Monday, March 16, 2009

Duct Joining and Sealing With Foilmastic Tape

What is an insulation installer to do, if a customer has not found and hired a competent HVAC mechanic, to fix defective ducts, in advance of his work? I gamely did what had to be done, in this first trial of methods. A 16" diameter return-air duct has been re-routed, with some new, stronger ducts and elbows. Several detached joints were made stable, and were locked and sealed using Nashua Polyken 367-17 Foilmastic UL 181B-FX listed tape. GENSCO will be a good supplier of all elements of the job. No one offfered me a wrap and goop method, and I would have found that unsuitable here. I needed to progressively, experimentally and securely assemble the ducts. Wrap and goop is both too permanent and too insecure.

Foilmastic is useful for all joining and sealing of solid ducts. It is also useful as a means of flaring and stabilizing a flexible duct where it joins a solid duct. This flexible duct was not large enough to engage the steel duct. I cut 120 degree “flaps” that were drawn over the steel ducts, with good control of engagement. I taped the flaps down with a couple of full turns of 3” foil tape. A full round or two of poly-skrim-kraft tape completes the mating with insulating wrap. Several dollars of tape are involved. It is worth it.

A crawl space might incude a large number of supply-side ducts of variable diameter, with perhaps a need of detachability for access. I note the following in a 1996 report:

  • Duct mastic works, although cold and wet weather make it difficult to apply.
  • Workers don't like mastic. It ruins clothes and tools, especially when users are novices.
  • Tape-applied mastic seals well. These are the aluminum tapes with 15-mil butyl backing.
  • Air handlers are leaky.

These duct elements had been bound with duct tape, which of course was dried out and detached. Joints were stable but not secure. There were no leaks more significant than that occurring at the elbow joints. Foilmastic is used to assure joining strength, and sealing was not the first objective. If simple sealing were the objective, brush-on mastic would do, and all joints should be painted. Here, near the crawl apace entrance and with relatively good head room, Foilmastic was not the only choice. Please see the next posting.

At 4/12/2015, upon seeing this as a prominent find in search of Nashua Polyken 367-17, add referral to my newer views that most steel ducts should be replaced with well-set flex ducts. 

The process employs 367-17 tape only at steel connector-fittings. All joining of flex ducts is with very excellent Nashua 557 tape, having just-right adhesiveness. Why aren't these excellent Nashua tapes in most stores, displacing and eliminating most sales of awful "duct tape." Choose black 557 Pro-Grade UL Listed Duct Tape. I doubt that where sold as silver 557 "duct tape" at Home Depot and a few other stores, that it measures up. Why do most stores insist on selling non-professional junk?


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Phil Norman said...

I return to this post by entering "Nashua 557" in the search bar. I'm wanting to link my writings here to a customer whose laundry-floor heat duct was detached for draining after his laundry sink overflowed. I was there for other reasons and didn't have that wonderful tape with me so must return. No other tape will do. Certainly not the silver awful duct tape at Home Depot and misusing name Nashua 557. Real Nashua 557 is a resilient cloth tape that never dries out. Its adhesive is perfectly tuned to permit non-damaging removal from HVAC flex duct liners and insulating jackets. To my tag-on commenters: I hope you are professional about taping and generally need that, taking ducts apart many places. I bet you do not. Are you really friends of energy conservation, wanting intelligent low-resistance ducts as I advocate? Most dirty ducts should be thrown away for intelligent replacement.

Real Nashua 557 is black, never silver. The junk will not have label UL 181B-FX. But beware that UL protection of consumers. "Film Tape" also meets UL requirement UL 181B-FX, but is not detachable, ensuring ducts will be destroyed in a take-apart. Film tape is especially a curse upon insulation jackets. Get the good tape at a distributor to HVAC mechanics if that is convenient, or just rely on, search Polyken 557-BK.

Steven Robinson said...

That being said, we strongly recommend metal dryer ducts, because they don't sag, making lint less likely to build up. Also, if a fire does start, a metal dryer vent is more likely to contain it than one made of plastic or foil.