As a Seattle Home Inspector I frequently find insulation tightly packed around B-Vent pipes that run through attics. (B-vent pipes are used to vent combustion by-products from gas burning appliances such as furnaces and water heaters.) Many builders seem to think that because materials such as rockwool insulation are used as fire-stopping, that it should be “good-to-go” right up against the hot pipes.
The manufacturer’s of these pipes typically require a 1″ clearance to combustibles to allow for air circulation so that heat is not conducted to those combustible materials.
So, even though rockwool is not considered combustible, it too must be retained away from the vent pipes because the insulation itself will conduct heat to things that can burn.
While this may or may not be a big deal, the air space also does something else—-it minimizes corrosion. Almost all B-Vent penetrations through the roof will leak over time. Wet insulation against the pipe can lead to corrosion of the pipe—-if it gets wet and stays wet. Sometimes this water that runs back down the pipe is condensate from the exhaust gases themselves. This water is slightly corrosive and makes the situation even worse.
This first picture shows insulation up against the pipe with some rusting apparent where the insulation covers the pipe. In this case a leaking flashing kept the insulation wet where it eventually rusted the pipe.
In this next picture the installer couldn’t run the B-vent through the space without touching the wood framing so it was changed to a single wall type pipe (this pipe itself is touching the wood) and then it was changed back to B-vent type pipe. The problem with this is that the single wall pipe requires even greater clearances (6″).
A proper vent with insulation baffle looks more like this:
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