductwork

Airflow in a duct run of any length can be compromised by sharp turns or even kinks. And each turn, kink, or compression in a duct run reduces airflow, which results in more comfort-related complaints from your customers. 

Unfortunately, on many jobsites, poor planning of the framing and lack of coordination among subtrades result in all manner of bends and kinks, some of which all but stop airflow completely [3A]

Once the drywall is up, these problems are impossible to locate, so carefully inspect flexible ductwork before the drywall crew begins its work. Kinks are comparatively easy to spot, but a sharp bend can constrict airflow and is more difficult to see [3B]. Where bends can’t be avoided, make sure that the radius of any turn is at least the same dimension as the duct diameter. For example, the center line of a bend in 8-inch–diameter duct should follow a curve with a radius of at least 8 inches [3C]. One easy way to ensure that bends meet this standard is to use metal elbows at these critical locations.

install flexible duct for heating and cooling

install flexible duct for heating and cooling

Lack of coordination among subtrades can result in bends and kinks to avoid piping, wiring, and framing components [3A]. To prevent the need for costly troubleshooting behind finished walls and ceilings, carefully inspect before drywall is installed to make sure there are no sharp bends or any kinks concealed behind framing [3B].

install flexible duct for heating and cooling

Illustration: courtesy U.S. Dept. of Energy

RELATED: Fresh Air: Rules for Good Indoor Air Quality

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. 

install flexible duct for heating and cooling

install flexible duct for heating and cooling

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.

install flexible duct for heating and cooling

install flexible duct for heating and cooling

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]. 

install flexible duct for heating and cooling

Illustration: courtesy U.S. Dept. of Energy


This article was adapted from a video series at ProTradeCraft.com, 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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About the Author



Recent CO inspection, it was noted Sheetrock dust, construction tape, small pieces of wood from subfloor, pieces of drywall. Who is responsible for keeping the duct clean, and who is responsible to clean the duct work. After construction. Is it a installer requirement to protect openings of forieng materials until use. Thanks

The ones that are your guilty parties are the plumbers and electrians.

your pictures of flying ducts are trash. the us government does not know how to install ducts. but now we must install ducts this way. because they gave money to our state governments. state governments adopted their codes. now we have stupid building inspectors and home inspectors and energy auditors, telling everybody ducts have to be way up in the top of the attic. ductwork people install now looks like Christmas lights strung way up high, strapped to the rafters and any piece of wood they can find to hold it up off the insulation. airtight is good. I believe in duct sealing. but you are driving the skilled workers out of this business, ( sheet metal workers who can fabricate any shape out of a flat sheet of metal and who used to run their flex ducts straight with solid metal insulated ducts.) now You all and are government are installing plastic ducts, that will not last 20 years in furred down sealings on slab houses. what you going to do when it needs to be fixed. tear out all the sheetrock. most builders now are scared to build a crawl space because they don't understand what a conditioned crawl space is. (its just a basement only about 3 feet tall.) the insulators and electricians who go back in the attic and add all the can lights, tare up all the plastic duct flex that is hanging everywhere in the attic. what do you except them to do jump up over the ducts, or crawl under them, through the nasty itchy insulation, while trying to balance on flimsy 2x4 trusses spaced 2 foot apart and 2 x4 s criss crossing everywhere. all because they say duct takes up space duct displaces insulation. well guess what, ducts are not a bad thing. ducts are the air you breath. the heat to warm your house, or to cool your house, to remove humidity, remove pollutants. all you need to do is let the ac man install his ducts to the best of his ability, test them for leaks if you want. build a walk way across the entire attic wouldn't hurt, because somebody will have to go up there, whether it be a duct smashing pest control guy, or a strangle wire duct standing on smashing cable, phone, alarm, satellite guy who crisscrosses wires everywhere blocking off every path through the attic with his little bitty wires. ducts need to last. they need to be run straight. they shall not leak. they shall be run in a manner in which they shall last. they shall not obstruct access to the attic, if they do then the next person who goes up there will tear them all up. believe me I see it every week. the government has it rules flying ducts to get more insulation to save a dollar. but in real life people tear them up costing 50 dollars a month. blown in insulation is cheap. just blow the attic to r 38 and over the top of ducts if needed. ducts are to be part of what is inside the house not part of. not part of what is in the attic. the attic is a hot dirty nasty place.. the higher in the attic the hotter it is. the lower the cooler it is. ducts up high get tore up. people can not carry ladders up into a attic and them place them onto 1 1/2 wide boards so they can climb over ducts. they just tear through them, smash them climb on them lay on them. and you the homeowner has to pay the bill. because sometimes it takes years or never to find the leaking ducts. remember the building inspector wont crawl around in your attic looking for leaks after the sheetrock has been installed and insulation blown in. do you think your builder will. how about your government official? not even a educated on the internet home inspector. he's not going to crawl around in that insulation, tip toe on joists try not to fall through the sheet rock in your 120 degree attic while jumping over your ducts that are now at waist level. because of the flying suspended out of the insulation rule.

No the ones responsible are the duct installers who don't bag openings until end of construction .

Is 4 inch flex duct harder to make?

Is it totally wrong to have all the duct sizes the same diameter like 9" unless dampers are used.. My installer has placed all run in 9" even those going to the bedroom and those going to the bathroom.

Duct design should be designed in accordance with Manual D. In some jurisdictions it is required to be. One reason why there is so much hot air heat instead of hot water. If a hot water system leaks, you know it. Too many hot air systems are junk. All you really know is that you aren't comfortable and you are paying a lot to condition your house, like everyone else who has a hot air system and thinks it is just the way it should be.

Thank you. Great info and the photos help immensely for the DIYer...

Just discovered that the TWO 14” flex supply and return ducts installed inside a 20” wide vertical chase (empty cavity) rising from the crawlspace to the attic switch places somewhere inside. Supply starts out in crawl on RIGHT, exits into attic on LEFT!

The house is 24 years old. Who is responsible?

Pretty sure it was Bob.

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