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What is a Fiberglass Duct?

By C.L. Rease
Updated May 17, 2024
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Fiberglass duct refers to two distinct types of ductwork. One type of fiberglass duct consists of a compressed fiberglass sheet with a metal foil backing, and the second type consists of fiberglass cloth bound with resin to form a strong solid material. Both types of fiberglass duct move air, but one has the capacity to withstand high pressures and exposure to outside elements, while the other provides an economical way to move air through a structure. Although both types of fiberglass duct move air, each type has a specific purpose, and one cannot be substituted for the other in a building's ventilation system.

Fiberglass-reinforced plastic (FRP) has a hard outer shell that resists crushing and does not react with liquids or air containing corrosive chemicals. The soaking of fiberglass in a hard-setting resin provides FRP ductwork with its strength. This industrial version of fiberglass ductwork is a common sight in chemical plants or other industrial facilities requiring movement of harmful byproducts from one area to another. Cutting FRP ductwork requires carbide cutting blades and abrasive grinding wheels to reduce material cracking and increase cutting blade life.

Another type of fiberglass duct uses a material that combines fiberglass fibers, lightweight glue and metal foil to form a flat semi-dense sheet. This material, referred to as ductboard, has a loose bond between the bonded fiberglass fibers, which allows it to be cut with knives and other sharp shaping tools. The cuts and notches used to form a ductboard fiberglass duct need to be exact, or air moving through the duct will cause the loose fiberglass to break down and fall from the sides of the duct. As fiberglass falls from the duct, its insulating quality is reduced, leading to increased inefficiency in the fiberglass duct. Loose airborne fiberglass particles have been shown to cause health problems, including lung cancer, in mice.

Knowing the function and limits of each fiberglass duct type before designing or renovating a structure will ensure the duct functions as designed. Improperly substituting one duct type for the other will result in a failed heating, ventilating, and air conditioning (HVAC) system. The signs of a failed HVAC duct are not always noticeable, but health effects resulting from the lack of air movement can include increased allergies, asthma and coughing. Visual signs of a failed HVAC duct include increased dust on tables and mold growing around air vents installed in ceiling, floors or walls.

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