On a duct system, what type of seam must be sealed to prevent air leakage?

• A. Circumferential
• B. Radial
• C. Butt seam
• D. Longitudinal

Answer: (D)

The air-tightness of a duct largely depends on sealing the seam that runs along the length of the duct. This longitudinal seam is created when sheet metal panels are joined to form the run, and because it extends the full length, any gap there becomes a continuous leak path. Sealing this seam with appropriate duct sealant or mastic (and tape where required) creates a tight barrier that preserves pressure and efficiency. Other seam types can exist, but they don’t represent the primary leakage path along the length of the duct, so the key prevention step is sealing the longitudinal seam.