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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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OctaviaQ
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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).

BMitch
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I just ran into this issue and after removing the fan into the 2nd floor void space I was able to see that the bathroom exhaust duct (hard metal piping not flex hose) ran directly against an air conditioning duct for about 10 feet. The cold environment from the hvac line was condensating the humid air back flowing from outside in the exhaust line. My fix was to raise the fan end of the hard pipe 3 inches and add a short 3 foot “pee trap” of flex ducting to work as a trap. Also ensuring I run the fan 15 mins a day to “dry out” the hard pipe run…. May not be the best solution, but it’s what I came up with and has worked wonderfully since… I also taped/sealed the loop connection…

Rodney L
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