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Roxul Insulation Batts

EightNineTen | Posted in General Questions on

Insulating the roof underside with 2 inch closed cell foam then putting Roxul under it.

The r30 roxul is difficult to work with.  Will 2x batts of the R15 Roxul be the same as 1 continuous r30 Roxul batt?

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  1. brandons | | #1

    First, what is your climate zone? Those reading and weighing in would find it helpful. In my opinion:

    -Yes same R-value but inevitably more voids with the installation (marginal). They fit snug but I would pay attention to all the seams.

    -Probably cheaper to do two R15 batts. Around here big box sells them at volume discounts for $.50/sq ft. The R30 batts are are closer to $1.50/sq ft

    -Absolutely cheaper to buy fiberglass batts, shop R30 batts. My local Menards has them at $.40/sq ft. I'm assuming your cavity space is 7" or so. A 10" R30 batt can be compressed into that space for a lower cost and easier install since it can be compressed to ensure a tight fit. You'll lose some R value but my opinion is that is a better economical and performance balance than the rock wool.

    Good luck!

    1. EightNineTen | | #2

      Nassau county, Long Island, New York

    2. Expert Member
      Dana Dorsett | | #5

      >"I'm assuming your cavity space is 7" or so. A 10" R30 batt can be compressed into that space for a lower cost and easier install since it can be compressed to ensure a tight fit."

      An R30 designed for 2x10s only performs at R25 @ 7.25" thickness of 2x8 framing, whereas an R30 rockwool batt is designed for 2x8s really performs at R30. Even "Cathedral ceiling" HD fiberglass is nominally 8.25", and only performs at R27 when compressed to 7.25". See:

      So yes, R30 fiberglass is cheaper, but it's also substantially lower performance, and is not an adequate thermal barrier against ignition for foam in a flash'n'batt stackup (but rock wool is.)

  2. GBA Editor
    Martin Holladay | | #3

    From an R-value perspective, two layers of R-15 are the same as one layer of R-30, as long as the batts are carefully installed.

  3. Expert Member
    Dana Dorsett | | #4

    R30s aren't tough to handle. Rock wool is a lot easier to trim than low density fiberglass. R30s can be cut/trimmed/sculpted with a 9"+ bread knife or any decently sized purpose made batt knife 9" or longer. (The Fuller Tool 320-0100 has an 11" blade, available for under $15 online, if your local suppliers don't have them. The 8" blade Grip-Rites sold at box stores would work, but are a bit on the short side for 7" batts- longer blades are easier.)

    A pair of R15s are 7.0" nominal depth, R30s are 7.25", and more likely to be a compression fit in a 2x8 cavity, though you could probably makes R15s work well enough.

    1. EightNineTen | | #7

      I do have a few questions now that the job is complete:

      1- when they completed the job, I spent about an hour reviewing everything. I inhaled the fumes obviously. How big of a problem is this?

      2- the golfball size holes, if there are just a few around, with r30 roxul under it, how big of a problem are they in terms of percentage leakage?

      BTW: We did 2.5 at the roof and 2x r15 roxul. Came out really nice from a design perspective. And performance wise, it's 9000% better than anything I've ever done for the few homes I've touched.

  4. EightNineTen | | #6

    Job is complete. Want to thank Dana for the guidance, and Martin and everyone else. It was a really awesome choice to do it the way we decided to do it. I noticed it was immediately super comfortable.

    That said, I'm not entirely happy with the outcome unfortunately because the foam company put the roxul in after the foam dried without giving us time to carefully inspect. Then he got pissy when we told his roxul installer to stop half way through. Of course, I did notice gaps, I noticed golfball size holes with wood exposed. I noticed at the end of the entire job the individual spraying wasn't even doing it right, he was zig zagging the spray and now spraying the edges first. Too late.

    I'm not going to have the roxul removed because that is senseless at this point even if there are a few spots not perfect or a few gaps/leaks.

    I am taking a FLIR today throughout and looking for big leaks I can insulate before we sheetrock.

  5. Expert Member
    Dana Dorsett | | #8

    With wood showing the thermal hit isn't going to be huge, but those spots are susceptible to moisture accumulation. Fix any spots you can find with the FLIR using FrothPak or can foam, then either install MemBrain before installing the sheet rock, or paint it with vapor barrier latex primer, either which will mitigate the risk considerably for any spots you may have missed.

    Most closed cell foam installers can install a pretty consistent 2" with no gaps in one or two arm motions. There's no taking it back, but the guy with the spray gun in his hand sounds like a raw newbie.

    1. EightNineTen | | #9


      The thing is, it's done. Done, done. 2x roxul r15 under the spray foam. So if there are holes and leaks, there will be leaks.

      What's worst case if theres some moisture accumulation?

      1. Expert Member
        Dana Dorsett | | #10

        The absolute worst case would be a rotting section around the hole, but the more likely scenario is a moldy spot, far more likely on a north facing or shaded pitch than an unshaded south facing pitch. The risk is low but real.

        The drier roof deck surrounding the holes in the foam will share that moisture burden a bit by wicking the adsorbed moisture laterally (if slowly). It's a much lower risk than if none of the roof deck had closed cell foam.

        With MemBrain or half-perm latex primer on the ceiling gypsum that low risk falls to effectively zero.

        1. EightNineTen | | #11

          Okay no biggie.

          What about the exposure to the fumes?

          1. michiganman18 | | #12

            What about the fumes? You already inhaled them. That's the thing about fumes/and the lungs. Theres not really any sort of treatment for major damage other than bronchiodialators and 02. That's why prevention is so important and you should have had a mask on.

            Fumes from foam, you should have a properly fitted respirator on to observe in a closed space.
            As the installer was hopefully wearing.

            From the roxul, dont huff it, at worse it may have a Formehldehyde binder it, but mostly it's just an irritant to some.

            This would have been a better question before the job was "done, done."
            I highly doubt an hour after spraying would cause damage away from blowing agents, but next time think about fumes in an environment.

            I would have had them take out the roxul to re go over the foam holes. Not paying spray foam prices to have holes in the wall.

  6. Deleted | | #13


  7. reesejoh | | #14

    Is it ok to compress the mineral wool into a smaller space than the depth of the wool? I am trying to reach 30 r value which is 7.25 inches thick for rockwool. My ceiling is 2x6... I would be compressing over an inch under the sheetrock. Will this negatively affect in any way?

    1. Expert Member
      BILL WICHERS | | #15

      You'll get a bit more R per inch, but a bit less overall R value. I think you'll have a very difficult time compressing the batts from 7.25" down to the ~5.5" available with a 2x6 though -- you'll probably be putting enough force on the drywall to cause problems if you try that.

      What might work better is to put an R23 mineral wool batt in the 2x6 cavities since those are a perfect fit, then use 1" polyiso behind the drywall which would give you R29 total. If you absolutely need R30 or better, using 1.5" polyiso would get you to R32, or you could use high density R21 fiberglass batts in the 2x6 cavities and that same 1.5" polyiso would get you R30 with slightly lower total materials costs.


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