Ducts must be sufficiently airtight to ensure economical and quiet performance of the system.

  It must be recognized that airtightness in ducts cannot — and need not — be absolute, as it must be in a water piping system.


Many manufacturers produce liquid sealants specifically for ducts. They have the consistency of heavy syrup and can be applied either by brush or with a cartridge gun or powdered pump.

Liquid sealants normally contain 30% to 60% volatile solvents; therefore, they shrink considerably when drying.

They are recommended for slip-type joints where the sealant fills a small space between the overlapping pieces of metal.

Where metal clearances exceed 1/16 in., several applications may be necessary to fill the voids caused by shrinkage or run out of the sealant. These sealants are normally brushed on to round slip joints and pumped into rectangular slip joints.


Heavy mastic sealants are more suitable as fillets, in grooves, or between flanges.

Mastics must have excellent adhesion and elasticity. Although not marketed specifically for ductwork, high quality curtain wall sealants have been used for this application.

Oil-base caulking and glazing compounds should not be used.


Durable materials, such as soft elastomer butyl or extruded forms of sealants, should be used in flanged joints.

For ease of application, gaskets should have adhesive backing or otherwise be tacky enough to adhere to the metal during joint assembly.

The choice of open-cell or closed-cell rubber gaskets depends on the amount and frequency of compression and on the elastic memory.


Nothing in this standard is intended to unconditionally prohibit the use of pressure-sensitive tapes. Several such closures are listed as components of systems complying with UL Standard 181 tests.

There are no industry-recognized performance standards that set forth peel adhesion, tensile strength, temperature limits, accelerated aging, etc., which are quality-control characteristics specifically correlated with metal duct construction service.

However, the SMACNA Fibrous Glass Duct Construction Standards illustrate the closure of a fibrous duct to metal duct with a tape system. The variety of advertised products is very broad. Some test results for tapes are published in the product directories of the Pressure Sensitive Tape Council, Chicago, IL.

The shelf life of tapes may be difficult to identify. It may be only six months or one year. Although initial adhesion may appear satisfactory, the aging characteristics of these tapes in service is questionable.

They tend to lose adhesion progressively at edges or from exposures to air pressure, flexure, the drying effects at the holes or cracks being sealed, etc. The tape’s adhesive may be chemically incompatible with the substrate, as is apparently the case with certain nonmetal, flexible ducts.

Application over uncured sealant may have failures related to the release of volatile solvents. Sea air may have different effects on rubber, acrylic, silicone-based, or other adhesives.

Tapes of a gum-like consistency with one or two removable waxed liners have become popular for some applications. They are generally known as the peel-and-seal variety and have been used between flanges and on the exterior of ducts.

Such tapes are typically of thicknesses several times that of tapes traditionally known as the pressure-sensitive type. Some may have mesh reinforcement. Others may have metal or nonmetal backing on one surface.

Heat-applied materials

Hot melt and thermally activated sealants are less widely known, but are used for ductwork.

The hot-melt type is normally a shop application. Thermally activated types use heat to either shrink-fit closures or to expand compounds within joint systems.

Mastic, embedded fabric

There are several combinations of woven fabrics (fibrous glass mesh, gauze, canvas, etc.) and sealing compounds (including lagging adhesive) that appear better suited for creating and maintaining effective seals than sealant alone.

Glass fabric and mastic (GFM) used for fibrous glass duct appears to adhere well to galvanized steel.

Surface, sealant, and safety considerations

Surfaces to receive sealant should be clean, meaning free from oil, dust, dirt, rust, moisture, ice crystals, and other substances that inhibit or prevent bonding.

Solvent cleaning is an additional expense. Surface primers are now available, but their additional cost may not result in measurable long-term benefits.

No sealant system is recognized as a substitute for mechanical attachments. Structural-grade adhesive systems are being developed to replace spot-welded and soldered connections of metals.

They have lap shear strengths of 1,000 to 5,000 psi or more. SMACNA is not able to comprehensively define their characteristics at this time; however, authorities are encouraged to monitor their development progress and consider their use.

Sealant systems may be flammable in the wet, partially cured, or cured state.

The contractor should carefully consider the effects of loss of seal and fire potential when welding on or near sealed connections. NFPA Standard 90A requires adhesives to have a flame spread rating not over 25 and a smoke developed rating not over 50.

These excerpts are from the Sheet Metal and Air Conditioning Contractors’ National Association’s (SMACNA) Hvac Duct Construction Standards Metal and Flexible manual. For more information about this or other SMACNA publications, call 703-803-2989 or visit www.smacna.org.