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Rgid Foam Installed on inside of Exterior Wall

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  • Rgid Foam Installed on inside of Exterior Wall

    The house located in Michigan. It is a 2x4 framing wall with r13 faced fiberglass in exterior wall.

    1. Can I remove the drywall and add 2" rigid foam from inside, then add a 1/2" drywall.

    2. Can I add 2" rigid foam from inside and add second 2x4 framing wall with r13 fiberglass then add a 1/2" drywall (double wall with foam in middle).

    3. If this installation is ok, where the vapor retarder shoud be installed (in our area, the vapor retarder is under the drywall).

    Thanks for any suggestion.

  • #2
    Re: Rgid Foam Installed on inside of Exterior Wall

    # 1 should be fine; the foam would be the vapor retarder if sealed well at seams & edges; or vizqeen over the foam, or seal& paint the drywall to achieve the vapor retarder.
    #2 would be risky(could trap moisture between the foam and drywall)and I can't picture a reason for putting foam in the middle?

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    • #3
      Re: Rgid Foam Installed on inside of Exterior Wall

      Thanks redronh.

      The reason for considering to add second frame is to help to hold drywall, same time may add more r value with r13 fiberglass.

      Some wall in the house is 12' high, without second frame I need to use 4" screw to hold the drywall on the studs. I am not sure (and uncomfortable) if that is a good ideal to have so many 4" screw on the wall, and is it strong enough.

      again, thanks for the suggestion.

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      • #4
        Re: Rgid Foam Installed on inside of Exterior Wall

        If you're going to remove the dw, why not remove the old pp wall insulation and have the wall spray foamed? I know it's not cheap, but neither is the 2" foam, as well as all the screwing around. You snap a line about a foot or so off the floor, cut the dw at that point so you don't have to remove the base trim. Cut high enough off the floor so the tape joint isn't too close to the top of base.

        Adding 2" of foam to the inside requires all sorts of extra work at any doors and windows on the outside wall, and I don't think the end result will be as good and clean as the spray foam.

        Kindly do not say your time doesn't cost anything because you're only spending time doing a lower end job versus remedying the sins of how we used to build, which wasn't very good back then!
        Take Care

        Jim

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        • #5
          Re: Rgid Foam Installed on inside of Exterior Wall

          Originally posted by James Eggert View Post
          If you're going to remove the dw, why not remove the old pp wall insulation and have the wall spray foamed? I know it's not cheap, but neither is the 2" foam, as well as all the screwing around. You snap a line about a foot or so off the floor, cut the dw at that point so you don't have to remove the base trim. Cut high enough off the floor so the tape joint isn't too close to the top of base.

          Adding 2" of foam to the inside requires all sorts of extra work at any doors and windows on the outside wall, and I don't think the end result will be as good and clean as the spray foam.

          Kindly do not say your time doesn't cost anything because you're only spending time doing a lower end job versus remedying the sins of how we used to build, which wasn't very good back then!
          Two issues with the closed cell spray foam. 1) very expensive. 2)You still have thermal bridging with the studs.

          Seems like putting 2" of foam over the studs will be taking a big chunk out of the room though. Then if you want to hang something a little heavy on the wall, it will be frustrating to find a solid backing. Not to mention fastening your outlet boxes. Those toggle thingy boxes just don't hold very well.

          Is the thermal bridging on the studs, say 2x4, very significant, in that the spray foam will still bring a decent ROI? Otherwise, one just might have to schedule a siding job and ad the foam on the outside.

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          • #6
            Re: Rgid Foam Installed on inside of Exterior Wall

            rigid foam over the drywall,strapping,then new dry wall is an option also,no?

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            • #7
              Re: Rgid Foam Installed on inside of Exterior Wall

              Originally posted by redronh View Post
              rigid foam over the drywall,strapping,then new dry wall is an option also,no?
              I suppose it could be, if it's ok to lose 3 1/4" along every outside wall. Dealing with the outlets, switches, window jams and trim will be an issue of concern.

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              • #8
                Re: Rgid Foam Installed on inside of Exterior Wall

                There's also going to be a gap in your foam everywhere an interior wall intersects with the outside wall... and at the joist band...

                Not for nothing, that the usual approach with rigid foam is from the outside, like Steve suggested.
                Francois


                Truth is just one man's explanation for what he thinks he understands. (Walter Mosley)

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                • #9
                  Re: Rgid Foam Installed on inside of Exterior Wall

                  I like Jim's idea. The foam is a bit expensive but is the most congruent. Protection of all exposed surfaces is a must.
                  Mark Parlee
                  BESI(building envelope science institute) Envelope Inspector
                  EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
                  EDI Seminar Instructor
                  Level one thermographer (Snell)
                  www.thebuildingconsultant.com
                  www.parleebuilders.com
                  You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

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