UL 181, UL 181A, and UL 181B: What Duct Tape and Mastic Markings Mean

Three different number families show up on duct tapes and mastic pails: UL 181, UL 181A, and UL 181B. They look interchangeable. They are not. UL 181 covers factory-made air ducts and air connectors themselves. UL 181A covers closure systems for rigid fiberglass duct board. UL 181B covers closure systems for flexible air ducts and connectors. The suffix tells you what the product is listed to seal, not how strong it is.

The markings matter because they are how the code gets enforced. Inspectors check duct materials and closure products for a printed listing marking that matches the duct type. The stakes are not trivial either: ENERGY STAR estimates that in a typical house, about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes, and poorly connected ducts.

Every listed closure product is marked with one of six designations. Match the marking to the duct type and you have answered the inspector's first question.

UL 181 is the Standard for Factory-Made Air Ducts and Air Connectors. It applies to the duct product itself: fiberglass duct board, flexible duct, and other factory-made duct and connector materials. The standard defines two classes by surface burning performance:

• Class 0: air ducts and air connectors with surface burning characteristics of zero.
• Class 1: air ducts and air connectors with a flame-spread index of not over 25, without evidence of continued progressive combustion, and a smoke-developed index of not over 50.

A roll of flex duct is itself listed and printed as a Class 0 or Class 1 air duct under UL 181. The tape or mastic that closes its joints carries a separate listing under UL 181A or UL 181B. Both listings have to be right for the installation to pass.

UL 181A is the Standard for Closure Systems for Use With Rigid Air Ducts. In practice that means fibrous glass duct board. It lists three closure types, each with its own marking:

• 181A-P: pressure-sensitive tape, applied to the duct board's foil facing.
• 181A-M: mastic, brushed or troweled over joints and seams.
• 181A-H: heat-activated tape, bonded to the facing by applying heat.

Duct board systems live or die by their closures, because the board itself has no mechanical joint strength comparable to sheet metal. That is why the UL 181A test program includes a 50,000-cycle pressure and temperature simulation, covered in the testing section below.

UL 181B is the Standard for Closure Systems for Use With Flexible Air Ducts and Air Connectors. Its markings are 181B-FX for pressure-sensitive tape, 181B-M for mastic, and 181B-C for nonmetallic mechanical fasteners such as duct ties. The IMC also points metal-duct tapes and mastics at UL 181B, which is why most quality foil tapes and duct mastics carry the 181B marking, and many carry both 181A and 181B listings.

Flexible duct has one extra rule that trips up a lot of installations: tape or mastic alone is not a complete closure. Per IMC Table 603.9.1, mechanical fasteners are required in conjunction with a listed pressure-sensitive tape or mastic on flexible ductwork.

The marking on a tape or mastic represents a specific battery of physical and application tests. Here is what UL 181A-P duct board tape and UL 181B-FX flex duct tape each have to survive:

The UL 181A duct simulation is the brutal one: a taped duct board assembly takes 30,000 pressure cycles at 165°F, then 15,000 cycles at 90°F and 90 percent relative humidity, then 5,000 cycles at 0°F. At the end, leakage has to stay at or below 15 cfm at 3 inches water column, with no tape displacement and no openings wider than 1/8 inch. For reference, a passing acrylic foil tape typically finishes the surface burning tunnel test with flame spread values between 5 and 15 and smoke developed values between 0 and 15.

IMC Section 603.9 requires longitudinal and transverse joints, seams, and connections in ducts to be constructed per the SMACNA HVAC Duct Construction Standards (metal) or the NAIMA Fibrous Glass Duct Construction Standards, and to be securely fastened and sealed with one of six closure methods: welds, gaskets, mastics, mastic-plus-embedded-fabric systems, liquid sealants, or tapes. Tapes and mastics on fibrous glass ductwork carry UL 181A markings; tapes and mastics on metallic and flexible ducts comply with UL 181B. The 2021 IMC consolidates the marking requirements into Table 603.9.1.

One exception worth knowing: continuously welded joints and seams, and locking-type longitudinal joints and seams, do not require additional closure systems on ducts operating below the 2 inch water column pressure classification. That is the code basis for leaving snap-lock seam pipe unsealed on low-pressure residential trunk, though energy codes often require sealing anyway.

California writes the listing requirement into its energy code. California Title 24 Sections 120.4 and 150.0(m) require duct connections to be mechanically fastened and openings sealed with mastic, tape, or aerosol sealant meeting UL 723 or a duct-closure system meeting UL 181, UL 181A, or UL 181B. Openings wider than 1/4 inch need mastic combined with mesh or tape, and cloth-backed rubber-adhesive duct tape is prohibited on joints and seams unless combined with mastic and drawbands. For the seal classes, leakage thresholds, and testing numbers behind these rules, see the duct sealing requirements guide.

Duct closure enforcement comes down to four checks:

• The printed listing marking on the tape roll or mastic pail matches the duct type: 181A markings on fiberglass duct board, 181B markings on metal and flex.
• Flexible duct connections have mechanical fasteners in addition to the listed tape or mastic.
• Openings wider than 1/4 inch are sealed with mastic plus mesh or tape, not tape alone.
• The tape is not a general-use insulation tape. Tapes that are only UL 723 Classified for surface burning are for jobs like seaming insulation facing. A 723 rating is not a UL 181A or 181B duct-closure listing.