A.Don Jackson responds:
No, you can't use gypsum board to reduce clearances from
combustion appliances and vent pipes. You may be thinking of
the use of drywall in firewalls and other fire-resistive
assemblies, which are intended to slow the spread of fire, not
to prevent initial combustion.
You'll need a heat shield to reduce clearances to a woodstove
pipe. One of the most common is a piece of 24-gauge sheet
metal, attached to the nearby wall or ceiling with standoffs so
that there's a 1-inch air space behind it (see illustration,
below). If properly installed, this can reduce an 18-inch
single-wall pipe clearance to 9 inches overhead and 6 inches on
the sides and rear, according to the IRC. You can also add a
protector shield to the pipe itself.
Another option would be to upgrade to a double-wall pipe. For
example, Simpson Dura-Vent (800/835-4429,
www.duravent.com) makes a double-wall
stainless-lined pipe that is rated for 6 inches to combustibles
on the sides and 8 inches above. This is available in a black
finish and might look better than a heat shield.
Most codes include a chart (see IRC Table M1306.2, for example)
that lists several options in addition to the sheet-metal
shield, but none of the other options is as simple. Check your
local code for specifics, and make sure you look at the listed
clearances for your woodstove. To have an approved
installation, you've got to meet those requirements, too.
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