Fiberglass duct liner is pinned to sheet metal during the fabrication of a sheet metal duct. Images courtesy of CertainTeed Corp.

The goal for high-quality indoor air in the design and renovation of commercial buildings has become one of the pillars of the sustainable design movement - and with good reason. IAQ not only affects the health, comfort and productivity of building occupants, but can also impact the “health” of the building.

Poor IAQ can result in a condition known as sick building syndrome, where a building’s air quality hits such dismal levels that the building’s intended use becomes impaired or impossible. Often, the cause of this condition is traced to widespread mold growth within the building, a product of an excessively damp interior.

Since this mold growth is most commonly found on interior walls, the building and design community is focusing ample attention on mold prevention strategies for wall assemblies. However, it’s important to also address another area with high potential for mold propagation and distribution - the ductwork of a building’s HVAC system.

Fiberglass duct board is fabricated into a rectangular section of a fiberglass duct.

The ‘lungs' of a building

The HVAC system is to buildings what lungs are to the human body - it is responsible for ventilating stale or contaminated air from a space and replacing it with fresh, new air. The ductwork of the HVAC system is the vessel that transports this air throughout the building. This is why it’s important to keep the inside of the ductwork clean. Any impurities, such as mold and microbes, present in the ductwork will be drawn into the interior air that building occupants breathe.

Mold and microbial growth can occur inside non-insulated sheet metal ductwork whenever excess moisture is present. This moisture is usually a result of condensation that forms on the inside and outside of ducts, caused by the reaction of the conditioned air to seasonal exterior temperatures.

During the summer, as cold air travels through warm, uninsulated ducts, beads of moisture form on the interior or exterior of the ducts. In winter, the same occurs when heated air passes through cold ducts. There is high potential for condensation on duct surfaces whenever the temperature is either equal to or lower than the dew-point temperature.

Condensed moisture becomes a threat when it combines with dust or dirt in uninsulated ducts to foster mold and microbial growth. If not prevented or caught in its early stages, the mold and microbial growth will quickly spread and pollute interior air with harmful airborne mold spores. Extended inhalation of airborne mold spores can significantly impact the health and comfort of building occupants, often causing headaches and irritability or more serious respiratory ailments, such as asthma.

The American Society of Heating, Refrigeration, and Air-Conditioning Engineers’ Standard 62.1, “Ventilation for Acceptable Indoor Air Quality,” lists strategies for improving IAQ through mold and microbial growth prevention. Two of the most effective recommendations are to properly seal HVAC ductwork and insulate it. These steps help prevent mold growth by allowing ductwork to maintain a constant temperature, which significantly reduces the possibility of condensation forming on duct surfaces. The most common duct insulation material, by far, is fiberglass. 

Fiberglass duct wrap can be used to reduce unwanted heat loss or gain and condensation during system operation. CertainTeed SoftTouch duct wrap is shown.


In addition to helping maintain a constant temperature, as an insulation material, fiberglass is inorganic and does not support mold growth or act as nutrients for mold growth. There are four types of fiberglass insulation commonly used in ductwork applications:

• Fiberglass duct liner, the most common duct insulation choice, is a layer of insulation applied to the interior of a rectangular sheet metal duct. It is designed to control heat loss or gain through duct walls, assist with quiet air distribution and minimize condensation. Fiberglass duct liners are resistant to mold growth and fiber erosion in accordance with industry standards and prevent the distribution of airborne mold or glass fibers. Fiberglass duct liners are also resistant to microbial attack.

Manufactured with mat-faced air stream surfaces, the duct liners resist damage during installation, maintenance and cleaning. They are manufactured in roll and board form and are available in a variety of densities and thicknesses.

• Fiberglass duct wrap is resilient blanket insulation with a vapor retarder facing that is made to fit snugly over rectangular, spiral, flat-oval or irregularly shaped surfaces. It can be easily cut and fitted for a neat, thermally effective installation and comes in various thicknesses and densities. Duct wrap, however, is usually not specified in conditions where ducts are exposed.

• Fiberglass duct board is a 1- to 2-inch thick, rigid board (manufactured from resin-bonded inorganic glass fibers) that is used to fabricate fiberglass ductwork. For commercial applications that require thermal insulation, condensation control and acoustic control, fiberglass duct board provides an efficient, lower-cost alternative to sheet metal. Since insulation is integrated into the duct board, fiberglass ducts don’t require as many fabrication steps as insulated sheet metal ducts. The outside surface of the duct boards should feature a factory-applied reinforced aluminum air barrier and a vapor retarder, which minimizes air leakage and moisture accumulation.

• Fiberglass commercial board insulation - available unfaced, with FSK or ASJ facings - is installed on the exterior of round, rectangular, oval or irregularly shaped ducts, plenums, chillers and other HVAC equipment. Ranging from flexible to rigid, commercial boards are used to reduce heat loss or gain.
Whatever the type, fiberglass duct insulation is an effective weapon in the defense against mold and microbial growth and poor IAQ. With properly sealed and insulated ducts and adherence to a periodic duct inspection and maintenance schedule, building owners can expect higher IAQ levels throughout their building for many years to come.

Properly sealing and insulating sheet metal ducts or installing fiberglass ductwork are excellent, proactive ways to improve the IAQ of a building. Good air contributes greatly to the sustainability of buildings, and provides a purer and more comfortable interior environment. As a result, building occupants will be healthier and more productive, which benefits the owners of the building. It’s a win-win situation.

Ken Forsythe is a product manager with CertainTeed Corp.’s HVAC group. He previously worked at Alcoa and Ford Motor Co. He earned a bachelor’s of science degree in mechanical engineering at the University of Toledo in Ohio and his master’s in business administration from the University of Michigan. He can be reached at