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Frost and condensation build up around the plumbing stack vent.

Frost and condensation build up around the plumbing stack vent.


  #1  
Old 01-08-17, 09:58 AM
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Frost and condensation build up around the plumbing stack vent.

Context information. The house is 40+ years old. I'm located in Eastern Canada, so fairly cold. There is no vapor barrier in the attic as far as I can tell.

So I detected what appear to be condensation around the plumbing vent stack. From looking at some of the water stains, the problem was likely there for a while. Probably got more serious when I got a new roof ridge vents 8 years back with much improved ventilation. it is not leaking into the walls or ceiling, but when ever it gets warm the insulation there would get wet.

The plumbing vent stack is only about 8 to 12 inches high before it goes right into the roof flange. So it is a short run. The plumbing vent is inside an interior wall, so it is surrounded by what appear to be 2 top plates.

In Dec 2016, I used some great stuff foam to air seal the gap in the plumbing vent bypass. The issue, while did improve, still had some frost build up on the decking board left of the vent (which is directly on top of an interior wall top plate). There is also ice build up in the flange itself as well.

http://imgur.com/a/SBf3s/layout/grid

So few days ago, I decided to apply some foam to the top plate's left edge, (well, applied foam to all the places I can reach, since the vent is very close to the overhang and space is very tight there). I also added a baffle right beside the vent, figuring more ventilation wouldn't hurt.

After doing this, the situation seem to have gotten slightly worse. now vent appear to be covering with even more frost, and I still have ice build up on the decking board.

Attic condensation near plumbing vent, air sealed edge of top plate as well in Jan 2017 - Album on Imgur


Not sure what is happening here. I can think of few explanations.

1. When I was moving insulation out of the way, i had to move some really old paper faced batt insulation (the yellow insulation), which caused the asphalt paper to be torn. Could this have allowed more vapor to get in?

2. The baffle improved ventilation, and the area even colder than before, so more condensation.

3. We had a week of fairly cold weather, 14 F (-10 C) during the day, and -4 F (-20 C) during the night, which may have contributed to it.

I'm at a loss at the moment. Hoping someone can point to me something I might have missed or done wrong. Could the pipe itself is leaking air? How likely is that?

Thanks.
 
  #2  
Old 01-08-17, 10:35 AM
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I'm not very knowledgeable on plumbing and suspect it may be something of an issue with the vent I can't comment on.

Instead, I'll comment on attic ventilation. Obviously at least part of your issue relates to moisture that's unable to vent from the attic. Yet in your post you state, "Probably got more serious when I got a new roof ridge vents 8 years back with much improved ventilation " . Not trying to be argumentative, but if the ventilation improved it's not likely the problem would also have increased unless it was only a coincidence.

All that begs the question, do you have gable vents that weren't closed when the ridge vent was installed? If so, it's possible the gable vent is creating a perpendicular air flow, which interrupts the proper flow of air from your soffit vents - allowing moisture to build and condensate at the pipe.
 
  #3  
Old 01-08-17, 10:48 AM
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Hi, there is no gable vent. Previously the house had one of those rotating air vents. Two maxi vent was put in, the soffits were also cleared. I can see quite a bit of daylight coming from soffit when I climb up there.

Could the air leak come from somewhere else? and just condense on and near the plumbing vent since it is the coldest part in that area? Although I don't see frost in any of the nails in other areas

Thanks.
 
  #4  
Old 01-08-17, 11:06 AM
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"Could the air leak come from somewhere else?" Absolutely, particularly on an older home back when heating costs were not the issue we face today. Colder air is typically relatively dry so moisture would probably be coming from the living area. Start first by seeking the most substantial sources. Make certain there's no bathroom, kitchen or dryer vent ending in the attic. If there are any such vents make certain they're sealed. If you have a chimney passing through the attic find out how to deal with that. How about access to the attic, itself? Do you have pull-down stairs? If so, we'll let you know about sealing it. Same with a house fan.
 
  #5  
Old 01-08-17, 12:41 PM
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Before looking at your pictures I thought perhaps your vent stack may be a copper line. After looking at pics and verifying that assumption I would offer a thought that the copper, being extremely efficient at conducting heat will probably be better dealt with if you can insulate the pipe itself to keep moist air from coming in contact with it. Since it is a relatively short run of pipe I would think that you can layer the foam from an aerosol can around the pipe to build it up or install a sleeve such as a cardboard tube around it and inject the foam so that it will have support until it sets.

Alternatively you could buy foam or fiberglass pipe wrap with a FSK exterior vapor barrier on it and install it as well as you can and then seal any gaps with the can foam. The foam pipe wrap would also prevent moist air from coming in contact with the pipe and you could also seal any gaps in that application with the can foam. You may have to go to a commercial plumbing supply dealer to get the appropriate size wrap in the foam or fiberglass.

Since attic temperature and moisture conditions will be infinitely variable, approaching this from the aspect of controlling those variables within the attic may become very frustrating.
 
  #6  
Old 01-08-17, 12:42 PM
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Ice in an attic is always from moisture escaping the house so you have some unsealed penetration that is bringing warm moist air from the house into the colder space. Most likely the hole in the top plate is not sealed as well as you think or there are other holes from can lights or electrical wires or something that need to be sealed. The paper facing on the insulation (which should always be facing the warm side of the insulation) is a vapor retarder and an air barrier, so tearing it could let more heat and moisture escape from the house. Might also check the rubber roof boot and make sure it's not an old gasket that's shot.
 
  #7  
Old 01-10-17, 08:24 AM
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Hm.... thanks for the feedback everyone.

Sigh, looks like my options are:

1. Find and seal every single little air leak in the attic, which would also include removing all the insulation between the trusses at where the top-plate/plumbing vent is and seal the interior wall top-plate.

2. Live with the condensation.

Have folks here removed all the insulation between trusses before? I think I'm looking at about 10 feet by 24 inch area worth of insulation. How much work would that be involved based on my photos? Would it be cheaper/easier to open up the drywall instead? The top plate should be right above a closet that's adjacent to the bathroom.

Thanks.
 

Last edited by Dimeron; 01-10-17 at 09:49 AM.
 

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