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Should Service cavity be vapor open to interior

smokey059 | Posted in General Questions on

I built a 2×4 wall to the interior of a 2×6 exterior wall.  I wanted the interior wall as a  way to break  sound transmission and as a service cavity to run all my  cables as the room will be a media/ living room. I live in zone 6a. The exterior wall has a layer of foil faced  2″ polyiso on the interior face of the studs and roxoul in the bays then a gap and then the 2×4 interior wall. I want to put fiberglass insulation in the service cavity bays (sound experts recommend) and then 2 layers of 5/8 ” drywall. The room is 400 sqft. 2 walls to the exterior. Now my question is since the polyiso is a vapor barrier and will stop vapor moving outward will I want the drywall on the interior of the service cavity wall to be vapor open as to allow moisture to dry to the interior since the mosture can’t move outward thru the foil faced polyiso.   I know the drywall will let any vapor in the cavity  dry inward but what kind of paint allows that? Or doesn’t it really matter if the drywall paint is also vapor retarder. My thought is I don’t want to trap moisture in the service cavity.  Air conditioner is only used here maybe 40 -50 days a year mostly for humidity control not temperature control. Any advice greatly appreciated.

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  1. Expert Member
    BILL WICHERS | | #1

    You want that interior wall to be vapor open. Unless you go really overboard with paint, it should stay vapor open enough to be OK.

    Note that by adding additional insulation in your 2x4 wall, you're changing the R ratio of the interior batts to the exterior rigid foam. This will result in colder exterior sheathing in the winter, and more potential for moisture accumulation within the wall. My first thought here is that a layer of vapor retarder (MemBrain, etc.) on the interior side of the exterior 2x6 wall, facing the gap between the 2x6 and 2x4 walls, might help here. This vapor retarder would help to limit moisture getting into the colder, exterior 2x6 wall. I haven't ever tried this, and will be interested if anyone else here has some ideas.


  2. smokey059 | | #2

    Maybe I wasn't clear enough in my post but the foil faced polyiso is on the interior of the exterior 2x6 wall. Since the foil face is a vapor barrier i assumed this should stop any moisture from the interior moving outward into the exterior wall assembly , but with the 2x4 service cavity wall also to the interior of the foil face I was wondering where any mosture would go once it passed thru the interior drywall on the 2x4 service cavity wall.
    If it can't move outwards because of the foil face polyiso vapor barrier and it can't come back inwards because of the drywall paint and primer it seems it would be trapped in the service cavity.
    If the paint and drywall primer won't trap any mosture that gets into the service cavity from moving back inwards my thought was the service cavity can dry to the interior. What type of paint would be best for that to happen? Thanks

    1. Expert Member
      BILL WICHERS | | #3

      Sorry, my default mode of thinking is "EXTERIOR rigid foam" and I completely missed that you mentioned your polyiso is on the INTERIOR side of the EXTERIOR wall :-)

      In your case, you would want a vapor open exterior siding or rain screen (rain screen is best), and you want vapor open on the inside too. You need drying towards both sides because you have a vapor barrier in the middle. The more "dangerous in winter" side is the interior wall, but with all that insulation, you shouldn't really have any issues with condensation on the polyiso in the middle of the sandwich. The exterior side would be an issue in the summer, but in your climate zone I'd expect that to be low risk too as long as you don't have a vapor barrier on the exterior side of that 2x6 wall.

      I wouldn't worry too much about paint and primer. Normal paint and primer doesn't create a vapor barrier. I would avoid vinyl wall paper though. I think you have a pretty safe assembly here. Do take care to air seal both the 2x6 and the 2x4 walls though.


  3. smokey059 | | #4

    Thanks for the input. I'll just prime and paint and air seal the drywall and not worry about it. Thanks again

  4. smokey059 | | #5

    Thanks for the input. I'll just prime and paint and air seal the drywall and not worry about it.

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