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Community and Q&A

Fire code concerns with rigid foam panels

aMmB72wGds | Posted in Energy Efficiency and Durability on

What are the fire code rules and general fire safety concerns when using rigid foam panels in the following applications:

– Exterior of wall with furring strips on top to form a vented rain screen behind siding.

– Exterior of roof with furring strips on top to form a vented roof plain

– Interior of roof to form an air tight vent baffle (a less traditional approach, but a possible retrofit)

– Applied to interior of ceiling and covered with drywall – the ‘top side’ would be exposed in the attic and covered with batts or blown insulation. (I know that this is not the typical use since you can usually handle all of your insulation needs in the attic by piling insulation on top of the ceiling joists. But I’m really just concerned with the ‘what if’ fire code concerns here.)

In each of these scenarios the foam is technically exposed in that it is not sealed between fire rated sheathing. Do the codes pose any restrictions on these uses? And above and beyond the codes are there any real world safety concerns with these applications? I’ve seen some of these approaches recommended from an energy efficiency standpoint, but I haven’t heard much discussion about the fire concerns. Any input would be much appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Good questions. Some local building officials have questioned whether vented rainscreens over exterior foam wall sheathing meet code requirements. For example, here's what Paul Fisette, the director of the Building Materials program at the University of Mass. in Amherst, wrote in 2005:

    "Building codes may not allow you to leave an air space behind the siding. Section R602.8 in the International Residential Code (and sections of other building codes) requires fire blocking at the bottom, between stories, every 10 feet, and at the top near the roof in concealed stud spaces, including furred spaces. Many builders and building code officials think the spirit and intent of this code provision is not directed at vented rain screens, but rather at furred interior spaces. However, other inspectors disagree and think the code section holds as written. Inspectors have ordered completed rain screens stripped and others have stopped jobs in progress demanding the addition of fire blocking. The code is not clear and the decision will be your local building inspector’s. So run your rain screen design past the inspector before building the walls."

    However, code officials and rigid foam manufacturers have made progress since 2005. New versions of the code specifically allow the use of foam sheathing to meet energy code requirements. It's now rare for local code officials to question exterior foam sheathing. In fact, a new coalition — the Foam Sheathing Coalition — has been formed to address the concerns of code officials about the use of foam sheathing. For more information on the Foam Sheathing Coalition, and to get copies of useful technical manuals, visit:

  2. aMmB72wGds | | #2

    Thank you for your response and for the resources. Code restrictions aside, is there a general consensus then, that using foam boards in exterior cavities is a safe practice as far as fire spread is concerned? Thanks again.

  3. Allison A. Bailes III | | #3

    Also, Thermax foamboard, a Dow product, has the fire rating to be left exposed without a fire-rated sheathing.

  4. Scott Heeschen | | #4

    I'm running into a similar issue with my building department - the roof assembly has a shear layer covered with a few inches of polyiso. We're laying 1x sleepers on top of the foam to be followed by 1/2 inch ply, then a metal roof. The idea with the sleepers is to leave a ventilation and radiant-barrier gap, but the building dept. is asking for fire blocking, defeating the venting purpose.

    I realize metal roofs don't need to be vented, but we're doing it for heat issues. The thing is, the city's objection seems like it would apply to any vented roof structure. How does that normally pass code?

  5. Robert Riversong | | #5

    Since most structure fires start on the inside, it may be that many jurisdictions would not treat exterior furred cavities the same as interior ones, with the exception of wildfire zones where you're not even allowed to have a vented roof without ember screens. I'd be interested in knowing what California wildfire codes have to say about vented rainscreens.

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