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Community and Q&A

Is this a good wall?

Andrew Snowdon | Posted in Energy Efficiency and Durability on

Can I use taped 1-2″ exterior XPS as my sheathing and drainage plane (with no OSB sheathing, racking taken care of with other bracing methods) and full cavity high density spray foam (2×6)?

In zone 6/7 where I am (cold) my exterior sheathing should be very thick (3″+) assuming OSB sheathing to avoid condensation issues on the interior surface of the OSB. BUT if the OSB is left off as planned and the spray foam and XPS essentially become one unit (really 6 or 7″ of total foam) am I OK on the condensation issue?

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Andrew,
    Q. "Am I OK on the condensation issue?"

    A. Yes.

    Note that the code requires the use of a water-resistant barrier (WRB). If you plan to use XPS as your WRB, note that not all brands of XPS have been approved for use as a WRB. Moreover, you have to follow the manufacturer's recommendations for seam sealing and flashing when using XPS this way.

    Here's more information: Using Rigid Foam As a Water-Resistive Barrier.

  2. J Chesnut | | #2

    Andrew,
    You should probably talk with a spray foam applicator and see if they are comfortable with your suggested wall. I've never seen spray foam sprayed up against anything other than OSB/plywood wall sheathing.

  3. Torsten Hansen | | #3

    Andrew,
    We have sprayed a lot of foam on extruded polystyrene and have never had a problem. Some of these boards come with a thin plastic film, which I would avoid if possible. The spray foam will have better adhesion with the naked polystyrene. That aside, if you spray 5 inches of closed cell foam, there is no way you will have condensation issues. The spray foam will be your vapor barrier.

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