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Community and Q&A

Expanding foam for sealing around flex duct through attic?

GuyintheSouth | Posted in General Questions on


I need to seal around this flex duct coming through ceiling joists and up through home made cutout above bathroom ceiling.
This is connected to a bathroom fan on the first floor and there is a lot of cold air being sucked through the square cutout around the duct.

I will also seal the fan cage perimeter inside the bathroom.
If I use silicone here the lip of the fan cage will be stuck to the ceiling and if the fan needs to be serviced then it will be difficult to remove it and will peel the ceiling paint.
I was thinking about using rope caulk there, as it will be held by the fan’s lip and will be squeezed between the lip and the ceiling.
Would that work for this or any other suggestions? (will the rope caulk get mold?)

Also, can you use a draft stopper (like those with a flap that close when air is not flowing) at the end of the flex duct to prevent air from coming through the flex duct into the bathroom? 
The fan is a strong 110cfm panasonic whisper quiet and I have recently cleaned it thoroughly and is blowing strong so I “guess” it can push the flap at the fan cage and the one at the end that I am thinking about? or is that overkill?

Objective is to air seal the bathroom fan in the bathroom. I will also need to seal 


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  1. Jon_R | | #1

    I expect that hard duct would allow a much more reliable seal. I'd avoid canned spray foam (poor qualities compared to gaskets and good caulks).

  2. Expert Member
    BILL WICHERS | | #2

    I agree with the comment about using some kind of gasket. I’d probably do something like this myself:
    Step 1: make something to go on top of the wood/drywall shown in that pic. Maybe some plywood, or some rigid foam. Cut a circle in the middle of a piece, cut the piece in half, glue the pieces such that one is on either side of the square hole and the round hole “halves” hold the duct tightly.
    Step 2: fill the void under the piece you made in step 1, and the edges where that piece meets the duct, with the “windows and doors” canned foam which is the kind that stays soft. The soft foam won’t have as much tendency to separate from the duct.

    It’s easier to seal around rigid ducting. The flex duct will move and deform as it’s pressurized/depressurized and bounces around, which will all tend to reduce any seal you make around the periphery of that duct. You definitely need a soft sealing material like the “window and door” canned foam.


  3. GuyintheSouth | | #3

    Bill, that's a great idea.

    I have a big piece of 12mm thick luxury vinyl tile that I can cut a circle in the middle the size of the duct and then use that to place on top of the cut out and sandwich the vent in between. I guess that would work too? although vinyl planks expand and contract with extreme temps especially in the attic, I may have to buy some thick foam to do the same.

    So, after I get these gasket figured you mean to say I should still fill the around the duct inside the cutout with foam to surround the duct and then foam between the gasket and the "floor" to basically glue it to the floor there? Sorry I ma trying to picture this correctly.

    Thanks a lot for your suggestions.

    1. Expert Member
      BILL WICHERS | | #4

      I’d glue the gasket to the top of the “floor”, using regular construction adhesive, which would leave the void underneath between the gasket and whatever is under the “floor”. I’d fill the void with the window/door foam and also seal around the duct/gasket edge with the same foam.

      The idea is to make the foam contact area against the duct pretty wide so that it maintains a seal when the duct moves. If you only have a small bead of foam against the duct, it will be more likely to seperate with movement due to the reduced contact area.


  4. GuyintheSouth | | #5

    Excellent! I will do that.

    I need to also seal inside the bathroom. This is the way the fan cage is mounted see pic.

    Is it a good idea to put some rope caulk between the ceiling and the lip of the fan cage to seal the cavity? I could use silicone but then to remove the fan for any maintenance will probably damage the ceiling when removed?

    How would you seal this?

    Pictures are not attching for some reason, so here is a link to the picture.

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