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15 minute thermal barrier

maggiebo | Posted in Building Code Questions on

I have had 3″ of Bayseal Closed cell foam sprayed into the exterior walls of my cabin. My intention was to apply 3/4″ interlocking car siding over the insulation but our building inspector is suggesting it does not satisfy the 15 minute thermal barrier required. Does anyone have any information on this? I could use 1/2″ drywall over the top or spray with a retardant but I’d rather not. If I use a retardant, any suggestions?

Let me know


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  1. GBA Editor
    Martin Holladay | | #1

    Your building inspector is right. So-called "car siding" (tongue-and-groove boards) do not qualify as a thermal barrier.

    For more information on thermal barriers, see Thermal Barriers and Ignition Barriers for Spray Foam.

    I think 1/2-inch drywall is the easiest way to go. Once the drywall is installed, you can install your boards on the interior side of the drywall.

    -- Martin Holladay

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