The best way to seal most duct seams is with mastic. However, some duct seams call for an easier-to-apply sealing method: tape. There are a great many types of duct-sealing tape, made with a variety of adhesives and backings, but the two most durable types are oriented polypropylene duct tape and butyl duct tape.

Ordinary rubber-adhesive duct tape may be great for a lot of tasks, but sealing ducts isn’t one of them. This tape dries out quickly and can peel off metal ductwork.

Before choosing a tape, consider that building codes require duct tapes to carry a UL 181 label, though in many areas of the country, this is not enforced. Also, according to tests performed by researchers Iain Walker and Max Sherman at Lawrence Berkeley National Laboratory, a UL 181 listing is no guarantee that a tape will last any longer than unlisted duct tape.

Oriented Polypropylene Tape

For repairing the outer jacket of flex duct or sealing its inner core to metal collars, many contractors choose oriented polypropylene (OPP) tape. OPP tape is a film-backed (as opposed to cloth-backed) tape resembling packing tape or housewrap tape. OPP tape is typically very thin (around 3 mil), with a smooth backing and an acrylic adhesive said to be more tenacious than rubber adhesive. The backing can be manufactured in many colors, including a shiny “metallized” plastic finish. Joints sealed with OPP tape also should be clamped for best performance.
Cost of a 2-in. by 120-yd. roll: $11 to $14

Butyl Tape

Butyl tapes, which have a thick (around 17 mil), aggressive adhesive, are often used for the best-quality flexible window flashings and are a good choice for ducts, too. “The cloth-backed tapes are the ones we see shrinking and failing,” says Walker. “Butyl tapes have much more adhesive on them, so they will take longer to dry out and will stay flexible longer. In our testing, we’ve done several different orientations over the years, and we haven’t found any failures in the butyl tape.” Butyl tapes are more expensive than OPP tapes, however.
Cost of a 3-in. by 100-ft. roll: $36 to $42

Sources

Berry Plastics (OPP tape, butyl tape)
Hardcast (butyl tape)
Intertape Polymer Group (OPP tape)
Permatite (butyl tape)
Shurtape (OPP tape)

Photos: Rodney Diaz

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