Johns Manville’s Spiracoustic Plus duct liner provides a unique liner system for spiral ductwork, the company reports. (Photo courtesy of Johns Manville.)

In regard to the various types of duct systems and the evolving technologies impacting these materials, providing acoustical and thermal control, as well as anti-microbial protection in duct systems are essential considerations for establishing healthy, sustainable building interiors. Developments within the HVAC industry, and the ever-budding tenets of the green building-sustainability sector include enhanced thermal performance with consideration given to noise abatement, IAQ, occupant comfort, recycled content, et al.

As noted by professionals across the industries, each duct substrate offers unique advantages inherent to the specific construction materials utilized.

According to Robert “Buck” Sheppard, AAA Heating and Cooling, Portland, Ore., and president of NADCA - who started out in the HVAC industry when he was 18 years old - at first, there was really no such thing as flexible/duct board. Today, he is kind of a hybrid guy.

“Rigid systems are inherently better for the ductwork,” Sheppard said. “However, flex and duct board came about for speed and ease of installation.”

“Time is money. If you cut down the time it takes to install, you can pass benefits on to the consumers.”

An ideal install job would run rigid, and then to a point, flex, Sheppard said, although he tries to avoid total flex jobs. “I like the hybrid systems. You can get longevity with rigid … but flex gives access to cleaning later on after installation and when in use.”

No matter which type of system you are dealing with, there is continuing progress relating to materials, performance, aesthetics and innovation.


Based in York, Pa., MKT Metal Mfg. Inc., provides sheet metal duct, HVAC products and services to contractors throughout the greater Mid-Atlantic region. According to the company, MKT’s primary function is the manufacture and distribution of product; however, it is equipped and experienced to provide other field services such as the measuring and field assembly of duct.

In May 2009, MKT Metal Mfg. Inc. announced a product line expansion to include spiral ductwork for use in residential, commercial and industrial applications, the company reports. The new line establishes MKT as a one stop shop for commercial and industrial sheet metal duct and accessories for both the HVAC and the architectural industries.

MKT has been a manufacturer and supplier of rectangular ductwork to mechanical and industrial contractors throughout the mid-Atlantic, and is equipped to make the spiral duct in both light and heavy gauge metal. Other capabilities include providing a double wall format for thermal and sound efficiencies, and coatings in different colors for aesthetic appeal. The company’s spiral duct comes in lengths ranging from 4 to 100 feet.

“Spiral duct rounds out our product line,” said MKT Production Partner Mike Sunday.

“It is all about choice,” said Sunday. “There is a place for both rectangular and spiral ductwork in the marketplace.”

Sunday said that spiral duct is attractive for various reasons, including versatility and ease of installation, as well as efficient movement of air.

To accommodate the new product line, MKT has added another 10,000 square feet of production and inventory area. For more information, visit

The Reeves Middle School in Olympia, Wash., part of the Olympia School District, used 400,000 square feet of ductwork to design a new mechanical and electrical system for the school. The engineers specified CertainTeed ToughGard fiberglass duct board. (Photo courtesy of CertainTeed Corp.)


Johns Manville participates in both residential and commercial HVAC systems, said Tanya Bradby, portfolio leader of Air Handling, Insulation Systems for Johns Manville. “The optimum duct system is obviously driven by specification requirements, type of building, design constraints, etc.”

According to Bradby, Johns Manville is a fiber glass insulation manufacturing company supporting the HVAC market with both internal (duct liner) and external (duct wrap) insulation systems for metal duct, blanket insulation for flexible duct systems and duct board for manufacturing duct systems from rigid fiber glass board maximizing acoustical and thermal performance.

“With regard to installation and cleaning of duct systems, the fiber glass original equipment manufacturers (OEM)s typically reference the following to ensure the industry standards are consistent among all fiber glass manufacturers:

North America Insulation Manufacturers Association (NAIMA) Fibrous Glass Duct Construction Standards (, and Sheet Metal & Air Conditioning Contractors National Association (SMACNA) HVAC Duct Construction Standards (

Bradby points to NAIMA, on their Website, which publishes multiple technical bulletins representing the Fiber Glass Industry recommendations on key subjects such as duct cleaning.

According to Bradby, some developments within the HVAC duct sector that Johns Manville is concerned with include IAQ, sustainability, and occupant comfort.

“Green building and comfort of the occupant continue to drive the need for products which maximize indoor air quality (products with low volatile organic chemicals or no added formaldehyde), support acoustical performance (indoor environmental quality), and address sustainability initiatives such as recycled content,” said Bradby.

“There continues to be tremendous focus on energy efficiency driven by code change.”

“The key to success is not only the code change, but the speed in which the new codes are adopted. Historically, the implantation phase (code adoption) has been a slow process.”

According to Bradby, all duct systems offer unique advantages, but proper installation is the key to maximizing the advantages in each system type. Acoustical and thermal requirements, space constraints, and air velocity all come into play when evaluating the advantages and disadvantages of one system over another, Bradby said. Johns Manville’s Spiracoustic Plus duct liner provides a unique liner system for spiral ductwork, the company reports. Other products produced by the company include: Microlite XG duct wrap, EnviroAire, duct board and Linacoustic RC and/or Spiracoustic Plus duct liner.

Bradby said that Johns Manville also manufactures Flex-Glas PC for use in flexible ducts, but are not the OEM of the flexible duct system that contains the company’s product.

For more information, visit

CertainTeed ToughGard 2 Duct Liner was specified to line the concrete plenums and shafts of the Cobb Performing Arts Center in Atlanta, Ga. (approximately 26,000 feet). (Photo courtesy of CertainTeed Corp.)


Headquartered in Valley Forge, Pa., CertainTeed and its affiliates have more than 7,000 employees and more than 70 manufacturing facilities throughout the United States and Canada. According to Michael Lembo, senior product manager with CertainTeed Insulation Group, Mechanical/ Industrial, and Ken Forsythe, product manager for Mechanical Installation, CertainTeed, in general, one innovation the company made with products is pioneering the use of airstream surfaces of duct boards and duct liners.

Composed of either rotary-type glass fibers, (TGR), or textile-type glass fibers (TG2), the company’s ToughGard duct liners are primarily used as an acoustical and thermal liner in HVAC duct systems. It also helps fight moisture and mold growth and improves the energy efficiency of heating and cooling systems.

The company’s “ToughGard product lines were developed to go to higher airflow rates (up to 6,000 fpm) and contain antibacterial and water holdout agents to prevent mold growth,” Lembo said.

“This allows users to increase their IAQ with features that not only improve the ability of ducts to be cleaned, but prevent mold growth and excess moisture buildup in the first place.”

All CertainTeed products meet GreenGuard criteria, including several that meet the Children and Schools standard, which is becoming a requirement in some markets.

“GreenGuard Children and Schools is the most demanding IAQ certification rating,” Lembo said. Having products with high recycled content is also a benefit for all construction materials. Lembo and Forsythe said that an estimated 12,000 tons of materials were diverted from landfills in the last five years by turning into a product rather than throwing it out.

According to Lembo, as part of IAQ concerns, you have to consider what you want to accomplish with a duct system.

“Thermal efficiency is one set of solutions. Looking for condensation control comprises an- other set of solutions. Noise abatement/quality of sound leans more toward liner solutions, for instance: fire prevention calls for still other solutions, and value systems are moving more toward a duct board system.”

“Myriad solutions exist for what you want to accomplish,” Lembo said. “Another concern is that any of the products - including the ducts - are manufactured and in- stalled by NAIMA and ASHRAE standards. Any product is only as good as the person that installs it.”

The four duct product categories from CertainTeed essentially cover liners inside the duct, a rigid (higher density) board that also lines the inside, a duct board which becomes the duct system and doesn’t require metal duct, and a duct wrap that covers the outside of metal ducts.

In addition, CertainTeed offers a duct wrap for thermal insulation and moisture control which works with most applications in commercial and industrial applications.

“A drawback to a duct liner is that the more insulation that’s in there, the bigger the steel duct needs to be in order to move the same amount of air. This, as opposed to wrapping a duct on the outside, where you can keep steel at the cross section needed for the airflow and still control condensation,” Lembo said.

For more information, visit


Whatever type of HVAC duct system contractors are dealing with, there are advantages and benefits intrinsic to each varietal. And, with regard to sustainability, IAQ and healthy interior environments, many options exist that place a focus on low/no volatile organic compound/formaldehyde and recycled content. Manufacturers are quick to point out these details and are adept at marketing them.

Personal preference and geographic region/location, coupled with practical research and future maintenance considerations, should help in guiding material selection, as well as scheduling ongoing upkeep and servicing, for any modern day duct system. Just be sure to keep them clean.

Sidebar: A Better System

Immediately following the creation and development of central heating and cooling systems, the debate between metal ductwork and flexible ductwork began. General prejudice against flex duct today is based on old information. “I don’t think 10 years ago anyone would have imagined the wide variety of new flexible duct products, and the quality improvements that are available today.

“Acoustical flex duct and commercial flex duct are a couple of examples. In fact, it’s been just over a year that we introduced AMGflex™ that is a natural extension to an HVAC system with IAQ components”, says Ardee Toppe, president of Quietflex, Houston.

As with any component of an HVAC system, installing flex duct correctly is the key to its performance. Improperly installed metal or flex duct may result in leaks, allowing conditioned air to pass into areas not requiring conditioned air such as in the attic or between walls that can lead to greater static pressures or impaired insulation values. Leaky ductwork can also detract greatly from the HVAC system’s efficiency and may cause harm to IAQ expectations of the homeowner.

AMGflex™, a Quietflex product, uses Agion® - a silver-based antimicrobial coating - on the inner core that acts to inhibit the potential growth of mold inside the duct. This coating offers the ability to inhibit the potential growth and possible spread of various bacteria, fungi, mold, and mildew when they are found inside the flexible ductwork. The product has a gold jacket with a polyester vapor barrier and heavy-duty rip-stop scrim reinforcement. Further, all Quietflex flexible duct products use formaldehyde-free fiberglass insulation that helps meet the U.S. EPA recommendation to minimize exposure to formaldehyde and California EPA’s recommendation to use insulation with no added formaldehyde. Quietflex was recently certified by the GreenGuard Environmental Institute (GEI). This certification is broadly recognized and accepted by a number of “green” building programs, including the United States Green Building Council’s (USGBC) LEED program.

Compared to hard ductwork, flex duct typically takes much less time to install properly and offers a lower material cost for an HVAC dealer/contractor. “Also, if any changes are made in the home such as remodeling or adding a room it’s a very economical solution”, advises Rick Young, vice president sales and marketing, Quietflex. “Even getting it in an attic is much easier,” he said. Still, the product is only as good as it’s installed. “A proper installation includes not having too much sag,” Toppe said, “and not having sharp turns.” Following the manufacturers installation guidelines will result in doing a proper installation where flex duct is well supported and joined together properly.”

For more information, visit or call 877-694-3669.

Publication date:10/05/2009