The North American Insulation Manufacturers Association fibrous glass duct construction standards are a guiding light for HVAC installation. As rigid fiberglass has gained a stronger footprint in the HVAC industry over the past few decades, modular fiberglass duct board systems have often become cheaper to fabricate than sheet metal systems in residential and light commercial buildings.

There are three closure systems for rigid duct that UL has certified; fiber ducts fabricated without UL 181A closure materials will not meet local building or mechanical codes.

  • UL 181A-P: pressure sensitive aluminum tape
  • UL 181A-H: Heat activated aluminum tape
  • UL 181A-M: Fiber glass reinforced mastic

Tested for adhesion, tensile strength, surface burning characteristics, temperature and pressure cycling, mold growth and humidity, all three options hold up well. UL has approved them for systems operating on up to, plus or minus, 2” water gauge static pressure and up to 5,000 ft. per minute internal air velocity. However, according to NAIMA, the mastic and glass fabric closure system is recommended on 1” water gauge static pressure sheet metal systems.

Mastic and glass fabric is popular among installers for its strength on sheet metal systems. It is also often the go-to method for high-pressure ducts.

While reinforcing mastic with glass fabric takes time, and time is money on the jobsite, the added strength to the adhesive makes it durable enough to last decades of vibrations and the occasional bump.