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Polyiso and ZIP

marnichol | Posted in Energy Efficiency and Durability on

We are looking to build next year, and have to build with a certain builder, wife wants a certain subdivision. We are in Prince Edward Island, Can. on the border of Cold/Very Cold zones probably around 4500 HDD. They will build with 1″ foam on exterior. I’m looking to maybe consider ZIP sheathing with hopefully 3″ polyiso (1.5″ over 1.5″) over top then blown-in in 2×6 wall. Will the ZIP system plus ridgid foam allow me to not have interior vapor barrier. Any ideas to help steer this ship.
Thanks in advance, Marty

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Replies

  1. mfredericks | | #1

    Hi Marty, the builders standard 1" of exterior foam is risky, so you're right to want to specify thicker insulation.
    If you install 3" of polyiso you wouldn't want an interior vapor barrier like 6mil poly. This would create a wall assembly that had no drying path either inwards or outwards because the rigid foam also acts as a vapor barrier. If you install air tight drywall and use latex paint, this might allow your building inspector to skip the interior vapor barrier, as painted drywall is considered a vapor retarder.

    If you haven't already seen these, here are 2 important articles:
    https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
    https://www.greenbuildingadvisor.com/blogs/dept/musings/cold-climates-r-5-foam-beats-r-6

  2. marnichol | | #2

    Thanks again, I think I will have to really try and go with my first wants and build using the REMOTE wall, but go with XPS. Hopefully I will be able to get the contractor on my side, since he father builds Passive style homes. LOL

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