The Pros and Cons of Flexible Ductwork

Ductwork is the infrastructure that moves conditioned air throughout the home. Essentially a system of empty pipes, these ducts connect the furnace and central air to the vents in each room for air distribution. There are two types of ducts suitable for the job: rigid and flexible ductwork. 

Like its rigid alternative, flexible ductwork has its advantages and drawbacks. In some ways, it’s indispensable. In others, it’s a liability. A deep dive into flexible ductwork will outline the applications as well as the pros and cons of this HVAC material. 

What is flexible ductwork?

Flexible ductwork is a long, hollow tube made of thin metal, a wire coil, and insulation. HVAC installers will use it to attach the supply and return sides of the main appliance (typically a furnace) and then run it to the main trunk. It’s commonly available in 4-inch, 6-inch, 8-inch, and 10-inch diameters, but can also come in sizes up to 22 inches. 

Due to its construction (and true to its name), flexible ductwork is easy to bend and manipulate into different spaces and shapes. It can make relatively sharp turns, conform to both round and oval flanges, and snake through walls.

For reference, rigid ductwork (the other form of duct) isn’t flexible and requires special elbows and fittings to turn or connect to certain flanges. It’s made from thicker sheets of rolled metal, as well. It does serve the same purpose as flexible ducts though: to bring conditioned air to and from a space. 

Where is flexible ductwork used?

Flexible ductwork is used most often on the supply side of a heating system or air conditioning system. It can attach the unit to the main ductwork system, or stretch from the main duct to the vents within the conditioned space. It’s also common to see flexible ductwork used on the return air sides of an HVAC unit, though they may collect more dust than rigid ducts.

The application within the home also matters. Flexible ductwork should not be used outside of the conditioned space—so it can’t be used in attics unless the insulation is between the roof rafters above (not in the joists below). Also, using flexible ducts in crawl spaces isn’t great either, as pests and insects can penetrate them and get in the home.

Advantages of using flexible ductwork

There are many advantages to using flexible ductwork to connect points within an HVAC system, not least of which is ease. 

  • Easy to install. Installing flexible ductwork is much easier than installing rigid ductwork. Flexible ductwork can snake around corners, fit in tight places, and adapt to a variety of flanges. It’s also very light so positioning it in place is much easier, and since it stretches and compresses, precise measurements aren’t necessary.
  • Great for tight spaces. When it comes to routing ductwork through joists or other tight spaces, flexible ductwork also shines. These ducts can bend and twist to fit in tighter spaces than rigid ductwork, doing away with the need for elbows and mastic (no, duct tape doesn’t cut it).
  • Affordable. Flexible ductwork is also very affordable. Since it’s made with inexpensive materials and doesn’t require elbows and connectors, the cost of materials to run between two points is much less than it would be with a rigid pipe. This means less time for the installer, making flexible ductwork even more affordable. 

Disadvantages of using flexible ductwork

Just as there are benefits to using flexible ductwork, there are just as many drawbacks.

  • Easy to damage. The main disadvantage of using flexible ductwork is durability. Due to its thin metal construction and where it’s often used, flexible ductwork is prone to damage. Nails, splinters, pests, tight spaces, and even cleaning can easily damage flexible ductwork. For this reason, it’s important to check on flexible ductwork more often than rigid ductwork.
  • Slows the airflow. Flexible air duct also increases friction over a rigid duct, which, in turn, reduces airflow. If a section of rigid duct isn’t as extended as possible or has slack in its length, the wire coil and material that make it so flexible bunch up inside the duct. As air passes through the tube, it hits the bunched material and slows down. This can create a scenario where an AC or furnace unit has to work harder than necessary to condition the space. The same scenario applies to bends and twists. While the flexibility is a benefit, it’s also important to note that each bend in a flexible duct creates more friction than rigid ductwork. 

Final thoughts 

Flex ducts aren’t a perfect material, but they do have their place in most duct systems. When used correctly, these ducts can make twists and turns that aren’t possible with rigid pipe. As long as these ducts are in conditioned spaces, installed correctly, and maintained, they can be a solution to many HVAC challenges.

MT Copeland offers video-based online classes that give you a foundation in construction fundamentals with real-world applications. Classes include professionally produced videos taught by practicing craftspeople, and supplementary downloads like quizzes, blueprints, and other materials to help you master the skills.

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