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Using polyiso foam and poly sheeting in one wall assembly

gwoloshyn | Posted in Energy Efficiency and Durability on

Hi, I’d appreciate any comments regarding my wall insulation details for my upcoming pole barn build. I’m located in Upstate NY, zone 6, and will be heating the building full time in the winter. I got my hands on some reclaimed 1″ thick polyiso foam sheeting that has a tar paper facing on both sides. For the additional R-value I wanted to install this directly behind my exterior metal panels in between the wall girts and fill the gaps with canned spray foam. Then, install 6″ fiberglass batts and finally my 6 mil plastic poly sheeting for the air barrier and interior OSB wall sheeting.

My main question is will the 1″ thick foam prevent my wall cavity from drying any condensed air that gets trapped? Or, since my interior plastic air barrier is blocking moisture from entering the wall, the wall will remain dry?

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  1. Expert Member
    Akos | | #1

    This is a common assembly here in Ontario (zone 5 an 6), no issues with it. Depending on which polyiso you have, some are vapour permeable, and can help to dry the wall to the outside. You will probably still need a WRB over the polyiso, the paper faced versions are not meant to be left exposed.

  2. user-723121 | | #2

    You may consider a variable perm membrane for your warm side air barrier to allow drying to the interior as well.

  3. Peter Yost | | #3

    Hi gwoloshyn (be great to have a real name for the GBA Q&A community) -

    Walls get wet from more than just condensation; bulk water leaks are among the most common. As important as drying potential is, water and air control layer continuity is one order of priority higher.


  4. gwoloshyn | | #4

    Thanks for the great information.

    With a water and air control layer, or WRB (if they are the same) does this refer to some sort of exterior "house wrap" to go on beneath my outside metal panels?

  5. GBA Editor
    Martin Holladay | | #5

    We still don't know your name. (I'm Martin.)

    To stop bulk water (rain), you need a protective layer under your siding. This is a combination of your WRB along with all of the flashing materials that are integrated with your WRB. (You'll need flashing at windows, doors, and all penetrations.)

    For more information, see these two articles:

    "All About Water-Resistive Barriers"

    "Where Does the Housewrap Go?"

    A common problem with pole barn walls that get insulated is an imperfect air barrier. As you insulate these walls, it's essential that you pay attention to airtightness.

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