Gap around furnace b-vent after roofing
Roofers often leave an open gap between the double-wall B-vent and the surrounding roof jack, which lets light and potentially water through. Standard caulk should not be used here because furnace vents get too hot; instead, you need a high-temperature rated storm collar and silicone flashing designed for gas vents to properly seal the opening without creating a fire hazard.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Installing a storm collar jammed down tight to the top of the roof jack and affixing a B-vent top directly to the pipe.
- 2applying new caulk to seal the gaps
- 3Using a curb adapter and a rain hood
- 4Flashing it with torch applied modified bitumen, granulated
- 5Using caulk or a piece of trim
In their words
Unedited, most upvoted first, each linked to the thread it came from.
“just got a house and there's a bunch of spots I need to redo the caulk on before it gets rainy again. I was wondering though, is there any reason why the tops of these vents would be left open like this?”source ↗
“Hello, I am looking for advice for what to fill these gaps around the chimney near the wood stove”source ↗
“the B-vent (cylindrical double wall from the top of furnace or boiler) runs inside the roof jack (brown part that is roofed-in) but has a gap, and the top, that is secured by tabs through the roof jack, is letting in light around through that gap.”source ↗
“Hello I am replacing the duct work in my kitchen exhaust and I've managed to get the new duct in and taped up. But I don't know how to seal it.”source ↗
“The original duct penetration was uninsulated and exposed to outside air.”source ↗
“Normally this would be done from inside the attic... But this is a flat roof so I have no attic.”source ↗
Where this came up
People with this problem also raised
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- 4How to patch an inaccessible vent pipe
- 18Should I use foam or caulk to seal gaps in exterior walls?
- 6Best way to seal gaps around pipes under the sink?
- 2Are open cavity vent returns and exposed pipes normal in new construction?