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  1. #1
    Join Date
    Jan 2010
    Location
    USA
    Posts
    74

    Sealing Duct Work From The Inside? Good Idea? Bad Idea?

    I have 9 flex duct lines coming off a sheet metal trunk in my attic. I started sealing it last year, but quickly lost interest I was doing it the way I thought was right, pulling the duct tape off, putting HVAC mastic on the supply box cylinder, using a zip tie to hold that in place, and then using another zip tie to hold the outer sleeve in place. I'm just finishing up spraying Great Stuff into every nook and cranny I can get to and I'm not hip on spending any more time up there than I have to.

    I have an idea motivated by laziness. However, I feel it's a good idea. What if I take the diffuser off from the ceiling and then just use some HVAC rated tape to seal the metal to the inside of the flex duct? It's 6 inches in diameter which doesn't give me a lot of room to maneuver, but if I can pull it off, would I really be losing out on a proper seal? I know I'll still need to seal the ductwork at the trunk from the outside, but will this work for the supplies?

  2. #2
    Join Date
    Jan 2013
    Location
    New Jersey
    Posts
    33
    A seal is a seal. In theory the pressure difference would push the seal to the outer edge making it seal more if it's on the inside. However the supply air at that point in A/C mode would have almost a 100% RH which could cause the tape to fail over time.

  3. #3
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,480
    I wouldn't use duct tape inside the duct. Mastic tape or brush on mastic works on the inside of duct.

  4. #4
    Join Date
    May 2011
    Location
    NC Foothills
    Posts
    73
    What they said. The tape won't hold after a while. You can use some mastic to seal it this way and. Have done this at times to insure a seal in the connection in areas that were hard to access any other way.

  5. #5
    Join Date
    Dec 2011
    Location
    Grass Lake, MI
    Posts
    254
    Nothing but mastic sealant. But what about the joint between the boot and the drywall? There is a great amount of infiltration here and it's impossible to get a good seal from the inside. I would seal the ducts and the boots from the attic side.

  6. #6
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,480
    Quote Originally Posted by jpbhvac View Post
    Nothing but mastic sealant. But what about the joint between the boot and the drywall? There is a great amount of infiltration here and it's impossible to get a good seal from the inside. I would seal the ducts and the boots from the attic side.
    mastic tape like hard cast 1402, will stick to the sheet rock and boot and stay there.
    Heating/Cooling Services Inc.
    www.andersonhvacservice.com

  7. #7
    Join Date
    Dec 2011
    Location
    Grass Lake, MI
    Posts
    254
    I like that product, but I've not had good luck on Sheetrock nor the mastic paste.

  8. #8
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Quote Originally Posted by jpbhvac View Post
    Nothing but mastic sealant. But what about the joint between the boot and the drywall? There is a great amount of infiltration here and it's impossible to get a good seal from the inside. I would seal the ducts and the boots from the attic side.
    Impossible?
    It is very easy to seal boots to the sheet rock from the inside with latex caulk...
    If more government is the answer, then it's a really stupid question.

  9. #9
    Join Date
    Dec 2011
    Location
    Grass Lake, MI
    Posts
    254
    Quote Originally Posted by mark beiser View Post
    Impossible?
    It is very easy to seal boots to the sheet rock from the inside with latex caulk...
    That does sound like the easy way, but I've never seen that approach last over the long haul. I recommend a tight seal that lasts the distance. My experience has shown mastic to fill the demand.

  10. #10
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Quote Originally Posted by jpbhvac View Post
    That does sound like the easy way, but I've never seen that approach last over the long haul. I recommend a tight seal that lasts the distance. My experience has shown mastic to fill the demand.
    There is no weather exposure, so latex calk sealing the boot to the sheet rock will basically last forever if applied halfway properly.
    They also package duct sealing mastic in calk tubes.

    I'll seal the boots from the attic if I'm having to go around to all of the boots to replace the duct, etc. anyway, otherwise they get sealed from inside when I'm replacing the registers with better ones, and/or cleaning the popcorn texture out of them.
    Sealing from inside is the only choice for most downstairs boots in 2 story homes.
    If more government is the answer, then it's a really stupid question.

  11. #11
    Join Date
    Jan 2010
    Location
    USA
    Posts
    74
    Thanks for the discussion, I've got some water based hvac mastic that I used on a couple to seal the flex line to the boot. I also have some in a caulk gun tube that I haven't tried yet. I also have great stuff cans. I thought about spraying great stuff in around the boots from the attic to seal the boot to the sheetrock. I also thought of using the caulk gun mastic from below and then using the metallic tape I have to go from the boot to the sheetrock on the bottom side. The register would then cover that up. Is Great Stuff a viable option here at least for the boot to sheetrock seal?

  12. #12
    Join Date
    Dec 2011
    Location
    Grass Lake, MI
    Posts
    254
    My concern is the off gassing

  13. #13
    Join Date
    Jun 2012
    Location
    Ahoskie NC
    Posts
    84
    it doesn't really matter wheather you seal it inside or outside just so long as it is sealed. IMO nothing beats a good old bucket of mastic and a 2" brush. It is messy but once up there and dry it will be there when the cows come home.LOL!!!

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