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
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
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.
If the ceiling is a closed cavity with no access, the drywall is the ignition/thermal barrier.
Thanks Peter, that would simplify things — unfortunately, our attic is large enough to require maintenance access per R807.1.