Most residential duct systems have numerous leaks that waste energy and lead to room-to-room pressure imbalances. Unfortunately, though, few building inspectors outside of California bother to enforce existing code requirements that residential duct seams be sealed with mastic or high-quality duct tape.
Most model codes, including the International Residential Code (IRC), include duct tightness provisions:
- The 2006 IRC section N1103.2.2 requires that “Ducts, air handlers, filter boxes and building cavities used as ducts shall be sealed,” while IRC section M1601.3.1 requires that “Joints of duct systems shall be made substantially airtight by means of tapes, mastics, gasketing or other approved closure systems.” Hardware-store duct tape is not an approved tape.
- Section 403.2.2 of the 2004 International Energy Conservation Code (IECC) requires that “All ducts, air handlers, filter boxes, and building cavities used as ducts shall be sealed.”
To learn how to test residential duct systems for leaks, see Duct Leakage Testing.
All about mastic
Most energy-conscious builders seal duct joints with mastic. Mastic is a gooey, non-hardening material with a consistency between mayonnaise and smooth peanut butter. Duct joints should always be secured with #8 sheet-metal screws before seams are sealed with mastic.
Sealing duct seams is messy work, so wear old clothes. The mastic is spread over duct seams with a disposable paintbrush, putty knife, or your fingers. (If you spread mastic with your fingers, wear rubber gloves.)
Gaps in ductwork or plenums that are over 1/16 or 1/8 inch wide can be sealed with mastic as long as the gap is first reinforced with fiberglass mesh tape. If you’re using mastic to seal seams in fiberglass board ductwork, use fiberglass mesh tape for all joints.
Sources of mastic
Among the distributors of AirSeal #22 mastic is AM Conservation Group.
All about duct tape
Since common hardware-store duct tape — technically known as cloth-backed rubber-adhesive duct tape — fails quickly when used on ducts, most energy-conscious builders seal duct joints with mastic. Although mastic works well on galvanized steel ductwork, it has its disadvantages: it is messy to apply and awkward to use on clamped flex duct joints.
According to section 503.3.3.4.3 of the International Energy Conservation Code (IECC), any tape used on duct board or flex duct must be labeled in accordance with UL 181A or 181B. In most regions of the U.S., however, local inspectors have little or no interest in the leakiness of residential ducts, and duct tape labels are rarely checked for UL compliance.
“I’ve been to wholesale distributors in Ohio, and I don’t see them displaying anything with UL markings,” says Mark Pulawski, former cloth tape market manager for Intertape Polymer Group, a duct tape manufacturer in West Bradenton, Florida. “When I ask them, ‘Where are your UL products?’ they say, ‘We don’t sell any of those.’ They hold up a roll of [cloth] duct tape, and they say, ‘This is what the guys use.’”
Does UL 181 duct tape perform any better?
In some areas, however, building inspectors insist that duct tapes sport a UL 181 label. Yet the UL 181B standard alone is no guarantee of long-term tape performance. “The UL 181 listing is more of a smoke-and-flame listing,” says Bob Davis, an energy consultant for Ecotope in Seattle. “The testing doesn’t have much to do with whether it will work as a duct sealant.”
At least four different types of tape have met the UL 181B standard, including some cloth-backed rubber-adhesive duct tapes, foil-backed tapes with acrylic adhesive, oriented polypropylene (OPP) tapes, and foil-backed butyl tapes. Unfortunately, according to tests performed at Lawrence Berkeley National Laboratory by Max Sherman and Iain Walker, a UL 181 listing is no guarantee that a tape will last any longer than unlisted cloth duct tape.
“In California, the duct-tape industry wanted the code to approve the use of UL 181-listed products,” says Walker. “But in our lab tests we have found that the UL 181 products fail. Just because it is UL 181 listed does not mean that it performs any better than non-UL 181 listed products. The listed tapes may be of a higher quality — the mechanical properties of the tape are better — but they are not any better in terms of longevity at high temperature. Under those conditions the UL 181 tapes failed as well as the non-UL 181 tapes.”
More important than a tape’s UL 181 label is the material category into which it falls. At least two new types of duct tape — butyl duct tape and oriented polypropylene (or OPP) duct tape — may offer better performance than cloth duct tape, without the messiness of mastic.
Oriented polypropylene tape
For sealing the inner core of flex duct to metal collars, as well as to repair the outer jacket of flex duct, many contractors have begun using oriented polypropylene (OPP) tape. OPP tape is a film-backed (as opposed to cloth-backed) tape resembling packing tape. (Housewrap tape is a type of OPP tape.) The tape has a smooth backing and an acrylic adhesive, said to be more tenacious than rubber adhesive. The backing can be manufactured in a variety of colors, including a shiny “metallized” plastic finish. “In new construction in California, we’re seeing more and more contractors using OPP tape and a clamp to seal the core of the flex duct to a metal collar,” says Walker.
At least three manufacturers make UL 181B-FX listed polypropylene duct tape: Intertape Polymer Group (manufacturer of AC698 tape), Shurtape Technologies (manufacturer of DC-181 tape), and Berry Plastics (manufacturer of FlexFix tape).
Manufacturers of OPP tape take pains to distinguish their product from the gray stuff. Although their DC-181 is a tape designed for use on ducts, Mark Hooks, a product manager at Shurtape Technologies, insists that “it’s not a duct tape.”
As long as joints sealed with OPP tape are clamped, it will probably perform better than cloth duct tape.
Butyl duct tape
Foil-backed butyl tape performs much better than cloth duct tape, although it isn’t cheap.
Foil-Grip 1402 consists of 12 mils of butyl adhesive (similar to the adhesive used in some flexible window flashings) with a 2-mil aluminum-foil top layer. Hardcast recommends Foil-Grip butyl tape for use with galvanized steel duct, duct board, or flex duct. The manufacturer claims that Foil-Grip 1402 is rugged enough to use outdoors or below grade.
Berry Plastics sells a butyl-adhesive duct tape under two different brands (Nashua and Polyken). Nashua 558CA is basically the same product as Polyken 558CA. The tape consists of a butyl adhesive on a polyethylene-coated cloth backing; it has a UL 181 BFX listing for use with flex duct.
Like OPP tape manufacturers, butyl tape manufacturers want to differentiate their product from duct tape. “We don’t like to use the word ‘tape,’” says David Barnes, a technical service representative at Hardcast. “We’re trying to overcome all of the perceptions associated with duct tape.”
Butyl tapes have fared well in the Lawrence Berkeley tests. “The butyl tapes come with a metal foil backing as opposed to the cloth backing,” says Walker. “The cloth-backed tapes are the ones we see shrinking and failing. The 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.”
Choosing between tape and mastic
Since all duct-sealing products, including mastic and all types of duct tape, have disadvantages, deciding on the best duct-sealing strategy is tricky. Chuck Murray, an Energy Specialist with the Washington State University Energy Program, sees no reason to abandon mastic. “I haven’t seen a tape yet that I like for use in a crawl space,” says Murray. “But we continue to monitor the situation.”
One of mastic’s chief advantages is that, unlike some tapes, it performs well without clamping. Yet mastic will not prevent a joint from opening up. “Mastic is not a mechanical fastener — you still need sheet-metal screws, and scrap metal or fiberglass drywall mesh for big holes,” notes Davis. “You need to be sure that everything will hold together on its own merits. But, unlike with tapes, you don’t have to worry about whether the surfaces are clean.”
Most installers don’t bother to clean their joints before applying a sealant, and Davis feels that mastic holds up better under the circumstances. Yet mastic manufacturers, like duct tape manufacturers, generally require joints to be clean. “You need to clean the joint with soap and water and a rag,” says David Barnes from Hardcast. “The same surface prep is required no matter which sealing system is used.”
High quality duct tape — not mastic — should be used to seal holes in a furnace or air handler. As energy expert Bruce Harley notes, “Mastic would render the cabinet unserviceable.”
Making good duct joints
Here are some tips for creating durable, airtight duct seams:
- Duct seams need to be mechanically fastened (using sheet-metal screws for galvanized ducts and compression straps for flex duct) before being sealed.
- To secure seams in round galvanized ducts up to 12 inches in diameter, use at least three #8 screws per joint. To secure ducts over 12 inches in diameter, use five screws per joint.
- For securing joints in rectangular galvanized duct, use at least one screw per side.
- In most locations, mastic is preferable to tape.
- Mastic is messy, so wear old clothes when you apply it.
- Install mastic “as thick as a nickel.”
- Cracks or seams wider than 1/16 or 1/8 inch need to be repaired with fiberglass mesh as well as mastic.
- Don’t forget to seal collar connections between plenums and duct take-offs.
Sealing joints in flex duct
Flex duct sections are usually connected with a beaded metal sleeve or coupling. Here’s the procedure for sealing flex-duct connections:
- The duct boot or coupling should be inserted at least 2 inches into the end of the duct. The fitting should be attached to the inner sleeve of the flex duct with a drawband (clamp) or #8 screws.
- Seal the joint between the inner section of duct and the fitting with high-quality duct tape or mastic.
- Seal the exterior vapor-barrier sleeve with a drawband and tape.
Last week’s blog: “Air-Sealing Tapes and Gaskets.”