SMACNA DUCT SEALING AND LEAKAGE CLASSES
Seal class and leakage class get used interchangeably on the job, but they answer two different questions. One tells you what to seal. The other tells you how much leakage the finished system is allowed.
This is a quick guide for mechanical contractors, engineers, and project teams reading a SMACNA spec and deciding how to build and seal their duct.
Published July 13, 2026 · JMC Fabrication
Seal class is a construction requirement. It defines which joints, seams, and connections have to be sealed on a given system, from transverse joints only up to all joints, seams, and wall penetrations.
Leakage class is a performance requirement. It sets the allowable leakage rate for the finished duct, expressed as leakage per unit of surface area at the test pressure. Seal class is what you do in the shop and the field. Leakage class is what the system has to prove when it is tested.
Pressure class drives both. The higher the operating pressure, the harder air pushes on every joint and seam, so the same small gap leaks more air on a high-pressure system than on a low-pressure one.
Because of that, higher pressure classes call for more aggressive seal classes and tighter leakage limits. Reading the pressure class first tells you what sealing scope and leakage target the rest of the spec is going to demand.
Most commercial sealing comes down to three approaches, often used together:
- Mastic — A brushed or troweled sealant that fills longitudinal seams, transverse joints, and penetrations. It is the workhorse for durable, long-term seals and holds up well on higher-pressure systems.
- Tape and mastic systems — Fabric or foil tape embedded in mastic to bridge joints. Tape alone is rarely enough for a demanding spec, so it is paired with mastic where the joint needs reinforcement.
- Gasketed connections — Factory-applied or shop-applied gaskets on flanged and slip-and-drive connections. Gasketed joints give repeatable results and are common where fittings are assembled and disassembled.
Testing is not automatic on every job. It usually shows up when:
- The system runs at a higher pressure class where leakage carries a real energy penalty
- The mechanical spec or energy code calls out a specific leakage class to hit
- The project team wants documented verification on critical or large supply runs
- The commissioning scope requires proof that installed duct meets the design intent
Field sealing happens in tight spaces, overhead, and under schedule pressure, which is exactly where seams get missed and mastic goes on thin. Shop sealing happens on a bench with clean access, controlled conditions, and consistent application.
Building and sealing seams and connections in the shop means fewer field joints left to seal, more repeatable coverage, and a better shot at passing a leakage test the first time. That is how JMC approaches SMACNA duct, and it is why we would rather seal in the shop than leave it to the ceiling.
JMC Fabrication is a Gulf Coast commercial and industrial metal fabricator based in Pascagoula, Mississippi, serving Mobile, New Orleans, and the Mississippi Gulf Coast. Send us the mechanical spec with your pressure and seal class, and we will fabricate and seal to it.
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