Joe Smith of Coastline Supply, Myrtle Beach, S.C., demonstrates how to splice together two pieces of flex duct during a seminar in Myrtle Beach.
Installed the right way, flexible duct is a great ventilation product for a great many applications. Sometimes, however, hurried installations create poor-quality results - as they can for any HVACR product.

In order to give contractors solid information on what needs to happen for a satisfactory flex duct installation, the Air Diffusion Council (ADC, the trade association of flex duct manufacturers), has published easy-to-understand guidelines. This helpful guide is aimed at contractors and technicians who are eager to demonstrate good workmanship practices and do the job right the first time.

The fourth edition of Flexible Duct Performance & Installation Standards - also known as the "Green Book" - contains helpful graphics and guidelines to maximize the benefits of flex duct when installing it either as a duct component of an air distribution system, or as the complete air distribution duct system itself.

Flexible duct is being installed in residential, commercial, and institutional buildings. The first step is for contractors to be aware of local codes, the authority having jurisdiction over those codes, and any regulations specific to the construction type. The new edition of the Green Book updates information on fire safety, use restrictions/limitations, duct sizing, leakage, R-values, field alterations, and exposure to natural and artificial sunlight.

The following information has been excerpted from the Green Book. For more information, contact the ADC at the address given at the end of this article.


Whether the installation is in an attic, basement, crawl space, plenum space, or wherever, route the flexible duct with the least number of bends and the least degree of bend at each turn. Avoid bending ducts across sharp corners or in any way that puts them in contact with metal fixtures, pipes, or conduits.

Where you do make bends, limit the bend radius to one duct diameter. Use the most direct route for any one duct run and the minimum duct length to make the connection. Do not install ducts in a compressed state or with excessive length; this will noticeably increase friction losses.

Avoid routing ducts under turbine vents, skylights, canopy windows, etc., where duct materials are exposed to direct sunlight, or near an ultraviolet (UV) light source (i.e., bio-treatment lamp). UV light, either natural or artificial, can degrade some duct materials.

Routing ducts near hot equipment (furnaces, boilers, steam pipes, etc.) that is above the recommended flexible duct-use temperature can damage the duct.

Ralph Koerber of ATCO Rubber Products Inc., Fort Worth, Texas, shows a well-made flex duct seal.


Support horizontal duct runs with 1-1/2-inch-minimum-width hanger/saddle material. Support intervals should be in accordance with the manufacturer's recommendations, or as your local code specifies, but at no greater a distance than 5 feet.

Long duct runs with sharp bends may require additional supports before and after the bends.

The maximum permissible sag is 1/2 inch per foot of length between supports. Ceiling joists offer an alternative to suspension straps for supporting ducts in attic spaces.

To prevent damage to the duct by the edge of the metal collar, the duct should have an additional support between the metal connection and the bend. This allows the duct to extend straight for a few inches before making the bend.

Some local codes may preclude any vertical use of flexible air duct. Where it is allowed, ADC guidelines say that factory-made flex air ducts may not be used for vertical risers in air duct systems serving more than two stories. Vertically installed ducts also should be stabilized by support straps every 6 feet.

Connecting And Splicing

Each manufacturer of UL 181-listed flexible ducts provides installation instructions with each packaged product, and these give specific methods on how to make proper connections and splices.

These methods and the required materials are part of the UL 181B Standard, "Closure Systems for Use with Flexible Air Ducts and Air Connectors." This standard defines test requirements for pressure-sensitive tapes, mastics, and nonmetallic mechanical fasteners to make a mechanically secure connection for air ducts.

Ralph Koerber (left) and Joe Smith show technicians the preferred method for splicing together two pieces of flex duct. Here, Koerber is applying a mechanical seal to the duct core.

Consult The Green Book

The ADC standard covers supply, return, fresh-air intake, and exhaust ducting as part of the comfort air distribution system. It does not cover industrial duct applications such as particulate conveying, corrosive atmospheres, etc., or outdoor applications where continuous exposure to sunlight and weather exist.

The standard discusses testing and listing of ducts to UL 181, "Factory-Made Air Ducts and Air Connectors," and tapes, mastics, and nonmetallic fasteners to UL 181B, which is mandatory for compliance to the 90A and 90B standards of NFPA, the International Codes of ICC, the Uniform Codes of IAPMO, and the various state and local codes.

The following topics also are covered in the standard:

Fire safety - The combustibility of flexible ducts and the four fire tests used to evaluate Class 0 and Class 1 flexible ducts are explained.

Installation restrictions and use limitations - Specific restrictions and use limitations due to existing codes, test standards, or end-use performance characteristics not designed for a given application are detailed; 10 important items are discussed.

Duct sizing - Combined friction and dynamic pressure losses in an HVAC system are discussed. The standard recommends what is needed to properly size a system when using flexible ducts.

Duct leakage - The inherent leakage properties of flex duct are discussed and the negative effects on a system when poor workmanship practices and/or improper materials are used are noted.

Thermal resistance (R-value rating) - The standard presents the three commercially available thermal resistance values (R-4.2, -6.0, and -8.0) and the method for testing ducts and calculating R-values. The UL adjunct test program for classifying, certifying, follow-up testing, and labeling R-values under the ADC Thermal Certification Program is discussed.

Field alterations - The standard emphasizes the UL 181 listing for flexible ducts and that these ducts "must be installed per the conditions of their listing." Any alterations made in the field (i.e., painting the duct, spraying internal sanitizers, adding additional insulation, etc.), may compromise the fire safety performance of the duct and void the manufacturer's product warranty.

Exposure to UV radiation and sunlight - It discusses degradation of polymeric materials from continuous exposure to natural sunlight or UV light.

Jack Lagershausen is executive director of the Air Diffusion Council, 1901 N. Roselle Rd., Ste. 800, Schaumburg, Ill. 60195; 847-706-6750; 847-706-6751 (fax);

Sidebar: Types Of Flexible Duct

The Air Diffusion Council (ADC) lists five types of flex duct:

1. Metallic uninsulated.

2. Metallic insulated.

3. Nonmetallic uninsulated.

4. Nonmetallic insulated, perforated.

5. Nonmetallic insulated, lined.

Sidebar: Contractors Speak Out On The Merits Of Flex Duct

Bucky Holland said that he installs flexible duct "as often as possible." The owner of Holland Heating and Air, Macon, Ga., said that 99 percent of his installations include flex duct.

"There are many reasons for that," he said. "No. 1 is packaging. It is also quicker and easier to install."

Holland said the fact that his business is a labor-driven company makes it a necessity to save as much time as possible. He said that the use of flex duct in many cases can reduce the labor costs of an installation.

His company uses flex duct mainly for turns and run-outs, which are tied into the main metal duct lines. "If used properly, flex duct makes a neat job," he stated. "It is abused every day, but if it's installed correctly, it will do as good a job or better than hard metal ductwork."

A Knoxville, Tenn., contractor pointed to the acoustic properties of flex duct as another advantage. "When installed properly, flex duct has a sound-deadening effect without the restrictions of other installation methods," said Mike Davis, president of City Heat & Air Conditioning. "It is also more cost effective to install than hard pipe with 2-inch ductwrap."

Michael Hays, general manager for Custom Air Concepts, Winder, Ga., said that a substantial number of installations in the Atlanta market include flex duct. He said his company offers customers a choice of sheet metal, ductboard, or flex duct products.

"Ease of use and speed of installation are the obvious advantages of flex duct," Hays added. "There are also fewer seams in flex duct, which means less potential for air leakage."

He noted that systems should be designed using flex duct specs, as flex duct is more restrictive to airflow than some other systems. "But if you design it properly, you could arguably make a better system," he said.

Hays emphasized that proper installation is essential. "It's easy to abuse because someone may decide to connect 5-foot runs with 25-foot lengths of flex duct," he said. "The product is fine - it's the installation that needs to be addressed. And flex duct manufacturers do a good job of providing installation information."

A Michigan contractor lists one more use for flex duct - moving make-up air. "We don't use flex duct much except for piping in make-up air," said Larry Paquette of Hutchison Mechanical Co. Inc., Macomb, Mich. "I buy a lot of 6- and 4-inch [flex duct] for make-up air. Make-up air should be insulated. Drawing in cold air causes condensate, which doesn't happen with flex duct."

- John R. Hall

Publication date: 10/25/2004