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Housewrap and XPS insulation or just XPS on exterior?

FULLCIRCLEJOE | Posted in General Questions on

Hello,

I am building a small house in TN with sips and am going to add 1″ of xps foam for additional insulation. I know that the xps foam can be used as a air/vapor barrier (if properly sealed) , but would it be recommended to use both house wrap and xps? I like the idea of the extra protection, but I am also concerned about creating somewhere for moisture to be trapped. As I am building in the rainy season, it would be convenient to get the wrap up quickly to avoid getting the sips rained on and not be rushed with the insulation. If I were to use both should I seal both? (I have read about using the proper tape to seal correctly.)

Just though I would bounce this idea of the community. Thanks in advance.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joe,
    There are lots of issues wrapped up in your question.

    First of all, every wall needs a water-resistant barrier (WRB). You can used housewrap for a WRB, or you can use rigid foam as your WRB, but you have to know which layer is your WRB because you need to integrate your window and door flashing with your WRB.

    It's perfectly acceptable to have housewrap directly under rigid foam, or rigid foam directly on top of housewrap. Putting these two materials close together does not trap moisture.

    I strongly suggest that you read the following three articles to learn more about WRBs:

    All About Water-Resistive Barriers

    Using Rigid Foam As a Water-Resistive Barrier

    Where Does the Housewrap Go?

    There's a totally different question that needs to be discussed in connection with your plan: namely, is it a good idea to cover the exterior side of a SIP wall with a layer of 1-inch XPS? If I had to rule on this issue briefly, I would say no. The exterior OSB facing of SIP walls is at risk of rot, so it's always a good idea to provide a way for this OSB facing to dry quickly. The best way to ensure rapid drying is to install a ventilated rainscreen gap on the exterior side of the SIPs.

    A layer of XPS will slow down drying to the exterior, and will increase the risk of OSB rot.

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