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Foamular over kraft faced fiberglass

sctrev | Posted in Green Building Techniques on
I have an existing 2 x 4 wall with kraft paper faced fiberglass insulation (no drywall has been installed yet) and would like to add a layer of foamular rigid insulation to the interior side of wall (On top of studs – not between studs) to add r value and reduce thermal bridging. I am concerned about the double vapor retarder ( but in this case, the vapor retarders would be immediately on top of (Touching) each other (ie, kraft paper touching foamular)) but wanted to know whether others think this would be okay. Please let me know your thoughts. Zone 4 Philadelphia area. Thanks.

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Replies

  1. Jon R | | #1

    Use unfaced EPS - it is less of a vapor retarder and better for the environment.

  2. Expert Member
    Zephyr7 | | #2

    EPS is also generally cheaper than XPS, but you’ll probably want to use the type-II EPS since it holds together better and won’t squish as much when you tighten up drywall screws over it.

    There is no issue with the Kraft paper touching the rigid foam. The condensation issues are all due to temperature differences and drying ability, not physical contact. Do you know if your wall can dry to the exterior at all?

    Bill

  3. sctrev | | #3

    I am not sure what you mean by whether my “wall can dry to the exterior. ” My concern is that the combination of the vapor retarder on the kraft paper and the vapor retarder on the rigid insulation is too much.

    Should I be concerned about this?

    1. Bradley Weingartner | | #5

      No, you shouldn't worry. You aren't creating a sandwich where you have two vapor barriers with a bunch of wood in the middle.

      However, I would advise you to take a step back and look at the bigger picture as I can see the path these sorts of things go. If your long term plans may include an exterior retrofit, I'd pass on the foam now as it will only complicate issues later.

      A good compromise would be to use foam only as furring strip to reduce thermal bridging without impacting the vapor perm.

  4. GBA Editor
    Brian Pontolilo | | #4

    Hi Sctrev.

    What is on the exterior of your walls for sheathing and siding? Is there a rainscreen assembly?

    The short answer is that you probably don't need to be concerned about adding the Foamular insulation. However, what Jon and Bill are offering are two good points:

    Firsts, walls need to be able to dry. So the more you slow inward drying with an additional vapor retarder, the more important it is that the wall can dry to the exterior. If, say, you had foil-faced exterior rigid foam on your walls, you may want to rethink the additional interior insulation.

    Also, XPS is one of the more environmentally-damaging types of rigid foam.

    EPS foam is both more permeable and more environmentally-friendly, so it would be a good choice that addresses both concerns.

    I hope that helps to clarify what Jon and Bill are offering. Again, they are good points to consider. You may find this article helpful to understand walls better: The Four Control Layers of a Wall.

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