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We are building a 700-sq.-ft. cottage on southern Vancouver Island, Zone 4 Marine

user-6030522 | Posted in Green Building Techniques on

The plan is to use 4 inches polyisocyanurate – R22.6 – (glass fiber covering) exterior insulation no sheathing except for earthquake walls. No insulation in the 2×4 wall cavities, tyvek on the exterior of the foam and vapor barrier between the polyiso and the framing. ls this ok or will we have problems. We have to present our wall assembly plans to the city and the city wants a vapor barrier but we are not sure if this is necessary.
Sandra Garraway

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  1. user-2310254 | | #1

    Sandra. This article and its embedded links will answer most of your questions.

  2. user-6030522 | | #2

    Thanks you, that article is very helpful but I am not sure of the vapour barrier placement between the studs and the polyiso in addition to the tyvek is ok.

  3. GBA Editor
    Martin Holladay | | #3

    To rephrase your description of your house in more conventional terms: You are planning to build a house with PERSIST walls, with all of the insulation on the exterior side of the studs. (For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.)

    Obviously, if you are building walls without conventional sheathing, you'll need to come up with a plan to brace your walls. (For more information on this issue, see Four Options for Shear Bracing Foam-Sheathed Walls.)

    You wrote that you are planning to install a "vapor barrier between the polyiso and the framing," and then you asked, "ls this OK or will we have problems?"

    You didn't tell us what you plan to use as a vapor barrier. In the traditional PERSIST approach, this vapor barrier consists of peel-and-stick membrane over OSB or plywood. But you don't want to install OSB or plywood, so there is nothing to attach your peel-and-stick membrane too.

    If you don't want to install OSB or plywood, you could choose a type of rigid foam that is also a vapor barrier. The easiest way to proceed would be to choose foil-faced polyiso -- a type of rigid foam with a very low vapor permeance, qualifying as a vapor barrier. That should satisfy your building inspector.

    There are a couple of other hurdles you need to face in addition to the vapor-barrier question. Here's a big one: How will you make your walls airtight? If you follow the conventional PERSIST approach, you've got an excellent air barrier (the peel-and-stick membrane attached to the OSB or plywood sheathing). In your case, you'll need to detail the polyiso in an airtight manner. That can be challenging, but it's possible.

  4. user-2310254 | | #4

    Martin. Would Sandra want to propose MemBrain to address code enforcement's desire for an interior "vapor barrier?"

  5. Expert Member

    Intermittent sheathing poses all sorts of problems. Depending on your exact location on Southern Vancouver Island, the amount of shear wall required will be either 25% or 40% of your exterior. Why not sheath the rest?

  6. Expert Member

    Sandra is fine with the placement of her VB as long as it is on the warm side of her insulation. There is no requirement for an 'interior" VB in Canadian building codes.
    Given that there will be no vapour movement through the foam there isn't any real advantage to substituting a smart membrane for poly.

  7. user-6030522 | | #7

    Thank you very much for the clarification and yes I see that sheathing the would be a much better solution and we will use 6 mil polyethylene over the sheathing if that is ok.

  8. GBA Editor
    Martin Holladay | | #8

    I'm glad you have decided to install OSB or plywood sheathing on your walls. The traditional PERSIST detail calls for the installation of peel-and-stick membrane on the exterior side of the wall sheathing, but as I noted in my article on the PERSIST technique, "Alaskan builders are experimenting with changes to the PERSIST system. To save money, some have switched from peel-and-stick membrane to 6-mil poly."

    From a building science standpoint, there is nothing wrong with using polyethylene in this location on a PERSIST wall. The only problem is that polyethylene doesn't make as good of an air barrier as the peel-and-stick membrane -- so if you use poly, pay attention to airtightness by sealing the OSB seams or the polyiso seams.

  9. user-6030522 | | #9

    Thank you very much Martin all is very clear now. Cost is a factor so we sill use the 6 mil poly and we will be very careful to seal all seams well.

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