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Community and Q&A

Poly Vapor Retarder on Interior Walls

mk10 | Posted in General Questions on

I am doing a full rehab of a second floor bath when i demoed the drywall on the interior of the bathroom i found plastic sheeting on the interior walls on the side facing the living space if that makes sense, i think but for some reason can’t remember that on one of these walls it was on both sides.

I also think it is original to the house (at least some of it) which was built in 1968. I am in Vermont, i thought (and not even sure if it is valid anymore) that the plastic would only go on an exterior wall with the plastic on the studs on the inside behind the drywall and not on the side that the sheeting/siding is.

I see no signs of mold anywhere.

So do i need a vapor barrier on the interior walls of a bathroom? Should i remove it?

thanks for your comments!

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  1. mk10 | | #1

    Anyone have any info on this? Thanks!

  2. Expert Member


    There is no reason to have poly on the interior walls of the bathroom. You can take it out.

    1. mk10 | | #3

      Thanks for the reply, so confusing I did some more searching before your response and found (i think on this site and on some insulation company sites ) that in the bath the interior should have a vapor barrier on the inside to keep out as much moisture as possible from entering the wall cavity

      1. Expert Member
        MALCOLM TAYLOR | | #5


        If you have enough moisture in the bathroom that it can enter and cause damage to the interior bathroom walls you need to deal with that by adequate ventilation, not vapour-barriers.

        That's a different issue from providing a good vapour-barrier on the insulated exterior walls, where it is definitely necessary.

        1. mk10 | | #6

          Thanks for the reply, this makes total sense now I get it! I do have a ventilation fan in the wall as I have a cathedral ceiling and i don’t think at this point I could put one in the ceiling.

          I actually had the one in the wall plugged for the past ten years but will use it now. I would like to upgrade as I don’t think it works well enough but it has this auto open door on it and can seem to find a replacement or way to upgrade the fan without a complete replacement, any recommendations would be appreciated.

          1. Expert Member
            MALCOLM TAYLOR | | #9


            The big advantage of most Panasonic fans, like the one Steve linked to, is they are quiet, and that makes them much more likely to get used. Replacing the switch with timer helps too.

          2. mk10 | | #10

            Thanks, i definitely want to replace it. This fan has an integrated auto open/close vent which appears to seal very well, i would like to keep that part installed but can’t find a fan that will work with it. If you look in the upper right of the picture you will see the motor for the vent.

            Added pic of outside wall cap that is motorized

          3. Expert Member
            MALCOLM TAYLOR | | #11


            Finding appropriate upgrades to fit in existing fan housings is a real problem. I've started putting the whole thing in a larger air-sealed plywood box so that the housing and all the fittings are accessible and can be replaced. Not much help in your situation though...

  3. user-2310254 | | #4


    You don't want moisture infiltrating into the walls and then condensing on the back side of the drywall or the exterior sheathing. This article may be helpful:

    It's also important to have a high-quality bath fan that vents to the outside.

    1. mk10 | | #7

      Thanks for the link and the info, makes more sense now. See bath fan comment #6

  4. mk10 | | #12

    Can’t seem to reply to some of the responses, the “reply” is missing.

    Steve, i did see that one looks like it could work (it’s expensive!) i really would like to keep the motorized wall cap but appears to be integrated into the fan And I can’t find anything like it.

    I guess my concern is how much cold air will come in or does a regular non motorized damper work well.

    1. user-2310254 | | #13

      Maybe this will work:

      You could always try Panasonic technical support as well. Many of the in-ceiling fans has a built-in damper, but I don't see one in the through-wall unit.

  5. bob_swinburne | | #14

    80’s and 90’s Vermont - they were using plastic in exterior walls. ‘68 is a bit early. They probably thought it would be extra insurance. My bath fan works very well but draws cold air in under the door so it doesn’t get used much during showers. Wonder about a single unit heat exchange like the eGo by fourSevenFive ? I think Broan might make one too.

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