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Community and Q&A

Closed cell spray foam and mineral wool insulation

hal_kepler | Posted in General Questions on

when insulating a 2×4 stud wall (16″o.c.) by first spraying a thin (1/2″ plus-or-minus) coat of closed cell spay foam insulation and filling the rest with 3 1/2″ Rockwool Comfortbatt insulation, How well does that work? Do the batts compress enough to not become a problem with installing 1/2″ drywall. Also does it change the r-value rating of the Rockwool when it is compressed that 1/2″?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Hal.

    I asked the folks at Roxul about this and they told me that compressing mineral wool batts has a similar effect as compressing fiberglass batts: the R per inch actually goes up, but you have less inches so the R-value of the batt, likely will go down. Mineral wool is also more difficult to compress than fiberglass. The best choice according to many builders, for this type of assembly is a blown in insulation, which could be fiberglass, mineral wool, or cellulose. For more on flash-and-batt or flash-and-fill assemblies, read this: Flash-and-Batt Insulation.

    1. hal_kepler | | #2

      Wonderful information, Brian. Thanks so much!

  2. Expert Member
    Akos | | #3

    You can use the SafeNSound insulation. This is 3" thick and should fit without issues over 1/2" of SPF. Although the data sheet doesn't show an R value, it is similar to regular mineral wool (R4.2/inch)

    1. Expert Member
      BILL WICHERS | | #4

      The manufacturer can send a data sheet showing an R value on request in case you use the sound attenuating variant and need to pass inspection for code required R values.


  3. Expert Member
    Dana Dorsett | | #5

    >" first spraying a thin (1/2″ plus-or-minus) coat of closed cell spay foam insulation..."

    First, it's nearly impossible to spray a half-inch of closed cell foam with ANY kind of consistency. The unevenness will almost surely create void spaces at the interface with a batt.

    Second, what is the purpose of such a thing flash of foam? It's not thick enough to provide a reliable air seal, nor is it thick enough to hit class-II vapor retardency.

    1. Expert Member
      Akos | | #6

      I've done about 1/2" with one of the SPF kits. Maybe not a true air barrier but does a decent job of sealing up old 1x6 board sheathing.

      Definitely not flat though.

    2. malibru | | #7

      Actually, it's totally possible. Not easy, but I just had it done for my roof, and the reason it was done answers your second question. You put on a thin coat to allow it to harden so that when you spray on the rest, it doesn't expand out and compromise the interface, which in our case was a 24-guage steel roof. Had we just sprayed on all 3" at once, the roof would have likely blown out. By putting a thin (.5") coat on first, however, we negated that possibility.

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