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Community and Q&A

Mineral Wool as a Fire Barrier

JayMart | Posted in General Questions on

Zone 5 – New Construction.

I’m thinking of a flash and batt application for my roof line to save some money versus all closed cell. The contractors I talk to are saying that if I went with all closed cell the foam needs to be painted to meet Thermal Barrier code.

Does anyone know if using a MineralWool batt could serve a substitution for this paint? This way, I save on the paint and with the flash/batt assembly?

Thank you,

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  1. Patrick_OSullivan | | #1

    Is the space accessible? If so, for what? Are you certain the space needs a thermal barrier (as opposed to an ignition barrier, which is different)?

    Mineral wool definitely suffices for the ignition barrier requirement. The irony is that some closed cell foams are also ignition barrier compliant sprayed as is, and do not need an additional coating.

    1. JayMart | | #3

      I think this is clear to me now after your question and reading ( Because this is an attic used for storage with a full set of stairs to access it I have to comply with a 15 minute thermal barrier.

      This to me sounds obvious but I'll ask to make sure. Since I have knee walls built into the attic trusses, the 15 min thermal barrier doesn't need to be applied right underneath the spray foam between the rafters, right? I would install the drywall as normal like I indicated below.

  2. Expert Member
    BILL WICHERS | | #2

    In some areas, you can leave the spray foam exposed in some areas. Check with your local building department to be sure, although I suspect your local contractors already know and that's why they're telling you the foam will need protection. Mineral wool should satisfy the fire protection requirements, but you'll need a way to keep it in place (wires, etc.). Note that if you only really need the mineral wool for fire protection and not R value, you can save a little money using the "safe'n'sound" variant instead of regular batts. Safe'n'sound still has R value, it's just a little thinner, but you get more square feet per dollar so it's cheaper for applications like this.


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