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Community and Q&A

Mineral wool over open-cell spray foam

Chad_T | Posted in Energy Efficiency and Durability on

I’m finishing up a 1980sqft single story house. I have 5.5″ of OS spray foam on the roof deck (conditioned attic) that needs to be covered for fire code reasons.
My options are:
Drywall  under $500
Sprayed on thermal barrier $2120
R15 mineral wool   $2000
I’m leaning to the mineral wool over spray foam. Is this a smart/safe assembly?
Will water vapor be a problem in the small gap between the foam and the mineral wool
I’m in zone 4A central NC

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  1. Expert Member
    Dana Dorsett | | #1

    Rigid rock wool at 1.5" is recognized as a thermal barrier for foam, I suspect 3.5" batts would be as well but can't point to a document for that.

    With or without voids/gaps between the rock wool and foam, open cell foam and rock wool are both vapor permeable, and potentially risky to the roof deck if installed without an interior side class-II vapor retarder or "smart" vapor retarder. Installing 2x4s perpendicular to the rafters and installing the R15s between them would reduce thermal briding, and give something to hang gypsum board onto painted with vapor barrier latex (about 0.5 perms, a class-II vapor barrier)

  2. GBA Editor
    Martin Holladay | | #2

    Your description of what is "over" and what is "under" is a little confusing. I'm going to assume that the open-cell spray foam is under the roof sheathing, and that you are planning to install mineral wool under the cured spray foam.

    I agree with Dana Dorsett that this type of conditioned attic may have moisture problems. From a moisture perspective, drywall with taped seams and vapor-retarder paint would help. So would the installation in your attic of a supply register connected to a forced-air heating and cooling system. For more information, see "High Humidity in Unvented Conditioned Attics."

  3. Chad_T | | #3

    Martin and Dana, let me just first say both of you are a huge asset to the building community.
    Thanks and yes, I'm now a subscriber.
    Martin, your correct mineral wool under OC spray foam.

    I should have added that I have attic room trusses. My concern is if I apply drywall with vapor barrier to the designed "room" I have essentially created 3 different rooms and only one will have a supply register. I assume since I have air circulation in the attic "room" its relatively safe. What is best practice in this situation? Also what happens in the spaces towards the eaves?

  4. user-456031 | | #4

    Hi Chad - I'm looking to do the exact same thing as you describe in your question. We are bringing the attic into the conditioned envelope of the building as my ducts and equipment are in the attic and no way to pull them out of there. Haven't done it yet but we are planning to have open cell foam sprayed between our 2x6 rafters. This will only get me to R20 so in addition to using mineral wool for a thermal barrier, I'm also looking to add some additional R-value to get my assembly up to R-33 whole wall (u-factor of 0.03).

    What did you wind up doing and how do you feel it turned out?


  5. Chad_T | | #5

    I put 2inchs of polyiso insulation on the roof deck, in North Carolina I could have got away with less but found used polyiso cheap on Craigslist. My only complaint is the spray foam or the iridescent coating is dusty. In the attic I'll wipe a surface clean and it dusty again in a week.

  6. user-456031 | | #6

    My preference is to put some insulation over the roof deck but the roof was replaced by the previous owners within the last 3 years so that's not going to happen for a while. I think I'll give the continuous layer of mineral wool boards (something like the Rockwool ComfortBoard product) a try.

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