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Will using air-sealing tape on the OBS sheathing joints make my new house walls unable to breathe?

Garth Cameron | Posted in General Questions on

I am building a new home in Eastern Canada. The exterior walls have OBS sheathing on them and will be covered in house wrap and then 1.5 in. Ener air breathable foam. On the inside for insulation I am putting R20 batt insulation with a Vapor barrier. Should I tape the seams on the OBS sheathing or will this make the walls to tight and not able to breath properly?

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    @garth. The experts on this site will want to know exactly where you are building. That will help them to advise you. In the meantime, you might wan to read this article:

    https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing

    It will provide some helpful information for understanding why you want to install the correct thickness of exterior foam while promoting drying to the home's interior.

  2. GBA Editor
    Martin Holladay | | #2

    Garth,
    Your question raises several points.

    One point is the one raised by Steve: Where are you? If you are relatively close to the U.S. border, you are probably in Climate Zone 6. But if you are in northern Quebec, you could easily be in Climate Zone 7. (Here is a link to a useful map: Climate Zone Map Including Canada.)

    The second point is: What is Ener-Air breathable foam? I did some research, and discovered that the product is a brand of rigid polyisocyanurate foam (from IKO) with a fiberglass facing. (Here is the link.)

    Here is the information you need:

    1. Installing air sealing tape to seal the seams of OSB sheathing is always a good idea. Air leakage is never good. If you think that "walls need to breathe" means "walls need to leak air," you are wrong. The best tapes for taping OSB seams are Siga Wigluv, Huber Zip System tape, and 3M All Weather flashing tape.

    2. If you are in Climate Zone 6, then the minimum R-value of any exterior rigid foam would be R-11.25 for 2x6 walls. So you need more than 1.5 inch of polyiso to reach that goal. In a cold climate like yours, where polyiso doesn't perform well, I would recommend at least 3 inches of polyiso or EPS foam for this purpose.

    3. If you plan to install exterior rigid foam on your walls, you can't use interior polyethylene (because walls with exterior rigid foam are designed to dry inward). Many Canadian building inspectors don't understand the building science behind this type of wall, and insist on interior polyethylene. If you meet this type of building inspector, the best way to proceed is to install a "smart" vapor retarder (for example, MemBrain) instead of interior poly. (The vapor permeance of MemBrain is variable, and this feature will protect your wall from moisture problems.)

    4. All of these principles are explained in these two articles:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    How to Design a Wall

  3. Garth Cameron | | #3

    Thanks for the info. The house is being built on PEI in Atlantic Canada
    Garth

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