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Community and Q&A

Fire Barrier

itserich | Posted in Building Code Questions on

I am installing rigid foam and am considering fire barriers.

Based on this chart, it appears the following are currently accepted fire barriers in occupied space:

Wood Panel / Particle Board / Plywood – 15/32″
Drywall – 1/2″
Steel Base Metal – .016 (?)
Aluminum Base Metal – .032 (?)
Mineral Fiber Insulation – 2″

Thermax insulation is also apparently an approved barrier in attics and crawl spaces, but not occupied spaces, so perhaps it is not as good as the other options.

Two questions. First, is there an overview of the barriers, as to what works best at preventing the spread of fire? Are there any other options?

Second, is steel or aluminum used indoor?This will be a large area, not a small area such as only the rim joists. Drywall is heavy, don’t know the weight of the wood.


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  1. GBA Editor
    Martin Holladay | | #1

    Most building codes, including the International Residential Code (IRC), require spray foam insulation (including foam installed on the interior of basement walls) to be covered with a 15-minute thermal barrier. However, if the spray foam is located in a crawlspace or an attic “where entry is made only for service of utilities,” the code permits the installation of a less stringent covering: an ignition barrier rather than a 15-minute thermal barrier. In sections R314.5.3 and R314.5.4, the IRC defines an ignition barrier as one of six permissible materials: 1 ½-inch-thick mineral fiber insulation; ¼-inch-thick wood structural panels (e.g., plywood); 3/8-inch particleboard; ¼-inch-thick hardboard; 3/8-inch-thick gypsum board; or corrosion-resistant steel having a base metal thickness of 0.016 inch. Presumably, code officials also permit the installation of thicker versions of any of the six listed materials.

  2. Expert Member
    Dana Dorsett | | #2

    " First, is there an overview of the barriers, as to what works best at preventing the spread of fire?"

    In rough order:

    Wood will eventually ignite and sustain flame on it's own

    Gypsum with paper facers can spread fire once the paper lights off, but has low fuel content on it's own.

    Aluminum & steel have fairly high ignition temperatures so your house is already toast if they light off.

    Rock wool will still be intact long after the house has gone up in smoke- it won't melt fall apart or burn even at VERY high temperatures (temperatures well above which fiberglass would melt.)

    I'm surprised that a mere 2" of rock wool qualifies though- I had always remembered 3".

  3. itserich | | #3

    Thank you Dana and Martin.

    Metal is epxensive and hard to work with (I think). Wool sounds like a great barrier but not rigid enough to leave exposed on the interior.

    So will probably choose between Drywall Type X or wood. I have not worked with drywall but it seems to be messy.

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