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Community and Q&A

Crawlspace foam board and fire code?

DennisDipswitch | Posted in Building Code Questions on

When installing rigid foam in an unfinished crawlspace, what is needed to have it comply with the fire code? Foam board to be installed on inside of foundation wall and on the underside of joists?

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  1. GBA Editor
    Martin Holladay | | #1

    The answer to your question varies, depending on your local building code and the way that your local inspector interprets the code. Contact your local building department.

    Most building codes, including the International Residential Code (IRC), require foam insulation (including foam installed on the interior of basement walls) to be covered with a 15-minute thermal barrier. However, if the foam is located in a crawl space 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.

    Thermax brand polyisocyanurate foam has passed certain fire ignition tests and is therefore approved by many building departments for installation in crawl spaces without any addition protection.

  2. DennisDipswitch | | #2

    Thanks Martin,I just needed clarification on the "service utilites only" area,to see if it needs protection,which apparently it does.The problem of many of those suggested materials is that many of them would be potential food sources for mold.

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