|Easy to install||Prone to sagging|
|Better for going around bends||Ridged inner surface affects airflow|
|Quieter than rigid metal||Less durable than sheet metal|
|Already insulated||Hard to clean|
|Cannot be used for dryers or range hoods|
Flexible air ducts are affordable, easy to install and bend, quieter, and often pre-insulated. They are, however, also easily compressed, prone to sagging, ridged on the inside (affects airflow), less durable, hard to clean, and disallowed for use with dryers and range hoods.
Pros of Flexible Air Ducts
One of the advantages of flexible air ducts is that you can expect the cost to be relatively cheaper than the alternative (rigid ducting).
The materials are cheaper and cheaper to manufacture. Furthermore, the flexibility and lightweight characteristics mean that they are less problematic to install, require less specialized gear, and less time. All of this contributes to a more affordable installation cost.
Flexible ducts are made from flexible polyester or aluminum. For the flexible polyester, you can expect a price that ranges from $1 to $4 per linear foot. The flexible aluminum is just slightly more expensive at, on average, $2 to $5 per linear foot.
So, while the cost will differ between the materials, but you should be able to lay flexible ductwork for about $10-$25 per foot, including installation costs.
In comparison, the alternative, which is rigid ducting typically made from stainless or galvanized steel, cost about $5-$20 for the material and up to $65 for the materials and installation.
Easy to Install
As mentioned briefly, flexible air ducts are easy to install, especially when compared to rigid ductwork. Unlike rigid ductwork, which almost always involves cutting, fastening, and sealing shorter ductwork runs, flexible ducts can essentially just be laid out.
The installation of rigid ductwork also requires much more planning and attention to detail. Once a rigid duct is installed, it’s most likely not going to be moved or changed anytime soon. Flexible ductwork on the other hand can be moved and manipulated as needed.
Another advantage to installing flexible ductwork as opposed to rigid ductwork is the number of tools needed for the job.
While rigid ductwork would call for tin snips, hammers, saber saws, and sheet metal screws, flexible ductwork can be cut with scissors or knife and connected with duct tape (marked “181B-FX) or mastic (marked “181B-M”).
With that being said, installing flexible ducting becomes a much more viable DIY project whereas rigid ducting should ideally be installed by a professional technician.
Better for Going Around Bends
As you would probably imagine judging only by its name, flexible ducting is much easier to maneuver around bends than rigid ducting.
Referring back to the installation process, this is where flexible ducting can save you quite a bit of time. While cornering with sheet metal requires precision cutting and attachment of elbows, flexible ducting can simply be bent.
It can also be slights curved to go around an obstacle, something that would be difficult with a rigid material.
However, you should note that you cannot just twist and curve your flexible ducting with abandon. A slight curve may make no difference, but a bend is equivalent to an elbow, affecting airflow and shortening the allowable ducting length.
Quieter Than Rigid Metal
A ventilation duct is not silent. The movement of air through the duct rattles the system. Sometimes, the noise is indicative of a problem, but some noise is definitely to be expected.
Because there is some give and stretch to a flexible duct, it can respond to the pressure of the air flowing through it in such a way as to absorb some of the impact exerted on it and reduce the noise of the airflow.
Flexible air ducts are useful in tight, cramped areas of the house such as attics and crawl spaces. These are often areas that are unconditioned. If you leave the ventilation system uninsulated, then water can start to drip from your vents as a result of condensation (hot pipes contacting cold air or vice versa) and you will also not be running the most energy-efficient system.
Rigid ducting most often requires separate insulation, whereas flexible ducts most often come with a layer of insulation when you buy them (amazon link). Even if they are not insulated, they are easier to insulate.
Cons of Flexible Air Ducts
The flexibility of these ducts gives them great qualities and abilities that we have already looked at, but there are some drawbacks as well. The more flexible a material is, the easier it is to squash or compress.
This can be helpful when you are trying to squeeze it past an obstacle and into the right position, but it should be able to return to its original shape once in position. If it is shoved into a spot that permanently compresses it, the airflow will be restricted.
This type of compression is a user choice (i.e., it is possible even though it is undesirable). With rigid ducting, there is no temptation to try because it is impossible to squeeze the sheet metal into these spots.
Then there is also the unchosen kinds of compression. If something falls against your flexible duct, then it can be squashed. Airflow is negatively affected and the chance of the duct being damaged or even compromised is high.
Prone to Sagging
Another issue with the flexibility of these ducts is a tendency to sag. Pulling the flexible duct to its full length will help mitigate this problem, but it cannot eliminate it altogether.
Ducting often needs to be suspended across beams and there will be portions unsupported by the house’s framework. With rigid metal, this is not an issue; the ducts hold their own weight. Flexible ducting will sag if not supported sufficiently.
Gravity does this, but it is exacerbated by the stretching caused by the pressure of airflow through the vent. The collection of water in the ducts will also weigh portions of the duct down.
When suspending a flexible duct, be sure to supply it with the proper supports and suspension equipment. Each support should be within 4 feet of each other with additional suspensions before and after a bend. Keep in mind that for every 4-foot interval there should be no more than ½” of sagging duct.
Whatever straps and suspension equipment you use should not constrict the body of the ducting. This would undermine the whole point of suspending in the first place as it would constrict airflow as well. Oftentimes the rafters of attics are used to suspend parts of the flexible ducting.
Ridged Inner Surface Affects Airflow
Rigid ductwork is typically very efficient when it comes to the airflow throughout the house because of its smooth interior surface. Conversely, flexible ducts are not as smooth as they have coils that allow them to be flexible. These coils increase the internal surface areas and create more friction when air flows through them.
Even when perfectly installed, flexible ducting is the less efficient option when it comes to maximum airflow.
The main issue with increased friction is it will cause your HVAC system to work harder than it needs to push air through the vents. This is also why the required fan size is different depending on what type of ducting you use.
Air that is exhausted to the exterior of the home through a flexible duct takes longer to reach the terminal vent as a result of the internal friction. Ultimately, this has the potential to decrease the quality of air in your home because “old” air takes longer to leave, delaying the inflow of “fresh” air.
Less Durable Than Sheet Metal
One of the biggest upsides to utilizing rigid ductwork is the durability and longevity of the sheet metal it’s made of. Rigid ducts have a lifetime between 20-25 years. However, with proper upkeep they can probably last you longer.
Unfortunately, flexible ducts are not so robust, with a maximum lifetime of around 15 years. Oftentimes, flexible ducts are used as a temporary fix and acknowledged as so. Very rarely you will see flexible ducts being used for the main trunk of a ventilation system.
Given that flexible ducts are used in tight, cramped spaces, such as an attic, some exterior factors may play a role in the attrition of the duct. One of these factors is rodents, which are drawn to these spaces. Although they may not chew through the entire duct, they will definitely want to sink their teeth into the fiberglass insulation.
Sharp objects of any kind are a problem for flexible ducting as well. Considering it can be cut with scissors or a standard knife, it’s not too far of a reach to think that it could be split open especially when stretched out completely.
All of these factors make flexible ducting unreliable to some extent as any of these issues would call for a replacement. In the long run, flexible ducting could prove to be more costly than installing a rigid duct.
Hard to Clean
The issue with cleaning flexible ducts is all of the ridges and curves inside the tubing. Unlike rigid ducts that can be cleaned with a vacuum or physically removed and cleaned by hand, flexible ducts are difficult to remove or get a vacuum into.
Another problem with cleaning the flexible ducts is getting them back into place. Even if you were to take them down and clean them out, you run the risk of many of the issues stated previously.
If not properly restored to the original firmness, the flexible duct could sag or be compressed resulting in reduced airflow and HVAC inefficiencies.
Cannot Be Used for Dryers or Range Hoods
Flexible ducts cannot be used to vent a dryer as they don’t meet the standards set out in the International Residential Code (IRC). Section M1502.4.1 of the IRC states:
“Exhaust ducts shall have a smooth interior finish and shall be constructed of metal not less than 0.0157 inch (0.3950mm) in thickness (No. 28 gauge).”
There are some flexible ducts that you may find that meet the standard thickness required by the IRC, however, the foil interior riddled with coils and ridges will definitely not pass as a “smooth interior finish”.
The reason this regulation exists is that using a flexible duct to vent a dryer can easily lead to lint build-up leading to dryer inefficiencies as well as the potential for a fire on account of the lint build-up.
Range hoods are the same. You cannot use a flexible duct to vent these as grease will build up, creating a fire hazard, and the ridges make the system very difficult to clean. The regulation for this one is found in Section M1503 of the IRC.
While the pros and cons of flexible air ducts are good to know, sometimes it is better when your options are compared comprehensively. If you are interested in more information, you can check out Flexible vs Rigid Ductwork: Which Should You Choose?