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Community and Q&A

Rigid foam over exterior stucco?

_matt_p | Posted in General Questions on

I would like add insulation to the walls of an addition to our mostly brick house in Climate Zone 4a. As best as I can tell, the walls in the later addition to the house have stucco on the exterior, I have no idea about insulation between studs and water barriers in others layers. Because of built in bookcases on the inside of the walls it would be hard to add insulation in the interior, so I was thinking of adding exterior insulation with rigid foam, but I am concerned about a moisture trap if I were to add that straight as an extra layer over the existing stucco on the outside. What are my options in this case? Would it be okay to simply leave an inch of gap between the existing exterior wall and the rigid foam for the moisture to “escape” to? Is this okay? Can I treat this assembly like adding exterior rigid foam over a brick wall, for which there are many good articles on this amazing site? I apologize if this question is ill posed and I appreciate your advice.

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  1. iLikeDirt | | #1

    You're fine, and you're right, it's just like adding stucco over a brick veneer wall. I did this two years ago on my own house with no problems. The existing stucco is not moisture sensitive, and it is either vapor-permeable (lets water out if water gets through the foam) or a vapor barrier (if painted; acts as a kind of additional WRB in this case). Proceed with confidence. And for some extra thermal punch, use foil-faced foam or mineral wool to create a radiant barrier under the new cladding in conjunction with the air gap on the ventilated rainscreen you're going to build between the new insulation and the new cladding.

  2. GBA Editor
    Martin Holladay | | #2

    Nate is right; you have no reason to worry, especially in your climate zone.

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