When taping a plenum, it is recommended to use a UL Listed foil tape. The plenum is usually made from sheet metal and is connected to the supply outlet of an HVAC system. The heated (or cooled) air is sent through the plenum to the ductwork, which will then disperse the treated air into each individual room of the building.
There are generally two types of UL Listed foil tapes – those that are UL 181A-P and 181B-FX Listed (shown as UL 181A-P/B-FX) and those that are only UL 181B-FX Listed. The difference between the listings is that tapes that meet both listings (UL 181A-P/B-FX) can be used on rigid fiberglass duct board, flexible air duct and sheet metal. Tapes that are only UL 181B-FX Listed may be used on Class 1 Flex duct and sheet metal, but not rigid fiberglass duct board.
Foil tapes that are UL 181A-P/B-FX Listed almost always have an acrylic adhesive. Acrylic adhesives perform well across a wide temperature range (usually below 20 F and over 250 F) and are robust enough to withstand the numerous thermal cycles of an HVAC system.
Foil tapes that are UL 181B-FX Listed usually have a butyl rubber adhesive. This type of adhesive is extremely thick and forms a strong bond to the surface it is applied to; these types of tape are also referred to as mastic tapes because they can be used in place of traditional paint-on mastic.