Flexible ductwork is easy to work with, and it doesn’t cost a lot. But it isn’t the “perfect” solution in all cases. There are flexible ductwork pros and cons to consider.
Read on to learn when using flexible over metal is a good idea.
Metal and Flexible Ductwork – A Brief Description
You’re likely familiar with the two types, but here are short overviews of each.
Metal ductwork is made of sheet metal; flat, thin sheets of galvanized steel formed into rectangular and round/oval tubes of varying sizes. Ductwork can also be formed by fastening sheet metal panels to studs and joists, especially to create return air ducts. Metal ducts are often uninsulated, but can also be insulated.
Flexible ductwork is made of a round wire coil usually covered in a bendable but very durable polymer (plastic). This polymer “skin” is usually surrounded by fiberglass insulation, then encased in a thin polyethylene or foil jacket. Less commonly, flex ducts are made from rubber, PVC, stainless steel, silicone and other materials which may or may not be insulated. Flex duct is made in many sizes from 3” to 22” in diameter.
Similarities: Both metal and flexible ductwork are connected to the heating and ventilation system by a main plenum. This distributes the air for heating and cooling through various runs which branch off to the different rooms in the building. Sometimes flexible ductwork is used for branches running off a flexible or sheet metal main trunk.
Differences: The main difference between the two types of ductwork is that the metal ducts are straight and rigid sections of tubes while the flexible ducts are soft and can be bent or turned in any direction. If the main lines are flexible duct, sheet metal ductwork isn’t used at all.
Flexible Ductwork Pros and Cons
Here are reasons to consider flex duct, aka flexible ductwork, and some cautions that might make you think twice about whether it is the right fit for your project.
Flex Duct Pros
Less Expensive – by almost 50%. By the time insulation is added to the cost of standard metal ductwork, the material cost is almost twice that of flexible ductwork that comes pre-insulated. Also, flexible ductwork is faster to install than metal ductwork. Properly sealing all seams, joints and elbows on metal is time consuming, and time is costly.
Easier to Install. Unlike metal ductwork that requires precise measuring and fitting, flex duct has a lot of “give and take” in the installation process. Sheet metal is more difficult to fit into the building’s framing dimensions, and the materials are also more difficult to carry and place in the right position.
Insulated. Flex ductwork comes with its own insulation built into its design. Depending on the type and size of the flex duct, the insulation factor can be between R-4.2 and R-8.0.
Quieter. It may surprise some, but as the air travels through the hard and rigid metal ducts, small vibrations, whistling in tiny air gaps and other sounds are transmitted to the registers. Because of this, some professional contractors use flex ductwork for all the final branches to the registers even if the main trunk lines are metal.
Fewer Seams to Leak Air. Metal duct sections are often only 3’ long. This means there must be a seam whenever two sections connect. Additionally, every time a metal duct makes a turn, there will be two more seams – one on each end of an elbow. According to the U. S. Department of Energy, it is estimated that the average home loses 20-30% of the heated or cooled air through leaks in the heating, AC and ventilation system. Flexible ductwork can be installed with much longer runs which translate into far fewer seams and joints to leak air. Learn more about the importance of sealing ductwork from the DOE’s Energy Star site.
Flex Duct Cons
Less Durable. Both the inner polymer tube and the outer foil jacket of flex duct can be easily pierced with any sharp object. Metal ductwork is basically impervious to punctures in all normal situations. Careful handling of flex ductwork during installation is very important.
Not as Long Lasting. Opinions vary widely, but unless metal ductwork is subjected to a very corrosive (rust-producing) environment, it will normally outlast flexible ductwork. Many manufacturers offer a 10-year warranty on flex ducts, with some as high as 20 years. Expect to have to replace it within about 25 years in most homes.
Prone to Installation Errors. Unfortunately, there are quite a few ways to screw up flexible ducting. While this is not the fault of flexible ductwork, because it is soft rather than rigid, installers are more apt to kink and restrict the airflow when turning corners than with metal ductwork. Another way that they install it improperly is by compressing it, like compressed slinky, which causes more inner surface area and friction inside the tube where the air travels. Still another way is by pinching the flex duct with various hangers when securing it between floor joists, etc. This has the same result as pinching a water hose – the airflow at the far end of the flex duct is greatly reduced.
Difficult to Clean. Because the inner lining of the flex duct is made of thin polymer, it requires much more sensitive tools and cleaning strategies than standard, hard air ducts. In other words, it takes more specialized cleaning tools to properly clean flex ductwork without damaging it. It is a job that even many professional duct cleaners do not accept.
When Flexible Ductwork is the Better Choice
So if you are considering replacing your HVAC ductwork, here are some reasons why you would want to go flexible:
1. When You Are On a Tight Budget. The materials alone will cost about half as much as rigid metal ductwork. And if you do it yourself, you will probably save almost as much as the material cost as well. Your total cost could be as much as 75% less than having metal ductwork professionally installed.
2. When You Plan on DIY. Flex duct is much easier to handle than metal. Fewer and less expensive tools are required to cut it and make all of the connections that are needed. The connections themselves are simple to do and do well. Just be sure to do some research, watch some videos, and follow the manufacturer’s instructions.
3. When You Have Some Hard-to-Reach Areas. Flex ducts are very easy to bend around roof rafter components in the attic, floor joists in the basement, as well as other difficult areas in the framing of a house.
4. When You Already Have Flex Ducts. If your home’s present HVAC ductwork system was constructed of flex duct, it will be that much easier to duplicate it if you have decided to replace it. Use the current layout to determine the correct size and length of runs all the way from the supply plenum to the return duct. The original system was probably planned by an HVAC engineer, so you will know that what you install will be according to all applicable codes.
Flexible Ductwork FAQ’s
What else would you like to know? Common questions are answered below. If we don’t cover your question, use the Contact form on this page, and we will reply asap.
Q – How much does flexible ductwork cost?
A - $1.30-$5.25 per linear foot depending on size (4”-16” typical). Cost is for flex duct only. Additional materials for making and sealing connections will be needed. Plus the cost of labor.
Q - Can flexible ductwork be repaired?
A - Absolutely! Flex ducts can be repaired by using similar methods as were used for making connections to a trunk or register box. A metal duct collar or elbow can easily be used to connect the ends of a torn piece that was removed, for example.
Q - I have heard that flexible ducts restrict airflow. Is this true?
A – Yes, flex duct causes a small amount of turbulence inside because of its inherent design with the coiled wire. Because of this, it will not allow quite as much air in cubic feet per minute (CFM) as the same size metal duct. For example, one company’s sizing chart shows that 6” round metal duct allows 85 CFM while flex duct allows 75 CFM, a reduction of about 12%.
Pro Tip: Some contractors recommend increasing flex duct by 1” diameter over metal duct sizes to get the same approximate CFM. Our pros agree that this is a good decision.
When to Use Flexible Ductwork
If you are thinking about replacing your HVAC ductwork, this might be a good time to go with flexible rather than metal. You will definitely save money. Even if you do what some HVAC contractors do and only use flex for the relatively short final runs to each register, you will see a savings. Additionally, you may experience a reduction of air noise.