Insights · Standards
A practical engineer's guide to SMACNA Seal Class A, B and C — what each class actually requires, the allowable leakage rates at each pressure class, and how the machinery that forms the duct directly affects whether the finished system will pass a leakage test.
SMACNA — the Sheet Metal and Air Conditioning Contractors' National Association — publishes the HVAC Duct Construction Standards — Metal and Flexible manual that the North American duct industry uses as its baseline. Chapter 5 of that manual defines three seal classes — A, B and C — and the associated leakage testing procedure. Every project specification in the United States, Canada, Australia and most of the Gulf will reference these classes by name.
Class A is the tightest. Class C is the loosest. There is no Seal Class D in the current SMACNA standard — unsealed duct is called "unsealed" and is permitted only on very low-pressure return and exhaust.
SMACNA expresses leakage in cubic feet per minute per 100 square feet of duct surface area at a stated static pressure. The table below is the commonly cited rule-of-thumb for rectangular metal duct. Project specifications may be stricter.
| Static pressure | Class A (cfm/100 sq ft) | Class B (cfm/100 sq ft) | Class C (cfm/100 sq ft) |
|---|---|---|---|
| 1 in. w.g. | ~3 | ~6 | ~12 |
| 2 in. w.g. | ~5 | ~9 | ~18 |
| 4 in. w.g. | ~7 | ~14 | — |
| 10 in. w.g. | ~12 | — | — |
Always confirm against the latest SMACNA printing and the project specification.
Designers generally pair pressure classes with seal classes as follows:
High-rise supply air, hospital isolation rooms and data center supply risers almost always call out Seal Class A by name. For machine selection and leak-rate targets by end-use vertical, see the HVAC duct machinery by industry hub.
In our experience, Seal Class A leakage tests fail for three reasons, in order:
Two out of three of the common failure modes are machinery related. An SBAL-V auto duct line holds ±1.0 mm on length and ±0.5 mm on width, which keeps TDF corners square and drops leakage dramatically compared to manual cut-and-brake duct.
The TDF transverse flange is rolled directly into the duct panel by the duct line. Because the flange is continuous metal — not a bolted angle frame — there are four fewer potential leak paths per joint than an angle-flange joint. Combined with a 6 mm mastic bead in the flange channel and four corner cleats, a properly formed TDF joint will hit Seal Class A on the first test.
SMACNA does not name a specific sealant brand, but requires the sealant to remain flexible within the operating temperature range of the system. Water-based mastic is the most common; silicone is used for high-temperature exhaust. Do not use duct tape as a seal — duct tape fails adhesion within months and is explicitly not accepted for any SMACNA seal class.
Testing uses a calibrated leakage tester (a variable-speed fan with an orifice plate) to pressurize a sealed section of duct to the design static pressure. The fan flow required to maintain that pressure is the total leakage. Divide by the surface area of the section and compare to the allowable table above. Sample size is typically 25% of the total duct system, selected by the commissioning agent.
Outside North America, similar leakage classes exist under different names:
Typical language in a mechanical specification reads something like: "All supply ductwork operating at 3 in. w.g. and above shall be constructed to SMACNA Seal Class A, with transverse joints, longitudinal seams and wall penetrations sealed with water-based mastic. Leakage testing shall be performed on 25% of the installed duct at design pressure, with allowable leakage not to exceed 3 cfm per 100 square feet of duct surface." When you see that paragraph, you know the whole fabrication tolerance conversation is about to happen.
Ask SBKJ how our duct lines hit Seal Class A →
SMACNA Seal Class A requires all transverse joints, longitudinal seams and duct wall penetrations to be sealed. It is the tightest seal class, used for high-pressure systems (3 in. w.g. and above) and critical applications such as laboratories, hospitals and clean rooms.
Seal Class A seals all transverse joints, longitudinal seams and wall penetrations. Seal Class B seals transverse joints and longitudinal seams only (no wall penetrations). Seal Class C seals transverse joints only. Class A is the tightest; Class C is the loosest.
SMACNA defines leakage in cfm per 100 sq ft of duct surface area at a given static pressure. At 1 in. w.g., Seal Class A allows roughly 3 cfm/100 sq ft, Class B allows 6 cfm/100 sq ft, and Class C allows 12 cfm/100 sq ft. Leakage rises with pressure.
Tight tolerances on TDF flange profiles, consistent Pittsburgh lock seaming, and accurate length cutting all reduce joint leakage. An SBKJ auto duct line holds ±1.0 mm on length and ±0.5 mm on width, which keeps TDF corners square and improves Seal Class A achievability on site.
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