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Frequently Asked Questions


What kind of components can be fabricated from flat fiber glass duct board sheets?
Straight ducts of any size or length. In addition, tees, offsets, elbows and transitions can also be easily fabricated.
Can you insert equipment into fabricated fiber glass duct board systems?
Equipment such as electric or hot water in-line heaters, manual or motorized dampers, registers and grilles, diffuser drops, and access doors can be incorporated into a fiber glass duct system. In most cases, sheet metal sleeves and extra hangers are required. Sealing of fiber glass duct board to sheet metal must be made with glass fabric and mastic except where operating pressure is less than 1" w.g. (250 Pa) and sheet metal surfaces are carefully cleaned, in which case pressure-sensitive aluminum foil t may be used.

When connecting fibrous glass duct systems to sheet metal components such as equipment flanges, fasteners such as sheet metal screws and washers are used to carry the mechanical load.

For step-by-step instructions, refer to NAIMA's Fibrous Glass Duct Construction Standards manual (Pub # AH-116).
What is the R-value of fiber glass duct board?
The thermal performance is determined by the thickness of the duct board.
  • 1"  fiber glass duct board has an R-value of 4.3
  • 1 1/2" fiber glass duct board has an R-value of 6.5
  • 2" duct board has an R-value of 8.7
Factory-controlled insulation thickness assures that specified R-values are met. R values many vary slightly by the manufacturer.
Can generic duct tape be used to seal fiber glass duct board?
No. UL 181 Closure systems are the only sealing systems that have been tested and approved. The longitudinal and transverse joints of fiber glass duct modules should be sealed using one of three UL 181A listed closures:
  • UL 181A, Part I (P) - Staples and pressure-sensitive aluminum foil tape. 2 1/2" wide tape may be used in making closures with 1" duct board. 3" wide tape is recommended for 11/2" duct board, and is mandatory with 2" duct board.
  • UL 181A, Part II (H) – Heat-activated aluminum foil/scrim tapes, 3" wide.
  • UL 181A, Part III (M) – Mastic reinforced with 3" wide glass fiber fabric (scrim).
This closure system may be required when the system is part of a fire-rated assembly.
What are the operating parameters of fiber glass duct board?
Rectangular ducts are designed to operate at 2" w.g. static pressure or less and 6,000 feet per minute internal air velocity or less.
Has the structural integrity of fiber glass duct board systems been put to the test?
Test programs conducted by NAIMA and the manufacturers demonstrate that fiber glass duct systems, including fittings and accessory items, are capable of maintaining their structural integrity through 50,000 cycles at 1-1/2 times system design pressurization. This testing demonstrates the reliability of properly constructed systems, but does not imply that systems should be operated at pressures above their reinforcement rating.
Are fiber glass duct systems code compliant?
Yes. Fiber glass duct systems qualify under UL 181 as a Class 1 Air Duct and meet the safety requirements of NFPA 90A and 90B including combustibility. They also comply with all widely used model energy codes including the ICC.
What is the most common mistake when installing ductwork?
The closure system. Air leakage is one of the biggest efficiency issues with any HVAC ductwork. Fiber glass duct board system can deliver virtually leak free performance when fabricated and installed properly, which means using a UL181 closure system. Closures made with UL 181A listed pressure-sensitive tape, heat-activated tape, or glass fabric and mastic, save energy by virtually eliminating air leakage.
What happens if duct board gets wet?
You should avoid situations where duct board can get wet with 'liquid' water.
  • When using either evaporative coolers or humidifiers, the immediate area around the device introducing water into the system, should be protected by using a drip pan and protective corrosion-resistant sheet metal sleeve.
  • Duct systems running through unconditioned space and used for cooling only must be tightly closed during the heating season to prevent accumulation of water vapor in the duct system.
  • Fiber glass duct materials that have become wet at the job-site before or during installation should not be installed.  Duct systems that, in service, are found to be wet should be replaced.
What helpful tips can contractors pass on to their customers as far as maintaining an efficient ductwork system?
Inspect and maintain. And, begin with the equipment itself. Check the heating and cooling units regularly. Make sure the unit is sized correctly for the system. Change the filters with each season. Make sure the dampers are working. Check the burners to make sure they are adjusted correctly. If the ductwork is specified, fabricated and installed correctly, there should be no maintenance, and perhaps no cleaning, necessary. If duct cleaning is necessary, fiber glass duct systems can be cleaned.
The air stream surfaces are designed for today's duct cleaning technologies. The North American Insulation Manufacturers Association (NAIMA) has developed recommended cleaning practices that have been adopted and used in the certification of duct cleaning professionals through the National Air Duct Cleaning Association (NADCA).


Pete Hays, instructor
Tommy Jackson, instructor
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