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Community and Q&A

Vapour barrier confusion

AngePEP | Posted in Energy Efficiency and Durability on

Hello from Canada,

I have been searching for a few days now for an answer about the use of a vapour barrier in conjunction with 1″ rigid foam and Tyvek on the exterior of a wooden frame house with 2″x6″‘walls insulated with fibreglass batt. I am more confused than when I started now!

We had wanted to use 2″ foam but the contractor refused so now we have used 1″ rigid foam with Tyvek and wooden clapboard siding. So that’s finished and now we are dry-walling and I was wondering if we needed a polyethylene vapour barrier on the inside.

It seems that first of all, the foam is not thick enough (I would be in probably between Zone 6 and 7 according to the charts, or ‘Very Cold’). So after all the reading I’ve been doing I fear that mistakes have been made that cannot be fixed easily (not without completely redoing the foam and clapboard) so I guess that leaves me with two questions:

1) is there anything I can do to fix this?
2) do I need an interior vapour barrier and, if so, what kind is best?

Thank you kindly for your help and my apologize s for the long-winded questions.

GBA Prime

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  1. GBA Editor
    Martin Holladay | | #1

    It would have been better if the rigid foam were thick enough to keep the wall sheathing from accumulating moisture during the winter. It's too late to fix that, however.

    There are three strategies you can do to lower the risk of this risky wall:

    1. Install a "smart" vapor retarder like MemBrain between your studs and the drywall.

    2. Install the MemBrain and drywall with attention to airtightness.

    3. Keep an eye on the interior RH in winter and strive to keep the RH at 30% or below.

    For more information, see these two articles:

    "The Exterior Rigid Foam is Too Thin!"

    "Rethinking the Rules on Minimum Foam Thickness"

  2. Jon_R | | #2

    I agree with Martin. Use blower door tests and on insist on better than code numbers. Consider Intello over Membrain - expensive, but somewhat better moisture and air sealing properties.

    If it's too late for a membrane, then your only choice is class II vapor barrier paint.

  3. AngePEP | | #3

    Okay, thank you for the advice.

    How less effective would just the paint be? And is that a specification that you look for in paint or would any latex paint do? What other considerations are there for ensuring air tightness with just paint?

    Would a combination of Intello or MemBrain and latex paint work?

    Thanks again.

  4. GBA Editor
    Martin Holladay | | #4

    Vapor-retarder paint (primer) is effective. Use as directed.

    Q. "What other considerations are there for ensuring air tightness with just paint?"

    A. You can't achieve airtightness with paint. The paint is simply a vapor retarder. If you are interested in airtightness -- and you should be -- you need to seal air leaks at penetrations (windows, doors, electrical outlets, hose bibs, etc.) and test your results with a blower door.

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