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Leftover mineral wool shavings

CTSNicholas | Posted in Green Products and Materials on

I have about 150 1.5″ thick pieces of 16″ x 47″ mineral wool insulation. From a prior post I mentioned I was going to try adding it to my attic as I began insulating it (new construction). However, I just got my 12″ thick fiberglass batts and began installing them between the trusses. The insulation is very airy..or fluffy. If I put these mineral wool scraps on top of the fiberglass running perpendicular.. I actually see the fiberglass shrink some from the weight! It compresses a good inch or two. I wonder if I am doing more harm than good. I know fiberglass needs to be loose. I know compressing it makes it lose r-value. Is it going to settle to the height anyways, or stay as fluffy as the day I install it as long as I don’t put the mineral wool on it?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nicholas,
    Compressing fiberglass batts reduces the R-value of the batt but increases the R-value per inch.

    Ideally, any insulation installed on top of the fiberglass batts would be a blown-in product like cellulose or blown-in fiberglass. What you want is a continuous layer that doesn't allow you to see the fiberglass batts. If you can do that, you'll end up ahead (with a higher total R-value).

    If you scatter a few pieces of dense mineral wool on top, and the mineral wool does not create a uniform layer without holes and gaps, you may be doing more harm than good.

  2. CTSNicholas | | #2

    That's what I was afraid of. Should I use them in the basement instead of a rigid foam, or assume they (~150 pieces mineral wool) are only useful for sound deadening?

  3. GBA Editor
    Martin Holladay | | #3

    Nicholas,
    If you want to insulate a basement wall on the interior, mineral wool insulation (which is air-permeable) cannot be substituted for rigid foam insulation (which is air-impermeable). For more information on this topic, see How to Insulate a Basement Wall.

  4. Expert Member
    Dana Dorsett | | #4

    They can be used in the basement foundation walls, but in addition to, not in lieu of the rigid foam.

    On basement walls you need sufficient foam-R on the exterior of the fiber insulation above grade for dew point control in winter, and below grade you need the foam against the foundation for it's vapor retardency, so that you can run without interior side vapor barriers that would potentially trap ground moisture in susceptible framing.

    While you can cheat the numbers a bit, a safe rule of thumb is to use the IRC chapter 7 prescriptives for above grade walls:

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

    When installing studwalls on the interior side of the foam, put at least a half-inch of EPS or XPS under the bottom plate of the studwall as a thermal & capillary break for the bottom plate. It's not holding up the house, so the compressive strength of the foam doesn't matter in this application.

    The most useful place for the scrap batt pieces would be the above-grade portion of the foundation, where the peak temperature differences are much higher.

  5. charlie_sullivan | | #5

    Dana's solution of adding it on top of foam in the basement is a good idea.

    I also think you could lay them out in the attic, neatly butting up against each other, and get some benefit. A denser layer of insulation on the surface of fiberglass in the attic can be useful for retarding convection, as well as adding to the overall R-value. If you are compressing the fiberglass by 1.5" when you add 1.5" on top, you are not gaining much R-value, but you are gaining a bit, so I think it's better than putting them in the dumpster.

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