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Thoughts on the Plastic Layer in this Wall Assembly

theregoesmymoney | Posted in General Questions on

Hi All,
I’ve sincerely enjoyed the many, many hours perusing this website and have learned so much.  So thank you everyone. First, some background…my home was built with old lumber. Specifically, I tore apart 11 barns at the historic Narragansett Racetrack in Rhode Island in 1978. The wood is tongue and groove southern yellow pine as well as many 4×4 and 6×6 beams. Using this wood, my house was built on a shoestring.
Fast forward, I recently removed a concrete deck on the gable end side of my house only to find out the sill plate was totally rotted (like cardboard ready to collapse). I fixed that. Moving forward, I want to strengthen and insulate the house from the exterior with plywood, then foam board (zone 5 so would be 1.5 or 2 inches) then  vinyl siding as the final exterior so I’m done with painting.  Here is my dilemma…much to my surprise, I have discovered that there is plastic on the interior wall. The wall sandwich goes like this: interior drywall–> insulation batts —>plastic on the exterior side of the batts—>exterior wood wall then vertical wood siding.

My questions:
1) Do I need to remove the plastic sheets? I really don’t want to because that would be a major interior overhaul.
2) Can I leave the plastic, remove the cheap vertical siding, add plywood and foam sheaths followed by vertical siding without worrying about moisture and rot? The wall sandwich would then be: interior drywall–>batts–>plastic sheets–>wood wall–>1.3 or 2 inch foam boards–>Tyvek (or similar) —>vinyl siding.
3) Am I asking for trouble down the road if I leave the plastic in place?
Thanks again for any help. BTW, I’m a 67 yo woman ready to retire but can’t until this is done!

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  1. Expert Member
    BILL WICHERS | | #1

    I’m assuming you’re in a heating dominated climate? If that’s the case, then this plastic is a “wrong side” vapor barrier, and may well be your problem with the rotting sill. Moisture will migrate through the insulation in the winter, then condense on the cold plastic sheet, run down and wet the sill. Then you get rot.

    I think your best option here is to remove that plastic (which is probably polyethylene sheet), but I’d wait for some more comments here before you start opening things up.

    Another potential solution would be to put up enough exterior rigid foam to bring the plastic layer up above the likely dew point in the winter, but that would still be a pretty big project.


    1. theregoesmymoney | | #4

      Hi Bill,
      Thanks for your thoughts on my issue(s). The sill plate rotted because of a failed 30 year old concrete deck that had no flashing and the plane was toward the wall rather than away so rain and snow simply went behind the deck. It was a mess. None of the other walls are rotted. If I had the energy I would probably rip out the interior walls and remove the plastic as you suggested however, I have cathedral ceilings with 16 foot walls. I envision a horror show trying to tackle that!

      1. Expert Member
        BILL WICHERS | | #6

        With all that rotting siding that will need to be replaced, you should seriously consider removing ALL of the siding, adding exterior rigid foam to address the problem with the plastic layer, then installing new siding. Pay attention to flashing as Akos mentioned too.


    2. theregoesmymoney | | #12

      Yes, I'm in Rhode Island. Very cold winters, hot and humid summers.

  2. plumb_bob | | #2

    The basic rule of poly sheet as a vapour barrier is to have the poly sheet on the warm side of the insulation. I think the best move would be to rebuild the assembly correctly. Unfortunately, this would mean a major interior renovation.

    1. theregoesmymoney | | #5

      Thanks plumb_bob. I have no idea why the plastic sheet was put on the cold side of the wall. I guess the thinking was to cut down on drafts.

  3. Expert Member
    Akos | | #3

    The simplest fix in your case is adding on the exterior rigid foam. This way the poly can be left in place.

    Provided you have the right amount of exterior rigid as per table in the link bellow your wall will work great:

    This also significantly bumps up your assembly R value, big plus for energy savings and comfort.

    P.S. Typically for this type of retrofit with existing flanged windows, the house wrap goes under the rigid insulation. Make sure you detail your window flashing properly to integrate with the WRB.

  4. GBA Editor
    Martin Holladay | | #7

    There Goes,
    Are you sure that the exterior plastic is polyethylene? Because if it is actually housewrap -- something like Tyvek or Typar -- you don't have any worries. Plastic housewrap is vapor-permeable.

    1. theregoesmymoney | | #9

      It's definitely sheet plastic and I don't know how many mils think - not something like Tyvek or Typar. The house was built in 1979 and everything was done in the cheapest way.

    2. theregoesmymoney | | #13

      Hi Martin,
      Yes, unfortunately the plastic is polyethylene and definitely not Tyvek or Typar.

  5. Expert Member
    BILL WICHERS | | #8

    Martin makes a good point! If that plastic is polyethylene, which is a vapor barrier, it is likely to be either clear (“clear” is sometimes a bit optimistic, it can sometimes look a little milky) or shiny black.

    If the material is white, and looks like a pressed tangle of threads, then it’s tyvek or one of the similar products.


    1. theregoesmymoney | | #10

      It's definitely clear - not white.

  6. Deleted | | #11


  7. theregoesmymoney | | #14

    Upon further investigation, the walls are a sandwich of: Pine boards on the interior lower half walls with drywall on the upper half; then batt insulation followed by plastic sheeting (polyethylene was used back in those days), then recycled 1/4-1/2 inch tongue and groove boards, followed by TAR PAPER then vertical T1-11 wood siding on the exterior.

    I'd like to remove the T1-11 and the tar paper and replace with rigid foam insulation, plywood, Typar (or similar) and vinyl siding. Does this seem reasonable? If so, in what order and should I still attempt to demo the inside walls to remove the plastic sheet (not looking forward to that!).

    Thank you all for your help. I'm finally ready to move forward just as the building materials have increased by probably 200%.

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