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Interior poly in southern Ontario, Canada

spencer_true_story | Posted in Green Building Techniques on

I’m doing a full gut renovation on our 1947 house in Prince Edward County, Ontario, which is on the north shore of Lake Ontario, and I’m curious about the near ubiquitous use of interior poly.

I’ve been combing through the articles here and elsewhere about the use of interior poly sheeting, but can’t seem to find any answers about our case. Most of what I’ve read says use it only in Canada/ Alaska, but isn’t specific, particularly when our climate isn’t any different than Maine or Buffalo (climate zone 5/6 – 7306 heating degree days. We may air condition in the future as well. Is an airtight drywall approach the only solution?

The exterior wall assembly will be a double 2×4 wall with fiberglass batts, cathedral ceilings with closed cell spray foam, plywood sheathing, tyvek, rain screen, and wood lap siding.

Cost is a huge factor, but more important is that the house doesn’t rot away. For the most part, all the exterior sheathing will be removed and the only things remaining will be some of the original 2×4 framing and the roof, which is newer.

Any help would be appreciated.


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  1. Expert Member


    How far you go to avoiding interior p0ly may be governed by your appetite for conflict with your local building inspector, and the complexity of meeting code by approved alternatives. The Ontario building code allows other materials to be used as a vapour barrier, but those alternatives aren't included in your proposed double wall. The one easy fix is to use a variable perm membrane in place of the poly. Something your local trades would be familiar with installing.

    Before deciding that poly is a inappropriate in your location, it's probably worth noting that houses in Ontario don't usually face problems associated with its use - as long as the rest of the wall assembly has good drying potential to the exterior - and any double wall assembly should include a vented rain-screen.

    Whether you use poly or a membrane, I'd air-seal your new exterior sheathing, and not worry about detailing the drywall for air-tightness.

    Sounds like a lot of work ahead. Good luck!

    1. spencer_true_story | | #4

      Malcolm, Thanks for such a swift reply! I have used MemBrain before and was leaning towards that method, and I appreciate the encouragement. Outward drying should not be a problem as far as I can tell. I would also note the building inspectors here seem to be pretty friendly, and I have included them in every stage of this project so far, mostly as general council, which they seem enjoy. Thanks!

  2. GBA Editor
    Martin Holladay | | #2

    First of all, if you are insulating your cathedral ceiling with closed-cell spray foam, then you already have a vapor barrier. So that assembly certainly doesn't need interior polyethylene. (Hopefully, even a Canadian building inspector knows that much.)

    I sympathize with on on the uncertainties surrounding your walls. While I think it is highly unlikely that interior polyethylene will cause any problems for your walls, even if you install air conditioning, my advice is to install a smart vapor retarder like MemBrain instead of interior poly. The small upcharge is worth it -- if only to lessen your worries.

    1. spencer_true_story | | #5

      Thank you Martin. I have used MemBrain before and have a few rolls left over, so that may be the way to go. Would you apply it like poly in colder climates, with acoustical sealant at all the joints? My understanding is that it would be my primary air barrier and as such such be installed as neatly as possible.

      1. GBA Editor
        Martin Holladay | | #7

        There are really three separate issues here. Many homeowners and builders (especially Canadian homeowners and builders) tend to confuse these three issues.

        Issue 1. What type of vapor retarder do I need on my wall?

        Issue 2. How do I satisfy my building inspector?

        Issue 3. What type of air barrier is best for my wall?

        With any luck, you can kill two birds (Issues #1 and #2) with one stone by using MemBrain.

        That said, I don't advise builders to try to use polyethylene or MemBrain as an air barrier. For walls, I favor taped exterior sheathing.

  3. Peter Yost | | #3

    Hi Spencer -

    There is a long history and tradition of using 6-mil poly on the interior of building assemblies in Canada, with the 6-mil poly detailed as both a vapor retarder and air control layer.

    There is nearly an equally long history of using 6-mil poly on the interior of building assemblies in cold climate US and NOT doing a good job of detailing as both vapor retarder and air control layer. As a builder/remodeler in NH in the 80s, I was as guilty of this as anyone. It simply was HARD to detail the 6-mil poly as an air barrier, at least the ways that I was doing it.

    I think a lot of the uncertainty around 6-mil poly is related to this issue.


    1. spencer_true_story | | #6

      Thanks for the reply. This makes sense, poly can be incredibly frustrating to work with through all the geometry of a house. The detailed approaches using acoustical sealant at laps seems like the best approach for air control. Since I may be using MemBrain as my air barrier, would you recommend this approach?

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