Mastic is a water-based paste that can be applied to seams in an HVAC system in order to help seal the system.
It is typically used to fill crevices that an HVAC tape can’t reach, especially because of its peanut butter-like consistency, which allows it to conform to any shape. Used to create airtight seals, mastic can withstand extreme temperature differences, and with proper application, can last for years.
Sometimes used over tape, this adhesive can be easily applied to joints of a duct system, with the only limitation being the ability to reach a joint. Codes will dictate when a mastic needs to be used.