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SMACNA and Mastic

Harry Thompson IVSeptember 25th, 2018

On Rectangular Duct

"Rectangular longitudinal seam leakage for Pittsburgh and Buttonlock seams is low compared to that in transverse joints." — SMACNA Duct Construction Standards 2005

However, SMACNA also states that “the designer is responsible for determining the pressure class or classes required for duct construction and for evaluating the amount of sealing necessary”.

SMACNA breaks down the sealing of commercial ductwork into three classes: A, B, and C.

  • Class A applies to static pressure of 4″ wg or more and requires all transverse joints, longitudinal seams and duct wall penetrations to be sealed.
  • Class B applies to static pressure of 3″ wg and requires all transverse joints and longitudinal seams be sealed.
  • Class C applies to static pressure of 2″ wg and requires only the transverse joint be sealed.


Low Pressure Duct with Slip and Drive connected, Duct Sealant Applied. Mastic is always required on Transverse Connection...
...but not on the longitudinal seam.
High-pressure duct with Hardcast CCW181 White, a heavily fibrous mastic applied at every seam, joint, and penetration. Some engineers prefer White Mastic as it is more visible, especially when the duct is high up in the air and gray mastic may be difficult to see.


All penetrations into the duct, including taps for takeoffs, transitions, and screw holes for hanging methods, are connected to the duct, like this large job in New Orleans.


Job site in Jacksonville, FL

Applying the duct sealant to the longitudinal seam is easily visible when done on the exterior of ductwork.

Rooftop duct in Greenville, SC

Are these handprints or...?

On this Jacksonville, Florida job, mastic was applied to every penetration; those are not handprints but the mastic applied to the points in the elbow where turning vanes are secured by drill screws.

Of course, some shops seal the longitudinal seam on the duct interior if the engineer prefers a cleaner look on the exterior.