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Community and Q&A

Tape vs. mastic

bobbomax | Posted in Energy Efficiency and Durability on

I’m posting this under both Energy Efficiency and Mechanicals, in the hope of catching more flak.
“Reputable” duct sealers all swear by mastic, but a good “duct tape”, e.g., Nashua 322 or 324a seems adequate to the task at hand. 324a has an application temp from -10 to 200ºF and a max temp of 325ºF. Even 322 is good for a max temp of 200ºF. I hope my ducts aren’t getting that hot.

Certainly one needs to be conscientious in the application of tapes, being sure to cover all the joints and pressing it down all over the joint, but mastic isn’t fool-proof either- I would argue that in all but the most experienced hands, tape is more likely to be effective than mastic, if it lasts.

AND, applying tape while squirming around under a house certainly seems easier than wrassling a tub of mastic. I’m visualizing using mastic covered gloves to drag my body through the crawl space, getting mastic on the plastic, getting mastic on my clothes, forgetting and wiping sweat off my face, adjusting my glasses, brushing my hair out of my eyes… you get the picture…

So, what does the community have to say on the subject?

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  1. jklingel | | #1

    The duct tape I used to seal in rim joists dried up and had to be replaced after about 24 yrs; no idea when it dried, but I happened to check at year 24 and it was completely toast. Crusty and pulled away from the poly, and apparently had bit the dust years earlier. That is my only experience w/ it long term, but I'd be cautious using it. The higher-end tapes are probably an entirely different animal; dunno. My gut and limited experience (just several years now) says that "mastic" like Tremco Acoustical will be there for the long haul, and it seems to "absorb" dust and chit far better than tape, so I'd lean toward mastic in cruddy conditions.

  2. GBA Editor
    Martin Holladay | | #2

    I've written an article addressing your question: Sealing Ducts: What’s Better, Tape or Mastic?

  3. Expert Member
    Dana Dorsett | | #3

    This is a classic "goop vs. tape-stickum" argument.

    I've seen plenty of 50+ year old duct mastic still doing it's job (must be the asbestos fiber reinforcement, eh? ;-) ). I've never seen 50 year old tape adhesive that still worked. YMMV.

    The state of the art of tape adhesives have probably improved, but the cleanliness of the surface would be critical for even the 3-month leakage performance let alone the 30 year or 100 year performance.

    Coefficients of expansion play a role too. Independent of the long-term flexibility and adhesion strength of the tape glue, putting aluminum FSK tape on aluminum ducts or facers on rigid foam will likely be sealing better after 50 years than aluminum FSK tape on different materials of differing COEs, at least in heating/cooling duct applications, where the temperature cycles are many, with substantial delta-Ts.

    Long term shrinkage on some rigid foam may cause both duct mastic or tapes to fail, but probably won't affect acoustic sealants. Hygric expansion/contraction cycles on OSB doesn't give me great confidence in the 100 year performance of tape-sealed OSB (not even Huber Zip) either, but fiber-reinforced mastic or acoustic sealant may still be OK in that time frame (TBD.)

    Mastic makes a FAR better hair-setting gel than any tape can muster too- it's not bad at all if you don't mind the color highlighting.

  4. bobbomax | | #4

    As far as the cleanliness, have to agree that's possibly an issue, although not usually terribly significant. Dare I say I've been going ahead with the tape...? My problem has been more unintended sticking than failure to adhere. Of course, this is very short term.

    CoTE is an interesting issue- that will stress the adhesive- steel ducts, aluminum foil tape, delta T something like 100F on a regular basis, as much as 125F on occasion.

    I really think a lot of it comes down to the long-term durability of the adhesive between the duct and the aluminum foil. Short term, I'm pretty comfortable w/ the tape. I'm gonna see what Nashua (Berry) can add to the discussion.

  5. Expert Member
    Dana Dorsett | | #5

    Heating ducts in fossil fired furnace system see multiple 50F+ cycles daily. Yes, the adhesive on better tapes tolerate it over many years. But many decades?

    Accelerated aging tests don't always conform closely to real world aging, but would be useful to see, if the manufacturers have some test data to share.

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