Duct Joining and Sealing with Mastic

Duct sealing can be done at lowest material cost, and with assured complete air tightness, using a two-part tape method, or a simple brush-on or spray-on of mastic. This photo shows one duct element solidly installed with materials I judge to be Hardcast Two-Part: RTA adhesive, and DT 5300 mineral-gypsum-compound-impregnated fiber tape. A two-part joint has alignment strength that is comparable to that with Foilmastic tape, but stable alignment must be achieved by other means, before tape application. The method was easily applied where a small-diameter duct was at rest on blocks. Brush-on mastic has its place where duct joints are not under bending stress. Mastic covering is not suitable where a duct must be lifted for subsequent insulation wrapping.

Hardcast materials are conveniently available at GENSCO.

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.
http://energyconservationhowto.blogspot.com/search/label/Better%20HVAC%20Flexible%20Ducts 

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?