Vent stack condensation

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Gavin Hindman

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I've got a vent stack from a 2nd floor bathroom that goes up a chase and into a finished 3rd floor exterior wall where it then vents out through a vaulted roof. There is a consistent, but infrequent drip from where the vent pipe exits the upper wall into the 2nd floor chase. There is no plumbing above where it is leaking, and I've had roofers out to check for a roof leak. The drip continues whether or not it is raining, and there is no water damage on the upstairs roof or wall, so I'm fairly confident it is not a roof leak.

There are gaps around the hole where the vent pipe enters the upstairs wall, and there is an uninsulated heating duct at the bottom of the chase, so my working theory is that warmed air from the chase is entering the upper wall and causing condensation on the colder vent stack that then drips out the bottom.

So my question is, can I just spray foam up into the gaps where the vent stack enters the upper wall so that it is sealed off from the warm air below? If I trap condensed moisture up there should it evaporate without causing mold? The other wildcard is we did install a minisplit in the 3rd floor room over the summer, so that room is consistently warmer than it used to be, but the vent pipe is inside a 2x6 exterior wall, and everywhere else I've pierced that wall it has been well insulated and has a vapor barrier.

Thanks!
 

Tuttles Revenge

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Is the vent stack metalic pipe? I've never seen that on plastics. But obviously there is warm and therefore moist air contacting a much colder vent pipe. However you prevent that warm air from contacting the cold vent pipe would prevent condensation. Maybe even insulating the vent pipe itself to create a vapor barrier.
 

Gavin Hindman

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Is the vent stack metalic pipe? I've never seen that on plastics. But obviously there is warm and therefore moist air contacting a much colder vent pipe. However you prevent that warm air from contacting the cold vent pipe would prevent condensation. Maybe even insulating the vent pipe itself to create a vapor barrier.

Thanks, yes, black PVC. Preventing the air from the chase from infiltrating up around the pipe where it is dripping from in pretty trivial. To insulate the pipe I would have to open up the upstairs wall, which I would rather avoid if I can, and in theory it is sitting inside a well insulated 2x6 exterior wall.
 
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