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Flash-and-Batt Roof Insulation as Ignition Barrier

don_lorenzo | Posted in General Questions on

Do ordinary fiberglass batts in a flash-and-batt roof qualify as an ignition barrier?

More specifically: Building a new house in WA state, outside Seattle. I have a limited-access unvented attic with a flash-and-batt roof above and a drywall ceiling below. The roof insulation is closed-cell spray foam and 10″ fiberglass batts covering the foam. Batts will likely be fastened (not friction-fit). I think an ignition barrier is required, per 2018 IRC R316.5.3. If the batts qualify as an ignition barrier, the requirement is already met, so obviously that’s my preferred outcome.

I suspect that this question has been answered but I can’t find anything definite. Martin’s barrier article mentions blown-in fiberglass as an ignition barrier option, but not batts. Dana has some posts suggesting that 1.5″ of fiberglass may function as an ignition barrier, but questioning whether friction-fit batts would qualify. Is there anything conclusive on this?

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    I know mineral wool will qualify, and you can use the slightly thinner "Safe'n Sound" version to do it to save a little money (compared to the full-depth batts). Mineral wool is used as a fire break commercially all the time.

    I don't know if fiberglass ever qualifies. Fiberglass will be harder to keep in place compared with mineral wool too.

    Bill

    1. don_lorenzo | | #4

      Thanks Bill. My understanding is that Safe 'n Sound has no certified R-value, so we'd still need the fiberglass (or something else) to insulate the roof. I'm hoping that the FG batts can do double duty.

  2. Expert Member
    PETER Engle | | #2

    If the ceiling is a closed cavity with no access, the drywall is the ignition/thermal barrier.

    1. don_lorenzo | | #3

      Thanks Peter, that would simplify things — unfortunately, our attic is large enough to require maintenance access per R807.1.

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