Sealing, Insulation, and Vapor Control
The Sealing, Insulation, and Vapor Control domain tests how to keep air, heat, and moisture where they belong. Items cover SMACNA seal classes, UL 181 tapes and mastics, leakage testing, insulation R-values, vapor retarders, and firestopping.
Core rules and concepts
- Seal class depends on pressure and specification: Class A seals all transverse joints, longitudinal seams, and penetrations; Class B seals transverse joints and longitudinal seams; Class C seals transverse joints only.
- Use mastic, gaskets, or listed tapes rated for the duct pressure and temperature. UL 181 tape is not automatically a substitute for mastic at every joint.
- Insulate to the specified R-value without compressing the insulation. In cooling climates, the vapor retarder goes on the outside of the insulation.
- Keep the vapor retarder continuous at joints, hangers, and penetrations; seal it with the approved tape or mastic.
- Use listed firestop sealants where ducts penetrate rated assemblies.
Worked example
A 2 in. wc duct is specified Seal Class A. The installer must seal every transverse joint, every longitudinal seam, and every penetration, including takeoffs and access doors. A joint taped only on the outside, with an unsealed Pittsburgh lock, does not meet Class A and will leak in a leakage test.
Traps that cost marks
Compressing insulation, putting the vapor retarder on the cold side, using ordinary duct tape on high-pressure joints, and skipping sealant at longitudinal seams because they look tight. Another is failing to seal around hangers and penetrations, which undermines the whole vapor barrier.