Throughout much of the 20th century, acoustics and indoor-air quality (IAQ) largely were overlooked in the design of classroom buildings on college campuses. As a result, one typically can find much room for improvement in those areas as older classroom buildings age and require renovation.

Efficient, low-noise HVAC equipment can go a long way toward providing a comfortable, healthy environment for learning, although perhaps not as far as well-thought-out duct design. While there are many fundamental principles and codes to research before designing an ideal duct system, the following guidelines can serve as a nudge in the right direction.


A variety of materials — each with advantages and limitations, depending on the application — are used to fabricate HVAC ducts.

Sheet metal

Galvanized steel is the most common type of sheet-metal ductwork and can be used in both low- and high-pressure systems. Aluminum alloy often is used in low-pressure, low-velocity air-conditioning systems; special exhaust systems; and ornamental exposed duct systems and for moisture-laden air and louvers. Stainless-steel ductwork can be used in industrial and laboratory systems to exhaust corrosive air and gases.

Though durable and widely used, sheet-metal ductwork, if not properly sealed or insulated, is prone to air leakage and corrosion. Additionally, it can be heavy and cumbersome, with pricing prone to sudden increases.

Fiber glass

Often lower-priced than sheet-metal duct, fiber-glass duct board is well-suited for schools and other budget-conscious customers requiring both thermal insulation and acoustic control. Fiber-glass duct board is durable, weighs less and is easier to install than sheet metal, and is resistant to corrosion.


Insulated flexible textile ducts are manufactured in a variety of sizes and can be used for connections between trunk ducts and room diffusers or registers.


The Sheet Metal and Air Conditioning Contractors' National Association1 (SMACNA) recommends that all duct design specifications include static-pressure classifications, local code requirements, material selection, and allowable duct leakage and sealing-system classification. However, before designing ductwork, including sheets, reinforcements, seams, and joints, HVAC engineers should consider the theoretical and practical limits of: air velocity, air-leakage control, vibration, dimensional stability, noise generation, transmission of air, exposure, support, seismic restraint, and thermal resistance.

The most important factor in duct design is static-pressure capacity, the amount of air pressure a duct can withstand. The combination of extremely high air velocities and large pressure drops can cause excessive noise and energy inefficiency. It is important for HVAC professionals to ensure that a quality duct material is used and that all duct joints are sealed carefully.

Leaky ducts release valuable cool and warm air, leading to poor building-temperature control and unnecessarily high utility bills. The proper use of fiber-glass duct liner or duct wrap with well-sealed ductwork can decrease duct leakage immensely, as air traveling through uninsulated ductwork cools quickly during winter and warms quickly during summer. For example, research has shown that air cooled to 55°F traveling through a bare sheet-metal duct in an 80°F space can gain 10°F in temperature after 100 ft. Conversely, the same air passing through a duct with fiber-glass insulation will increase no more than 2°F in temperature.

Insulation varies in thickness, depending on the needs of an HVAC system and the amount a customer is willing to pay. When specifying duct-work for a system expected to operate at high temperatures or involving large surface areas, it is a good idea first to conduct a detailed analysis of “economic thickness,” which takes into account the amount of energy saved, the cost of insulation, and other financial factors. The most important point to remember is the higher the R-value of insulation, the greater the amount of energy conserved.

The next factor to consider is acoustics. In recent years, research has linked excessive classroom noise to poor student performance. Research performed by the Acoustical Society of America2 reveals that many classrooms in the United States have speech-intelligibility ratings of 75 percent and below, meaning students with normal hearing abilities hear fewer than three out of every four words spoken clearly by an instructor.

Uninsulated ducts often act as large speaker tubes, amplifying whatever noise they come in contact with, such as aerodynamic noise, breakout noise, and mechanical-equipment noise. Aerodynamic noise is the sound of air moving through elbows, dampers, branch wyes, pressure-reduction devices, silencers, and other duct components. Breakout noise, sometimes called crosstalk, usually is noise from human speech.

Fiber-glass duct board and liners absorb sound waves that metal ductwork otherwise would amplify. According to American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Technical Committee 2.6, Sound and Vibration Control, “Fiber-glass duct-system insulation continues to be the most cost-effective solution to noise control in most HVAC air-duct systems.”3


In 1994, the Occupational Safety and Health Administration (OSHA) issued a proposal to regulate IAQ.4 The proposal targeted the 30 percent of non-industrial work environments OSHA believed to have poor IAQ and led to the creation of a detailed inspection and compliance program. The objective of the program was to significantly reduce the occurrence of severe headaches and acute respiratory illnesses, such as Legionnaires' disease and asthma. ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, provides guidelines on improving IAQ.

Condensation can form when cool air passes through warm uninsulated ductwork during summer and warm air passes through cold uninsulated ductwork during winter. If dust or dirt is present inside of the ductwork, harmful mold can develop.

A 1998 Harvard University study showed that the binders and glass in fiber-glass duct board and duct liner do not support the growth or spread of mold.


The most effective means of ensuring a duct stays mold-free and performs efficiently is periodic maintenance, which many engineers have begun to add to specifications. Ducts should be inspected regularly for moisture and contaminants. If mold growth is present, ducts should be cleaned in compliance with North American Insulation Manufacturers Association (NAIMA) and National Air Duct Cleaners Association (NADCA) standards. The three most common methods of cleaning ducts are:

  • Contact vacuuming, which involves the conventional vacuum cleaning of interior duct surfaces. The hose of a high-powered vacuum cleaner with a high-efficiency-particulate-air (HEPA) filter is inserted through holes cut into ductwork.

  • Air sweeping, by which compressed air is introduced into a duct through a hose capped with a skipper nozzle, placing the ductwork under negative pressure. The compressed air propels the nozzle through the ductwork, dislodging dirt, dust, and debris. The dislodged particles become airborne and are drawn downstream through the duct and out of the system.

  • Power brushing, by which a pneumatically or electrically powered rotation bristle brush is used to loosen dirt, dust, and debris and draw it downstream and into a vacuum collector. When dealing with fiber-glass ducts or sheet-metal ducts lined with fiber-glass insulation, duct-cleaning professionals must take special care to use only brushes with soft, flexible polymer bristles that will remove debris without putting extra stress on surfaces.


Errors in design can lead to leaky ducts and mold growth, which can result in low productivity and ill health among building occupants. With plenty of care taken to meet the requirements of SMACNA, ASHRAE, OSHA, NAIMA, and NADCA, however, HVAC professionals can design and fabricate ducts that perform efficiently in every respect.


  1. SMACNA. (2006). HVAC systems-duct design (4th ed.). Chantilly, VA: Sheet Metal and Air Conditioning Contractors' National Association.

  2. Seep, B., Glosemeyer, R., Hulce, E., Linn, M., Aytar, P., & Coffeen, R. (2000). Classroom acoustics I. Melville, NY: Acoustical Society of America.

  3. ASHRAE. (2001). ASHRAE handbook-HVAC applications (ch. 47). Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

  4. OSHA. (1994). Indoor air quality (59:15968-16039). Washington, DC: Occupational Safety and Health Administration.

Renee Chesler is general manager of CertainTeed's HVAC and industrial division, overseeing product development and engineering, marketing, and customer partnerships. She can be contacted at