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I've seen a lot of questions (including one on this site) about condensation on bathroom vents. However, they all seem to take place in winter, with consensus being that as the warm air from the bathroom travels slowly to the outdoors, it is cooled off and condenses in the pipe and drips back down to the fan.

However, I am having this problem in the peak of summer, where it's intensely hot and humid outside, and nice and cool inside. This happens in the downstairs bathroom. I believe that it is vented outside, to the side of the house, not the roof. If I am correct, then the exhaust pipe has to traverse the length of the living room to get there, maybe 10 feet or so. This is only a half bath, so there isn't even a shower creating humidity.

Any ideas would be appreciated! The dripping is driving me nuts.

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I would assume that it's the warm moist air from the outside condensing on the cool surface of the vent. No idea how to fix it though - sorry. –  ChrisF Aug 14 '11 at 21:11
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Where is the drip coming from, as far as you can see? (a pic may help). Is it possible it's water getting in through the vent outside? Does it happen all the time, or only when the outside humidity is high? –  gregmac Aug 15 '11 at 0:30
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This problem is why the duct work is supposed to slope down towards the outside so if you get this happening the water drips outside. –  auujay Aug 15 '11 at 2:04
    
@auujay: I'm curious how the line would be run like that, since the fan would be mounted in the ceiling, the line comes out the side of the fan, and has to run along the ceiling to get to the side of the house. I've also seen a fair number of them run vertically through the attic to the roof. –  BMitch Aug 15 '11 at 11:35
    
Short answer is I don't know. I have read the suggestion (I think I even heard Mike Holmes mention it on a show) but I know exactly what you are talking about. In fact I just installed a fan in my bathroom yesterday and it slopes "up" (the "wrong" way) because of where the duct is on the fan unit and the hole in the wall to the exterior has to clear the top plate of the wall. I think it is more important to have a straight run than get this slope so I opted to just use a straight pipe and insulate along the length of it. –  auujay Aug 15 '11 at 15:30

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up vote 5 down vote accepted

I think Chris is right, warm moist air from outside is coming into the vent and condensing on the cooler inside surfaces.

Fixing this could be as easy as fixing or replacing the outside vent damper so it closes properly. You could also go for a vent damper that installs within the vent itself. And finally, you can insulate the pipe so that the vent pipe stays close to the temperature of the air flowing through it rather than the inside or outside of the home (depending on where the pipe runs).

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I recently installed a vent in my bathroom, and though the line slopes down anyways (so any water will drip outside anyways), the vent itself has molded styrofoam inside, and a large ball in a channel that acts as a damper. When air is blowing, the ball lifts up slightly and lets air out. When it stops, gravity pulls the ball back down and it completely blocks the pipe. I've only had it in for a couple months, but it seems to me a much more reliable design than the typical plastic flappers that have air gaps and are easily blocked or held open if any debris gets in/on them. –  gregmac Aug 15 '11 at 22:29

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