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Effectiveness of Roxul “flash and batt” – irregular closed-cell spray foam causing air gaps?

Chuck Swanberg | Posted in Green Building Techniques on

I have a spray foam crew coming Thursday to convert my attic to “unvented” (2×8 rafters). They want customers to do 5″. My plan though was to do 4″ and then 3.5″ Roxul batts to fill followed by a layer of the Roxul board to reduce bridging on the rafters.

I’ve spent way too much time researching and cannot find anything decisive on this. Specifically the question is about the air gaps that would be present between the ccSPF and the Roxul. I’ve seen this question come up several times in comments here, but it’s never addressed. The closest I found was Martin referencing someone to Joe L.’s “Mind the Gap” article on the “foam sandwich”, which states that a 1/8″ gap would reduce the thermal effectiveness of the outer foam sheathing by 5%. That seems reasonable.

The “flash and batt” assembly is documented by Building Science and they don’t call out this issue as a warning. However, flash and batt is mostly documented with fiberglass which seems like it might contour to the irregular ccSPF better than Roxul.

I am nervous now because more often than not when this assembly is discussed in forums/comments someone calls out the issue of the irregular shape of ccSPF and that the batts will be less effective. For the cost of my Roxul materials I could upgrade from 4″ to 5″ and let them flash coat the exposed rafters to reduce bridging. Less R than my flash and batt, but it’s a “proven” approach. Any advice/opinions?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    With any batt job it's important to have a compression fit, and with flash'n'batt on an uneven surface that's even more the case. A continuous 1/8" gap is a thermal bypass, but isolated "bubble" spaces won't hurt, since it doesn't create a large convection path.

    It's true that low density fiberglass is more springy & compliant than rock wool, but high density fiberglass is pretty similar in that regard.

  2. GBA Editor
    Martin Holladay | | #2

    Chuck,
    Your "Mind the Gap" analogy is imperfect. The type of gap that Joe Lstiburek was talking about in his article was a ventilation / drainage gap on an exterior wall -- a type of gap that is connected with the exterior air, and therefore subject to convective loops and air exchange.

    If you do your job right, you aren't going to have air leaks in your rafter bays -- because your construction practices should be paying attention to airtightness.

    Any minor air gaps between the closed-cell spray foam and the batts that are part of a flash-and-batt job should be pockets of air that aren't connected with exterior air -- so these air pockets will not degrade the thermal performance of your assembly.

    I wouldn't worry about it.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    If you spray the roof and spray the rafters in days of old that would be all you would do. All the other work will add cost and not pay back IMO.

  4. Chuck Swanberg | | #4

    Martin/Dana, thanks. Your opinions hold more weight than the random Internet "I would never" comments I saw on this subject.

    AJ, you hit a nerve. Lazy me would like to punt the DIY project and just do the extra ccSPF. This is another one of the debated subjects that drove me crazy as I read for hours on end. "r-value myth - diminishing returns" vs. "physics - r value is r value". Martin, correct me if I'm wrong, but my perception is that the typical GBA stance on this is that you don't listen to the spray foam contractors and you get the R value at least up to code (and i assume in my case, attic, as high as possible). In my case I'm comparing 5" of ccSPF (R34.5) to 4" of ccSPF + 3.5" batts + 1.5-3" boards (R48.6-R54.6).

  5. GBA Editor
    Martin Holladay | | #5

    Chuck,
    In spite of the fact that there are diminishing returns for increasing R-values, I'm in favor of at least hitting code minimum requirements. It's possible that AJ disagrees with me on that.

    Of course, because of environmental concerns, one wouldn't want to try to achieve R-38 or R-49 with only spray foam. But I imagine you know that.

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