Frequently Asked Technical Questions

Cleaning of air ducts, including flexible, wire-helix type, has become a common practice as more-and-more companies offer this service.
Typical cleaning tools may include simple brushes and a number of pneumatic style agitation and cleaning devices. Accumulated debris is loosened from inside ducts by power brushing or manual brushing. Pneumatic devices such as blowguns, air skippers and air whips are often utilized to drive agitated debris to a collection device which is typically a vacuum device used to create negative pressure within the HVAC system that collects contaminants during the cleaning process.
ATCO recommends that flexible duct be cleaned by qualified technicians following the procedures outlined by the National Air Duct Cleaners Association (NADCA).
We caution that flexible duct inner liners may potentially be damaged by overly aggressive contaminant removal techniques, such as high-pressure blow guns, air skipper, and air whips. If used to clean flexible duct, these devices should be handled appropriately in order to prevent damage to the inner liner during the cleaning process.
ATCO has evaluated the cleaning of our flexible air ducts and air connectors following the rotating brush and vacuum process per NADCA procedures. During this evaluation we found this cleaning process to be effective in dust and dirt removal and the process did not exhibit any apparent damage to the interior liner of our duct during the cleaning process.
If cleaning of ATCO flexible duct is needed, care should be taken in the selection and application of any cleaning process. When cleaning is finished, any connections that required opening must be properly resealed in order to maintain air tightness and duct thermal properties.

ATCO Rubber Products does not recommend screws be used to fasten the polyester core of the air ducts because they weaken the polyester. Polyester is a very strong material as long as there are no holes or tears in it. As soon as a tear or hole is introduced, its strength drops. To maintain our UL approval status, our air ducts must pass a tension test (25 pounds hanging from one end of the duct), a torsion test (one end rotated 180° or to 25 foot-pounds whichever comes first) and then a leakage test. In all of these tests, both ends must be connected to collars per our installation instructions. Flexible duct connected with screws would not pass these tests.

For ATCO’s Installation Instructions, click here. When hanging or supporting flexible duct, there are a number of different materials which are used regionally. ATCO doesn’t specify a particular material in our Installation Instructions, however ATCO and the Air Diffusion Council do make the following statement: “Hanger or saddle material in contact with the flexible duct shall be of sufficient width to prevent any restriction of the internal diameter of the duct when the weight of the supported section rests on the hanger or saddle material. In no case will the material contacting the flexible duct be less than 1 1/2″ (38 mm) wide”. As in all cases, local jurisdiction has final say in these matters.

UL (Underwriters Laboratories), in their 181 Standard for Factory-Made Air Ducts and Air Connectors, defines two categories of flexible “ducts”.

The UL Listed Air Duct must pass all of the tests in the UL 181 Standard. Air Ducts are labeled with a square or rectangular shaped label showing their respective listing. There is no limitation on the length of runs when using UL Listed Air Ducts.

The UL Listed Air Connector must pass only a limited number of the UL 181 tests, and is labeled with a round shaped label, which states “for installation in lengths not over 14 feet.”

All UL 181 Air Ducts and Connectors have been tested by Underwriters Laboratories, Inc. for mold growth and humidity, and are considered by the manufacturer as acceptable for use in conjunction with evaporative coolers. Please refer to Chapter 6 and Appendix B Testing, Listing, Reporting and Certifying of the ADC Flexible Duct Performance & Installation Standards (Greenbook). www.flexibleduct.org for further information.

Condensation or “sweating” is a complex problem that is primarily seen in high humidity areas. There are a large number of factors that can lead to condensation. Condensation occurs on any surface that is colder than the dew point temperature of the air surrounding it. In air ducts it is most common to see this on the outer vapor barrier of the duct. However, it can also be on the inner core in flexible ducts, or the sheet metal of a sheet metal system. We also see condensation at the fittings or plenums. Any place where the temperature of a surface is colder than the dew point temperature of the surrounding air you will have condensation.

To prevent sweating it is necessary to either raise the temperature of the sweating surface, or lower the dew point temperature of the air. It is only possible to lower the dew point temperature by decreasing the amount of moisture in the air (dehumidify) or by bringing in (mixing) drier air from the outside or inside. It is usually easiest to increase the temperature of the sweating surface by adding insulation.

When it is necessary to route an insulated duct through a truss/joist support, care should be taken to prevent tearing of the vapor barrier and insulation as this will allow moisture into the duct wall and reduce the thermal performance..

Localized compression of the fiberglass should have minimal, negative effect on the total, thermal losses of the duct. However, when the installation is in a critical condensation zone, localized compression could result in localized sweating.

The A/C contractor or designer should consider this information in view of total design performance.

Read this for more information.

Fire Resistance-Rated Systems are evaluated under the ANSI/UL 263 Standard “Fire Tests of Building Construction and Materials.” A complete list of Fire Resistance-Rated Systems can be found in UL’s On-Line Certifications Directory. By entering the System “design number” you can determine what materials have been tested as part of the system and are allowed to be used as air ducts in that rated system. In many Fire Resistance-Rated Systems, Class 0 or Class 1 Flexible Air Ducts are allowed to be used. But this may not always be the case. The designer and installer must verify by individual design number what duct materials are allowed within that particular rated system.

Flexible air ducts and air connectors do not have “plenum ratings” similar to those for wiring and plumbing pipe. The requirements for materials allowed within plenums is covered in NFPA Standards 90A & 90B and the applicable Mechanical Codes (IMC, UMC). UL 181 Listed and Labeled Class 0 and Class 1 Flexible Air Ducts and Air Connectors meet the requirements in these referenced standards and codes for installation within plenums. AS always, local jurisdictions have final say.

ATCO does not have a duct specially designed to reduce noise within the duct system. However, ATCO does have independent test data on the acoustical properties of our standard products. The data shows Net Insertion Loss on 10′ lengths of different diameters of ATCO duct, as compared to sheet metal duct. Tests were done on straight lengths, and duct with a 90 degree bend, with and without airflow, and are recorded at a variety of octave bands. Click here to download the acoustical information on our products.

No, flexible duct must be protected from the weather, including sunlight. In the late 1980’s, manufacturers of insulated flexible duct discovered that the products with gray polyethylene jackets could fail when exposed to direct or indirect sunlight. The ultraviolet rays in the sunlight would break the chemical bonds in the gray polyethylene jackets causing them to disintegrate over a period of several years. To alleviate the problem, manufactures began using black polyethylene jackets and metallized polyester jackets. The carbon black used to color the black polyethylene jackets has an inherent resistance to ultraviolet light as so is the metallized polyester. Since the change, the problem has been eliminated. However, most manufacturers still require that insulated flexible duct of any type not be installed in areas with direct or indirect sunlight. We are confident of the quality of our flexible duct products.

If you have a duct that will be exposed to Ultraviolet light from the use of a Bio-treatment lamp or UV lamp, we suggest you use our Ultra-Flex Duct Kit. It is specifically designed to be used where UV light from a Bio-treatment lamp is emitted onto the core of a non-metallic flexible duct.

ATCO provides insulated flex duct with three different R-Values (4.2, 6.0, and 8.0). The thickness of each is:
R-4.2 = 1 ¼” C-Value of Approx: 0.238
R-6.0 = 2” C-Value of Approx: 0.167
R-8.0 = 2 ½” C-Value of Approx: 0.125

All fiberglass insulation is supplied by Owens Corning.

In an attempt to standardize, the flexible duct industry no longer states insulation values in thickness and density. The Air Diffusion Council, an industry organization of manufacturers, certifies R-Values based on insulation only, at installed wall thickness, based on ASTM C-518. Underwriters Laboratories classifies these tested R-Values based on plant inspections. These two third party verifications provide an accurate and reliable method of assuring that you are getting the thermal performance that you require.

Rated Positive Pressure:

10” w.g. per UL 181

Recommended Operating Pressures:

6” w.g. pos., 4”-12” dia., 4”w.g. pos., 14”-20” dia.; ¾” neg., all dia.


5,000 FPM

Flame Spread/ Smoke Developed:

25 max/ 50 max

Standard Lengths:


ATCO’s UL Listed pressure ratings are determined by testing ducts in straight lengths, at ambient temperatures. Testing is done in a static condition (no airflow). The test sample is sealed airtight on both ends and pressurized to 2 ½ times the manufacturer’s desired rated pressure for a test period of one hour. A minimum of 1 1/4″ of positive pressure must be achieved in order to pass the test.

ATCO performs some additional tests in accordance with the Air Diffusion Council, FD 72-R1 Flexible Duct Test Code. The purpose of these tests is to simulate a variety of realistic conditions, which may exist in actual installations. The ADC tests include a 90 degree bend, heated air at continuous and intermittent temperatures, for a test period of 168 hours. The resulting “Recommended Operating Pressures” are intended to give the contractor and/or engineer a responsible guideline within which to design the duct system.