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6 mil Poly and Exterior Foam Insulation

james_murray | Posted in Green Building Techniques on

Polyethylene vapour barrier and exterior foam insulation?! I know, I know… trapped moisture, don’t do it. BUT… I see tons of walls built this way everywhere in Canada! Typically: drywall, 6 mil poly, 2×6 stud cavities with pink batts, cheap housewrap and ~R5 continuous exterior foam insulation.

And as has been previously mentioned on this topic, we do not see widespread failures of these in regular houses (without pools, greenhouses, etc.) “up north”.

I am in climate zone 6. My question is: if I have more than R5 continuous insulation, say R10 or R15, does this get more risky? Why don’t the R5 walls have more issues? Does it make a difference to your answer if I am taking the time to carefully install the materials myself (as opposed to a builder on a fixed budget) or are the risks inherent to the system as opposed to poor installation?

Vapour permeable membranes, better WRB’s, vapour open Comfortboard… these are all great products but with considerably higher price tags, so I am trying to dig in to the risks of the “cheap” products if used well.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    J. Murray,

    These assemblies seem to work because they have:

    - Good air-sealing and an interior vapour-barrier, both of which limit the amount of moisture that gets into the walls.
    - Thin en0ugh exterior foam that it still allows some drying to the outside.

    So to answer your questions:

    - Yes, the better you install the interior poly the safer the assembly will be.
    - And yes going with slightly thicker foam may make the assembly more risky. If you increase the foam beyond the R-5, you should go the whole way to the recommended thickness for your climate.

  2. james_murray | | #2

    Thanks for the response Malcolm. A bit of a tangent thought on "going the whole way to the recommended thickness", as I have read on here, the IRC would require a minimum of 36% of the insulation to be exterior foam to avoid condensation in CZ 6. The Canadian residential building code (NBCC Part 9, adopted with slight variations in each province) Table 9.25.5.2 would only require 16.7% in the same zone, or 23% in CZ 7.

    I have attached the code discussion of how the Canadian table was developed, if any one is interested. Is many regards, Canadian standards tend to follow American ones pretty closely, but this seems to be an exception.

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