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Community and Q&A

Supporting 5″ panels of Roxul Comfortboard

415irwin | Posted in Green Building Techniques on

I am looking for thoughts on whether Roxul comfortboard applied on the outside of a wall would need a leger to support its weight. Our tallest wall is 23 feet high and will have 5″panels of Roxul comfortboard from top to bottom. Any thoughts on needing to support the weight of all this Roxul or do you think that the furring strips would be adequate?
Also interested in any experience people might have with the detailing of water management around windows with thick “outsulation”.

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  1. GBA Editor
    Martin Holladay | | #1

    Your wall can easily support the weight of the insulation, in my opinion. The real concern is not really the weight of the insulation; it concerns the limits of the screws holding the furring strips.

    For more information on this type of calculation, see Fastening Furring Strips to a Foam-Sheathed Wall. If you remain concerned after reading that article, you can always consult an engineer.

  2. LucyF | | #2


    Patrick Walshe has a blog where he writes about building a house with Roxul Comfortboard - A Green Hearth (I can't look up the link right now). Also one blog post specifically talks about the flashing around the windows.

    We are thinking about using "Dudley Boxes" for detailing around our windows. Foam outside of the rough opening, Dudley boxes taped to the foam, then roxul abutting the foam.

    Search for Dudley boxes on GBA website.

    I hope that helps a little bit.

  3. LucyF | | #3
  4. albertrooks | | #4


    Here is a design we did based on 6" of Mineral Wool on a wood framed wall using Heco Topix Therm screws. They are made for exterior insulation applications of 4" to 12" (which is still a bit novel in North America).

    It's pretty simple to get an evaluation done by a Heco engineer. Contact us at The Small Planet Workshop if you'd like detailed help on your application.

  5. LucyF | | #5


    I was wrong about one of the details. Dudley boxes are useful for a retrofit with replacement windows. We are using wood windows with aluminum cladding so we will be detailing around the windows similar as described in the "A Green Hearth" blog.

    The main problem with Roxul as an exterior insulation is that you can't tape the weather resistant barrier to it so the windows will be "faced" with foam for the flashing details.

    We are also planning on using the Heco Topix Therm screws for fastening the Roxul, but we're only putting on 2 inches of the Roxul.

    Let us know how your project is going.

  6. 415irwin | | #6

    Thanks everyone. Lucy have you begun installing your windows? One of the tricky parts of this process is integrating the window box details (sloped sill for example) with the window. We haven't chosen a window yet. The detailing is highly dependent on the window brand. Lucy which window have you chosen?

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