Sheet Metal Air Ducts
The most common type of ridged air duct is constructed of galvanized steel or aluminum. Aluminum in particular is relatively light and easy to install but needs to be thick enough to avoid getting crushed if someone walks on it. They may be round, rectangular, or in the form of a spiral oval. These are the types of ducts that you’ve see in movies – usually with an action hero crawling through them! Sheet metal air ducts are the most durable type of air duct construction, and because of their non-porous surface, are the least likely to have mold or biological growth.
Fiberglass Lined Air Ducts
Some sheet metal air ducts are lined with an external or internal fiberglass duct liner. Fiberglass lining is used to insulate air ducts from heat loss or to avoid condensation in cases where the supply air is very cold, or there is a high ambient humidity in the plenum. Fiberglass lining also provides sound attenuation, reducing the noise of the HVAC system. Because of the dampened sound benefit, this type of duct is common in office and commercial buildings. However, the fiberglass in these ducts can deteriorate and eventually release fiberglass particles into the air – which is a major health concern, especially with long-term exposure. Fiberglass lined ducts also have the potential to become contaminated with molds and bacteria. Fiberboard ductwork is also convenient because it can be fabricated to any configuration in the field, which is helpful when the roof framing is blocking a typical installation.
Fiberboard Air Ducts
Fiberboard air ducts are constructed with boards of compressed resin bonded inorganic glass fibers. They have a foil face on the outside that serves as the air barrier and water vapor retarder. The interior of fiberboard air duct is sealed to prevent fiberglass fibers from entering the air stream. Fiberboard air duct also provides acoustical and thermal benefits, and are typically the most inexpensive air duct systems to install. This type of duct is good for cooling and heating systems because it is well insulated by itself. However, it is not recommended for ventilation because it can become a breeding ground for mold and mildew in humid climates. Also, because the surface is rough, it can affect airflow and efficiency.
Flexible Air Ducts
Flexible air ducts are constructed with a spring steel wire helix, encapsulated in a 2-ply, polymer plastic. Flexible air ducts are inexpensive, lightweight, and easy to install. It is important that the lengths of the flexible air ducts be kept as short as possible as the amount that they bend greatly effects system performance. Typically, flexible ducts are tube shaped and are the second least expensive type of ductwork. This type of ducting is best in tricky spaces where rigid ducts are just not possible to install, or used to attach non-flexible ductwork to an air supply outlet. Kinks, bends, and turns need to be minimized in installation, as these issues reduce air flow and could hamper the efficiency and effectiveness of the air conditioner.
Flexible air ducts are the easiest type of ductwork to install, so easy in fact that almost anyone can install them... which also happens to be their downfall. Since "anyone" can install them, best practices go out the window and installers show little care to how the airflow will actually be in the home when they put 6 wye splits before a duct reaches the last room, a room with three exterior walls or a room sitting directly above the garage.
Flex ducts are also the worst type of ductwork for airflow. The inner liner ribs restrict the airflow more than sheet metal ductwork, but not significantly. Flex ducts are good for air quality because you are not breathing in fiberglass insulation lined ductwork like the Fiber Air Ducts and they are mold and rust resistant unlike galvanized sheet metal ductwork.
When installing an air conditioning system, consult with Green ID to find out which type of duct is best for your home.