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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Charlie, I have been reading your blogs, I make it a point because you always have good reminders for all of us. I think these should be submitted for credits with the state.
I also see be vents packet tight to the insulation.
Dennis, you are too funny 🙂
Great article Charlie, I’m always learning. I previously had thought rock wool was the solution but you shot some holes in my theory. Thanks for helping us all.
Thank you! This was the definitive answer I was looking for about rock wool in contact with B vent. Rather than using a thin metal flashing material around a flue horizontally where it penetrates into the attic, can thicker cement board or a leftover piece of wall protector stove board be used in direct contact with the B vent and then caulked with a bead of fire caulk? I’m hoping to keep more heat in the living space by doing this.
Metal collar is recommended. Getting away from wood burning is the only real solution to heat loss in this area. I know this is not what you want to hear but all approaches to heating and cooling our homes comes with down sides.
If someone really wants insulation up to the pipe, could they use triple-wall pipe in that section? I’m just curious.
p.s. Thanks from this homeowner about rockwool–I’m just trying to figure out how to fix exhaust pipe problems on the house we bought about 2 years ago–it’s amazing how many roofing project managers don’t understand clearance issues!
I know of no type of vent or chimney that does not have clearance to insulation requirements.
The context of your comment lets me know that my wording was misleading. Wood burning is not part of the equation with this property. Thanks for your website!