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This weekend when the snow started melting on my roof, I noticed a small water spot developing around all three bathroom exhaust fans in my house. I'm guessing the water leaked in around the exhaust fan flashing on the roof and ran down the pipe. Is there another possibility? What is the best way to fix this?

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Each vent has its own exhaust port in the roof. The water on one fan in particular seemed to be more significant than what would come from condensation. –  SchwartzE Jan 12 '11 at 17:21
    
What are the key things to look for on the roof to make sure the exhaust vents are installed properly? –  SchwartzE Jan 12 '11 at 17:22
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3 Answers

up vote 7 down vote accepted

You may have condensation running down the vent pipes into the fans; given how a vent pipe attaches to the back of a fan, the condensation will tend to run to the side(s) of the fan fixtures.

  • Are the vent pipes uninsulated and in a cold space?
  • Could condensation have frozen inside of the pipe, and then melted at the same time as the snow?
  • Would the snow have stop the wet air getting out of pipes?

If it is condensation then you need to insulate the pipes and/or add a condensation trap just above each fan.

Assuming that all 3 exhaust fans vent via different pipes, you are looking for a design fault, as I cannot think of anything else that will be in common between all 3 fans.

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I got a chance to go up in the attic today. It is thin walled straight metal pipe through an uninsulated area, so I think this might be the problem. I bought insulation for the pipes today, so I just have to go up to install it. There wasn't any sign of water damage in the roof sheathing, so I believe it is the condensation. –  SchwartzE Jan 15 '11 at 18:26
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This is a very common problem. The person above who mentioned a condensation trap is right on the money. No matter how powerful a fan, how well insulated the duct, you are blowing warm, MOIST, air into cold air which makes that moisture in the air condense into water droplets.

A condensation trap, which can be as simple as a bit of a loop in the flexible ducting coming out of your fan, much like the traps under your sinks for plumbing, will collect and contain that moisture, and over time just evaporate. It goes against the conventional wisdom to keep the duct as straight as possible so as not to restrict air flow, but if you just have one small little bend, right near the fan and within the attic insulation, your problem will stop.

It might seem as though there is so much damage done to your drywall that this solution won't be helpful, but the amount of condensed moisture at any one time is minimal, but a slow drip over time ends up looking like a massive leak.

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Ok, if the snow fairy didn't land on every fan individually, then I think we have a common problem between fans. If your roof vent has a common back flow air baffle, I'd check it to see if it is stuck open, allowing snow to blow into the vent pipes. Second and not really likely is a leak around the flashing. A leak around the flashing would appear on the ceilings no in the fans.

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