This is what the supplier says about that. "One of the fire safety features of double-wall Type
B gas vent is that its outer wall, insulated by the
airspace beneath it, requires only 1" clearance to
combustible material in the structure". There is a firestop spacer manufacturers part number 40FS that may be ordered from the manufacturer to mount between the 2x4 studs.
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4 years ago
Lincolnshire, IL, USA