It’s natural for contractors and specifiers to be concerned about the safety of any building material that has the potential to give off particles or fumes that can be inhaled and ingested. For this reason, fiber glass continues to be one of the most studied materials in the building construction industry.

Fiber glass duct board and duct liner are tested for maximum design air velocities in accordance with UL 181 and ASTM C 1071 standards, respectively. The tests are designed to ensure that the maximum-rated airflow will not dislodge glass fibers into the airstream.

To meet the UL 181 Class 1 standard for duct board, manufacturers are required (on a continual basis) to test the product at 2.5 times the rated air velocity without showing any evidence of fiber erosion after initial blowdown.

To comply with ASTM C 1071 specifications, duct liner is also tested for fiber erosion at 2.5 times the rated velocity. Most manufacturers offer a fiber glass duct liner with a rated velocity of 6,000 fpm, so they are required to test the product at 15,000 fpm.

Specification compliance for fiber erosion is measured after the initial duct system blowdown. When any duct system is installed, proper procedure calls for a “blowout period” during which the system operates at full design capacity. This will help blow out any fibers and other debris loosened during the fabrication and installation process. Most fiber glass duct insulation manufacturers also apply a mat facing or an acrylic polymer coating to the airstream surface for added surface protection.

The Elbow Test

To satisfy the requirements of either standard, products must pass a demanding air erosion test in a 90-degree elbow configuration without turning vanes. The elbow creates a tremendous amount of turbulence, which severely challenges the ability of duct board and duct liner to resist erosion. A filter is placed at the exit of the duct section to collect any debris.

Duct board must be fabricated and installed in accordance with North American Insulation Manu-facturers Association (NAIMA) fibrous glass duct construction standards; lined sheet metal must meet Sheet Metal & Air Con-ditioning Contractors’ National Association (SMACNA) duct construction standards.

In order to pass the test, the duct section cannot show any evidence of fiber erosion over the duration of the procedure — a 5-hr period that includes four shutdown-startup cycles.

The primary factor that prevents air movement from dislodging fibers from ducts and blowing them into a conditioned area is the low velocity of the airflow along the inside surfaces of ducts. There is little or no air movement at the surface to dislodge the fibers.

This is why most duct surfaces have a fine coating of dust. During the 30-plus years that fiber glass duct insulation has been in service, a number of tests by manufacturers and independent researchers have shown that, after initial startup and blowdown of a duct system, the number of airborne glass fibers is well within the ranges found in ambient air.

In fact, fiber counts in rooms served by ducts containing fiber glass have typically been lower than those found in outdoor air.

For more information, visit NAIMA’s website at www.naima. org or call 703-684-0084.

Brower is the vice president of research and development for Knauf Fiber Glass and can be reached at 800-825-4434, ext. 8212; or visit (website).

Publication date: 10/09/2000