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Install interior vapor barrier if foil faced poly on ext walls?

plat | Posted in Green Building Techniques on

I removed about 5 linear feet of plaster from a half bath on exterior wall with 4×4 studs, plank boards 1906 house.  On the exterior is foil faced ½ inch polyiso board which is covered by vinyl siding.  I am installing rockwool in the stud cavity and the ceiling portion that contains a roof that sticks out 2 -3 ft from the house.  Should a vapor barrier be installed for the ceiling portion or walls? I live in eastern part of PA.

At some point in time I plan to remove the faced poly iso and vinyl siding from the house for some shingles but that will be many years in future.  Thanks for this great resource

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Plat,

    Typically, with foil faced exterior insulation, you would not want to include a class I or class II vapor retarder because the wall cannot dry to the outside. However, you have a minimal R-value with only 1/2 inch of exterior insulation and you are planning to remove it. You may consider using a smart vapor retarder like the Intello or Membrain products over the mineral wool insulation. This would give you some flexibility with your exterior material choices when you get to that part of your project. I wouldn't install interior polyethylene, though.

    Also, you might find this helpful: The Four Control Layers of a Wall

  2. Jon_R | | #2

    Here is a calculator you can use to get a robust wall. A smart-retarder is a good idea, but unfortunately the tool doesn't give it any extra credit.

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