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Community and Q&A

Spray foam over XPS on external wall

Jeff M | Posted in Energy Efficiency and Durability on

I purchased about 600 bdf of spray foam, and had planned on using it for my rim joists of my baasement (about 200 board foot of it). My house is a walk-out, so I have a combination of poured concrete walls and above-grade 2×6 stud-framed wall in the basement level.

I have sheets of 1″ XPS available, so I was thinking about layering the XPS rigid foam sheets on my 2×6 walls, at 2″ thick (possibly 3″) and then covering that with a 1″ layer of spray foam to air-seal it.

I was wondering if there were any issues with applying spray foam on top of XPS on an external wall. Since there would no longer be fiberglass in that wall, would I have to worry about consensation or mold issues?

Thanks for any feedback!

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  1. Richard Beyer | | #1

    Tough question since no one here knows if you already have water and mold issues. My comment is assuming your installing these products over your interior exposed walls. If you do you first need to manage the water or moisture problem. The foam itself will not cause mold, but the foam has to be covered by a fire retardant barrier per code. This means 1/2" drywall or intumescent paint applied over the foam at the manufacturers mil thickness. That is unless the area is always unoccupied and no mechanicals are in the space. By the sound of it you need to follow code.


    Also keep in mind... VENTILATION... See...

  2. GBA Editor
    Martin Holladay | | #2

    The method you describe is (informally) called the peanut-brittle variation of the cut-and-cobble method. For more information on this method of insulating walls, see Cut-and-Cobble Insulation.

    As long as you pay attention to airtightness, your method will work fine.

    I have no idea why Richard raised the question of mold, since there is no reason to believe (from your question) that you have a mold problem.

  3. Richard Beyer | | #3

    Martin, I'm not the one who raised the question.

    "Since there would no longer be fiberglass in that wall, would I have to worry about consensation or mold issues?"

    I addressed the concern. ;)

  4. Jeff M | | #4

    Thanks for your responses. I don't think I phrased my questions in a clear way, but my primary question was "would this work" and my secondary question was "would this lead to problems (like condensation... thus leading to mold)".

    Since this is a fairly new construction (5y old), there aren't any mold or moisture issues right now. I'm working to finish my basement and would like to improve on the R19 fiberglass batts that the walls currently have.

    I appreciate the link Martin, the blog post was very informative. I was worried that this was a problemmatic approach, because I've seen XPS+spray foam recommended in rim joists (as an alternative to spray foam) and I've seen recommend spraying foam over XPS against a concrete wall, but in the context of an above-grade wall, I haven't come across any information yet.

    It sounds like I won't be introducing many problems, but will still be suffering from thermal bridging, just as I was with the R19 batts. I guess I will continue with the peanut-brittle appraoch for now and look into exterior insulation down the road.

    Thanks again!

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