CSLB C-20 HVAC ContractorVentilation and Air DistributionEasy
Per the California Mechanical Code, which of the following materials is NOT approved as a primary sealant for joints and seams in metal ductwork?
- AStandard cloth-backed "duct tape"
- BUL 181B-FX certified tape
- CWater-based duct mastic
- DUL 181B-M mastic with fiberglass mesh reinforcement
Show answer & explanationAnswer & explanation
Correct answer: A. Standard cloth-backed "duct tape"
Standard cloth-backed duct tape degrades quickly under heat cycling and is not UL 181-rated, so it is prohibited as a primary duct sealant. Mastic and UL 181-listed tapes are required.
Why the other options are wrong
- B. UL 181B-FX is a listed, approved flexible duct tape.
- C. Mastic is a standard approved sealant.
- D. Mastic with mesh reinforcement is approved for larger gaps.
Approved Duct Sealants
Only UL 181-listed tapes and mastics are approved for sealing ductwork joints; ordinary cloth duct tape is prohibited.
- UL 181B-FX = listed tape for flex duct.
- Mastic + mesh used for larger gaps or seams.
- Cloth duct tape dries out and fails within a few years.
Memory trick: Cloth tape cracks, mastic sticks.