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Community and Q&A

Using Duct Mastic for general air sealing

Curtis Dean | Posted in Energy Efficiency and Durability on

At a building conference last week, one of the sessions was on air sealing in attics. The presenter explained that duct mastic is a good option for sealing top plates and around drywall in knee walls. I had never thought of using duct mastic for this purpose, but it does seem to make sense.

Is anyone else using duct mastic for this application? What has been your experience?

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  1. Lucas Durand - 7A | | #1

    Curtis, as it happens there has been a lot of conversation lately about using tape for air sealing.
    I recently brought up mastic tape as an option in a thread of a slightly different vein:

    There is also Martin's blog on air sealing tapes and gaskets:

  2. Curtis Dean | | #2

    Actually in this case, he was spreading the mastic directly on the crack between the top plate and the drywall. No tape, although he said you could use tape if the gap were too wide to work the mastic into. He was using a brush to "paint" the mastic and force it into the gaps. It just made so much sense to me I'd like to use the technique myself (or at least advise it to my customers), but I'd like a second opinion.

  3. Robert Hronek | | #3

    I have thought of the same thing. Have wonder about durability.

    Spray applied mastics are available. We have talked about using the spray applied. Painting or smearing it on seem like a time consuming method. If spray applied were a solution a sprayer with an extension to reach the top plate along an outside wall would be easier than have to crawl out and seal with foam.

    Any chance mastic would bleed through drywall

  4. Chris Brown | | #4

    Robert, check out the Knauf Insulation website. They have a new system called the EcoSeal system. I have used it in both new construction and retrofit applications. Uses airless sprayer so wands, extensions, nozzles are readily available at paint stores. I have actually applied the product with 2 six foot extensions on the end of the gun. Makes reaching those exterior wall top plates much easier. Click on the EcoSeal tab on their website. There are a few videos to watch. The 15 minute application video is pretty informative though largely new construction oriented.

  5. John Semmelhack | | #5

    On multi-family job sites (where general contractors make the trades seal up their own holes!) I sometimes see the HVAC folks use duct mastic to seal around exhaust ducts, fresh air ducts, etc. where they penetrate the band/rim joist. When applied to the proper thickness, the mastic should hold up really well for this purpose. It's a lot better than a can of spray foam.

    The technique you describe sounds solid, with the caveat that you might need a mesh tape or backer rod if the gap is too large.

  6. Robert Hronek | | #6


    What sprayer do you use. The one recommended by Knuaf isabout $3900.

    There is another product I have thought about. I dont know the brand but it is a spray applied material that is applied to the exterior of OSB or plywood sheathing. It is meant to replace housewrap.

  7. Aaron Vander Meulen | | #7

    Robert, it sounds like you are referring to Grace Vycor EnV. Keep your eyes open on Craigslist to find a used airless pump. You can probably get away with a smaller pump for light use. I wouldn't be afraid to use our Graco ultra 395 for either, but for extended use I'd get something bigger. You also may be able to rent a pump from your local paint supply store.

  8. Chris Brown | | #8

    Robert, the machine recommended by Knauf is the Graco 795 Ultra II series and the 3900 includes hoses, nozzles, extensions tips etc. You can find a plain 795, or even a 695 machine for far less than 3900. Aaron's suggestion is also a good one. Any machine that is a piston pump (not diaphragm) and has psi capacity up to about 3000 psi will work. Brand doen't matter. Must be strong enouigh to pump elastomeric material. Lightweight latex paint sprayers won't work.

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