Condensation on vent stack

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Reyber340

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I'm having an issue with the vent stack for the kitchen sink. I live in Canada and its now starting to get cold outside -16 C tonight and I've noticed over the last few days there has been quite a bit of condensation on the vent piping under the sink.

So far I've cut open the wall from the outside to see if there was an insulation problem and all was sealed up. I also put the hose down the stack to see if there was a leak anywhere and nothing. I've also tried extending that stack (vented through roof around a window separate from the main stack) around 14" high incase it was drawing air in but nothing seems to stop that vent from getting cold inside and condesating. Tonight I removed the 14" piece and stuck on a 36" high piece to see if that'll do anything.

Does anyone have any ideas as to what would cause this and/or any remedies to try?

Thanks in advance for any help
 

Dgeist

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You didn't mention what material the vent is. If it's Cast Iron, I would expect much more heat (cold) transfer from the roof penetration. Also, how far up from the condensation is your roof? How far does it travel on the outside wall? I know even living in the northern US, we didn't usually have pipes in the outside walls if we could avoid it. You mentioned that there's insulation, but what type, and is it an air barrier? Water will condense when it reaches its dewpoint regardless of where it came from, so even if your vent is KINDA warm, a little bit of humidity in the wall cavity goes a long way.
 

Reach4

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In cold areas, there should be a vapor barrier that keeps room humidity from the insulation. You may have cut open the vapor barrier when doing some plumbing stuff. Try to restore that vapor barrier. Keep the water vapor away from the cold pipe, and your insulation. The humidity from, the house reaching the insulation makes the insulation perform poorly. The vapor barrier may be suitable paint over drywall, after you have caulked and sealed the openings. It may be a sheet of suitable plastic.

However if your insulation is inadequate, in the coldest temperatures, open the cabinet doors under the sink to warm that area to avoid pipes freezing.

Been doing a lot of cooking? That can generate extra humidity in the kitchen. Until you get that vapor barrier working again, if you use forced air heat, putting the system in fan-on mode can move that humidity to where it can do some good. Or use the exhaust fan, but that sucks in cold air through the cracks in the walls.
 
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Jadnashua

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Get a humidity meter and see what your room humidity is. It's somewhat unusual for a home to have high humidity in the winter, but it can happen if the house is well sealed and not ventilated.
 

Reyber340

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You didn't mention what material the vent is. If it's Cast Iron, I would expect much more heat (cold) transfer from the roof penetration. Also, how far up from the condensation is your roof? How far does it travel on the outside wall? I know even living in the northern US, we didn't usually have pipes in the outside walls if we could avoid it. You mentioned that there's insulation, but what type, and is it an air barrier? Water will condense when it reaches its dewpoint regardless of where it came from, so even if your vent is KINDA warm, a little bit of humidity in the wall cavity goes a long way.

Material is ABS. Pipe is insulated with spray foam and rock wool insulation. I opened it up from the outside and ensure the vapor barrier is completely sealed around the pipe penetrarion. I'll draw out a picture of the pipe and add.
 

Reyber340

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Humidity In the house is around 49% which is a little high. The poly is 6 mil and is sealed well. I sealed around the penetration with spray foam from the outside. I've attached a couple of pictures of under the sink.
 

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