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Ductwork - Seal Classes

Ductwork air leakage classes.

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ASHRAE and SMACNA standards specify the duct sealing requirements in terms of seal classes A, B and C.

  • Seal Class A - All transverse joints and longitudinal seams and duct wall penetrations shall be sealed. Pressure sensitive tape shall not be used as primary sealant. Max. 2 to 5 percent total system leakage.
  • Seal Class B - All transverse joints and longitudinal seams shall be sealed. Pressure sensitive tape shall not be used as primary sealant. Max. 3 to 10 percent total system leakage.
  • Seal Class C - All transverse joints shall be sealed. Max. 5 to 20 percent total system leakage.
  • Unsealed - 10 to 40 percent total system leakage.
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