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Sheet Metal Duct 

The most popular type of air duct is made of Galvanized steel or aluminum. These air ducts may be round, oval, or rectangular. Sheet metal HVAC ductwork is the most durable and least likely to harbor mold, and this is due to its non-porous surface. They tend to be the most durable type of air duct construction because they are practically rustproof, especially galvanized steel with its zinc coating preventing any biological growth. 


Flex Duct 

Flex ducts are made from a steel wire spiral, wrapped with a polymer (a bendable plastic). These types of ductworks are more affordable because they are lightweight and easy to install. They fit in awkward places, are held in place by insulation and they fit hard-to-reach angels where more rigid ductwork would not fit. 


Fiberboard Duct 

Fiberboard air ducts are constructed with boards of compressed resin bonded to inorganic glass fibers. They have a foil face on the outside that serves as the air barrier and water vapor retarder. The interior of the fiberboard air duct is sealed to prevent fiberglass fibers from entering the air stream.  


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. Unfortunately, old fiberboard ducts aren’t treated to prevent mold or mildew and should be treated immediately if infected with mold or other dangerous toxins. 


Fiberglass Lined Air Ducts 

​Sometimes the sheet metal air ducts are lined with an external or internal fiberglass duct liner. This fiberglass lining is used to insulate air ducts from heat loss or to avoid condensation in case the air is very cold or if there is a high level of 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 with time and release fiberglass particles into the air – which is a major health concern, especially with long-term exposure. Fiberglass lined ducts also have a high risk to become contaminated with molds and bacteria and should be treated immediately. 

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