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Community and Q&A

Suggestions on locating 2.5in R10 Mineral Wool Insulation for Flash & Batt??

aston01 | Posted in General Questions on

I have remodeling to do on some 2×4 exterior walls and would like to replace the existing fiberglass batts with a 1″ polyiso & 2.5″ mineral wool assembly.

In my climate zone (Dallas, TX) the 1″ of polyiso is sufficient to avoid condensation and readily available.

The issue I have run into is the 2.5″ mineral wool batts. Rockwool’s website shows the main suppliers being Home Depot/Lowes which neither even show a 2.5″ R10 product listing on their website.

I could probably special order what I am looking for, but most of the listings I have seen are for 16″ wide pieces meant for use with metal studs (I have wood), so every piece would have to be trimmed.

Has anyone else that has been through this with a 2×4 wall found any better  way to go about this?

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  1. Expert Member
    BILL WICHERS | | #1

    I would just put in regular R15 mineral wool batts and a smart vapor retarder on the interior. Is there any particular reason why you want to include the polyiso layer in what I assume is a cut’n’cobble installation?


    1. aston01 | | #2

      Because it is a 1970s brick house with leaky gypsum sheathing, I was trying to stick with a happy medium alternative to pulling the brick off to reseal it from the outside or spray foaming the walls.

      I'm not a huge fan of spray foam hence the "cut'n'cobble" option.

  2. Expert Member
    Michael Maines | | #3

    Are you proposing polyiso on the outer portion of the stud bays, with mineral wool batts filling the remaining, interior portion of the stud bays?

    In Dallas, you don't have minimum exterior insulation requirements for dewpoint control, because you have so little heating required. You do have a reservoir cladding (brick = moisture-absorbing), lots of sun and no vapor retarder in your wall currently; I would be concerned about adding a vapor-closed element that will guarantee that solar vapor drive (and regular vapor drive) pushes moisture from the bricks into your wall assembly without giving it a place to go, or a continuous barrier to prevent it from causing damage. I would consider using mineral wool alone.

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