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Foil-faced polystyrene and WRB

Robert Brown | Posted in Energy Efficiency and Durability on

I believe it’s suggested that the WRB be placed as close to window plane as possible. With the shrinkage problem associated with rigid foam , a house wrap may be a better choice. So…if you use foil faced polystyrene then cover it with a house wrap…..then have a rain screen space…..doesn’t the wrap negate any benefit from the foil facing.

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure that I understand your question.

    The foil facing on the exterior side of foil-faced rigid foam (usually polyisocyanurate, although foil-faced EPS is also available) serves many purposes. It protects the foam from abuse during installation; it provides a surface that is easy to tape; and (in the case of foil-faced polyiso) it limits the escape of trapped gases (blowing agents). Adding a plastic housewrap on the exterior side of the foil facing doesn't "negate any benefit from the foil facing."

    The rigid foam and the plastic housewrap serve different functions. The rigid foam is an insulation layer; the housewrap is a water-resistive barrier (WRB).

  2. Robert Brown | | #2

    Thanks Martin. I thought the foil face was attached to act as a radiant barrier.

  3. GBA Editor
    Martin Holladay | | #3

    It's possible to use the foil facing as a radiant barrier -- and you are right that you won't get any thermal benefit from a radiant barrier unless it faces an air space.

    If that's what you want to do, you can either (a) install the housewrap on the interior side of the rigid foam, or (b) use the foil-faced rigid foam as a WRB. For more information on these issues, see:

    Where Does the Housewrap Go?

    Using Rigid Foam As a Water-Resistive Barrier

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