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 heat loss. So the
only way you can technically meet code is by combining sheet
metal — acceptable next to a chimney, despite its absence
from the list — with glass or mineral wool, which should
be placed over the top of the metal without completely filling
the gap between the masonry and framing.
When the chimney is exposed, an alternative is to cut back the
ceiling drywall and patch in strips of noncombustible cement
board, butting them to the masonry and sealing the gap with
fireproof caulk (see illustration, left). Fire blocking is
required at each level, at either the top or bottom of the
joists, whereas draft-stopping is important primarily at
insulated ceilings and floors that are part of the home's
thermal boundary.
In parts of New England, it's common to strap the underside of
sawn floor joists before hanging drywall; although the gaps
between the bottom of rough openings and the drywall should be
sealed, they are often missed. Foam sealants are fine here,
since these areas are outside the 2-inch clearance zone. Other
potential trouble spots are the large openings where the top
plates of the surrounding walls fall outside the rough openings
in the joists; here you may need a larger plywood fire
block.
The metal-and-caulk method you describe is one that I typically
use in attics, but an alternative approach is to frame tightly
around the chimney with steel studs, then use solid steel
channel at the top as a fire block. Of course, you'll still
need to seal gaps with fireproof caulk.
The same rules apply to prefabricated metal chimneys. These are
easier to seal because manufacturers offer sheet-metal
fire-block kits that fit their chimneys and provide the
necessary clearances.