by Trevillian Highter

A duct system is a network of tubes — located in a building’s walls, floors and ceilings — that direct air from one location to another. Each type of duct material has characteristics that are beneficial in specific applications, and improper ductwork installation can result in inefficiency and increasing costs.

That’s why knowing the types of ductwork is important. Read on to learn more about their individual materials, uses, pros and cons.

1.  Flexible Duct

Known as flex duct, these types of ductwork are made for HVAC applications. They usually consist of a flexible plastic inner liner which a metal helix wire coil supports on the inside. A layer of fiberglass insulation covers the duct. 

Although flexible ductwork is typically installed in a single continuous piece, it is possible a leak can occur at the seam when it is connected to rigid sheet metal ductwork. Flex ducts must be installed properly and carefully to avoid restricting air flow. Improper installation of a flexible duct system that allows tearing, sagging, sharp bends, kinks and crimping can result in service problems and possible equipment failure.

2. Galvanized Steel

Perfect for sustainable buildings, sheet metal ductwork like galvanized steel (and aluminum, which we’ll cover next) are created from recycled materials and easy to clean. Sheet metal has a number of other advantages: it is sturdy, non-combustible and non-porous. Due to this non-porous material, sheet metal ducts restrict the growth of mold.

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Galvanized steel, used as a duct material for most air handling systems, is high-strength, durable, rust-resistant, widely available, easily changed to a new shape and able to weld with similar materials. Round ducts are recommended for maximum air-carrying capacity rather than rectangular ducts, and they have lower installation costs, which may result in substantial cost savings. (Rectangular ducts are most flexible when it comes to fitting within limited spaces.)

A disadvantage of sheet metal ductwork is they are more likely to leak air where seams or two sections are joined. 

3. Aluminum

Most commonly used for cleanroom applications, aluminum ducts are light in weight and easy to install. These ducts have a resistance to corrosion from moisture, but the basic cost per pound is more than galvanized steel. 

Limiting characteristics of aluminum ductwork include its low strength, cost of materials, thermal expansion and weldability.

4. Fiberglass Ductwork

In low-pressure systems, it is common to find fiberglass ductwork. Sheet metal ducts frequently use fiberglass insulation having a metal foil to act as a vapor barrier. This lining provides insulation to prevent the formation of condensation. Fiberglass ducts are insulated as part of their construction, and their applications include underground duct systems.

The advantages of fiberglass ducts are their resistance to corrosion, strength, and ease of modification. Fiberglass duct board provides excellent sound attenuation, reducing sounds created by an HVAC system, which is ideal for commercial buildings.

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 The main disadvantage of this type of air duct is that over time, the liner can deteriorate and expose building occupants to fiberglass particles. To combat this, most sheet metal ducts are lined with fiberglass on the outside of the duct. In addition, fiberglass-lined ducts are more difficult to keep clean. Other limiting characteristics include cost, weight, range of chemical and physical properties and brittleness.

 5. Fiberboard Ductwork

This type of rigid ductwork is constructed of fiberglass stands bonded by resin. The exterior surface of the board is covered with foil, acting as an air and moisture barrier; the interior surface of the fiberboard — in contact with air flow — is not as smooth as metal. Fiberboard ductwork is often used for heating and cooling ducts, but it is not recommended for ventilation ducts. 

One advantage of this type of ductwork is that fiberboard is inherently insulating. Fiberboard ductwork is airtight, contingent on the care taken in constructing the sections and cutting and sealing the joints and seams. An unnoticed crack in the bonded fiberglass over time can lead to erosion of the glass fibers, creating a durability issue and compromising the insulation value of the duct.


These five types of air duct systems can be used to determine which type of ductwork is right for your building. Even though a duct system is simple in appearance, it is important to the comfort and efficiency of a building. Do you need fabrication services at your existing building or new construction? Contact Therma today to work with our expert fabricators.

Trevillian Highter has a master’s degree in computer science. Her experience includes analytical research.


Instructor: A. Bhatia, B.E – HVAC Ducting – Principles and Fundamentals  

Refrigeration Service Engineers Society – Duct System Design Considerations     

Department of Natural Resources: State of Louisiana – Duct Design and Sealing 

Design and Trade Show Presentation – Duct Construction    

Building America Solution Center (BASC)Sealed and Insulated Fiber Board Ducts