Which method is NOT a recommended sealing technique for metal ductwork?

The method identified as not recommended for sealing metal ductwork is electrical tape. This type of tape is not designed for the high-temperature and airflow conditions present in duct systems. It lacks the properties required to create a durable and airtight seal, which is essential for maintaining energy efficiency and preventing leaks in HVAC systems.

On the other hand, UL 181 mastic, foil tape, and silicone sealant are all suitable options for sealing metal ductwork. UL 181 mastic is specifically formulated for HVAC applications, as it adheres well to metal surfaces and remains effective in various temperature ranges. Foil tape is resilient and provides a strong, moisture-resistant seal, making it ideal for ductwork. Silicone sealant is versatile and can handle temperature fluctuations while providing excellent adhesion and flexibility. These attributes make them preferable choices for ensuring a proper seal in duct systems.