A. Bruce Harley, technical director of Conservation Services Group in Westboro, Mass., responds: Canned foam might seem like a quick, easy way to provide both fire blocking and air-sealing, but unfortunately there's no such thing as fireproof spray foam. All urethane foams (including several brands labeled as "fire block") are combustible; they can't be used next to chimneys, and they can't be used for sealing penetrations in fire-rated assemblies. These "fire block" foams differ from other canned foams only in that they've had a service evaluation to qualify for "fire blocking" in type V (combustible) construction, which is typically limited to sealing annular spaces of limited size around wiring and plumbing penetrations in top plates and — by extension — other holes of limited size between wall cavities and attics.
Interestingly, the list of code-approved fire-block materials (2006 IRC, R602.8.1) specifies mostly combustible materials, which can't be used in contact with a chimney. The only noncombustible materials on the list are fiberglass and mineral wool, which don't stop air flow and thus are ineffective both as fire blocks and as air barriers against...
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