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Attic – Exposed Mineral Wool

user-7653783 | Posted in Green Building Techniques on

Hi

i am in the middle of insulating my attic and have 3” of closed cell insulation on the underside of the roof sheathing. My plan was to install 3 inches of mineral wool on top of the spray foam and leave it exposed to the attic interior to act as an ignition barrier and add to the r value. 

My question is does the exposed mineral wool add to the r value of the assembly if there is no air barrier between the mineral wool and the attic interior?

Or do I need to add drywall or another air impermeable material for the mineral wool to add to the assembly’s r value? If so any inexpensive material suggestions? 

thank you

Greg

 

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Replies

  1. Expert Member
    Zephyr7 | | #1

    Convection currents within the insulation aren’t as much of a problem with mineral wool as they are with fiberglass due to mineral wools higher density. Ideally, you still want some kind of air barrier for best insulation performance. In your particular case, and partly because you’re using mineral wool, I probably wouldn’t worry too much about it. Since you want to use the mineral wool as ignition barrier, you need to make sure anything you cover with it preserves this function. That means you’d probably be limited to drywall or steel sheet.

    Bill

    1. Jay M | | #5

      I had planned on doing something similar here but for purely r-value reasons. Are you saying that adding the mineral wool on top of the closed cell will not only improve the r-value of the whole assembly but also provide a substitute to the intumescent paint that I was told I needed over the foam if I left my attic unfinished?

      1. Expert Member
        Michael Maines | | #6

        Jay, yes, if you meet the requirements in this section of the IRC you can use1 1/2" mineral wool as an ignition barrier in place of a thermal barrier: https://codes.iccsafe.org/content/IRC2018/chapter-3-building-planning#IRC2018_Pt03_Ch03_SecR316.5.3

      2. Patrick OSullivan | | #7

        Just a helpful note for others who might come across this...

        Some foams (Demilec HFO High Lift being one) have additional testing and certifications to be used *in certain areas* without the prescriptive ignition barrier and without an intumescent coating. You have to read the code, manufacturer technical documents, and sometimes documents with fun names like "AC377 Appendix X", but there's some interesting information out there.

        However, like marriage, sometimes you have to ask yourself "would I rather be right or happy?" when it comes to your relationship with your code enforcement official. :-)

  2. Expert Member
    Malcolm Taylor | | #2

    Greg,

    An interior air-barrier might be beneficial for other reasons, but not improving R-value. Mineral wool is hardly affected by wind-washing, and you don't have any wind up there anyway.

    See table on page 56: https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf

  3. user-7653783 | | #3

    Thank you!

  4. 1910duplex | | #4

    Hi Greg, I did this project, with 2 inches closed cell foam and 5.5 inches of rockwool, which also meant we had to fur out the rafters with polyiso strips.

    Just for your info, 5.5 inches is needed to be a thermal barrier rather than an ignition barrier. We felt that would be safer, since our attic is used for storage. Plus you get more R-value that way!
    https://www.rockwool.com/siteassets/o2-rockwool/documentation/technical-bulletins/residential/thermal-barrier-technical-bulletin.pdf

    We didn't put membrain or anything over the rockwool in the slanted parts of the roof, but put .75 inch drywall over the eave walls, that had foam but no rockwool.

    Mara

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