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During the winter, I experienced "Chinese water torture". The bathroom exhaust fan is located above the toilet. While seated on the throne, drops of water would hit my head every so many seconds.

I traced it to the roof vent. It is a metal cap that gets cold in the winter. The mylar duct runs from the fan through the attic to the roof vent. Looking at the roof vent, I found this:

enter image description here

That's an icicle on the right side, and little frozen rivers of water running from the base of the cap. Another roof vent sits to the left of it. A different view:

enter image description here

There is some water vapor frozen on the roof from the left one, but no icicles, and no interior dripping from that fan.

My working assumption was that humidity from the shower was condensing inside that hood. Some of it ran back down the duct and dripped out (I actually saw the water inside the duct), and some dripped from the vent to form an icicle.

I thought I had solved it by waiting for a warm day and then running the fan to dry out the residual water in the duct, then ensuring that the fan ran for a long time after a shower.

It's now mid-October, the weather is still warm, and I just experienced the dripping again. The attic is still hotter than the house, so I wouldn't expect humidity from the shower to be condensing in the duct at this time of year.

The only water source I can imagine is rain blowing into the vent. Perhaps there's a dip in the duct to the left vent, so the water stays trapped in that one and eventually dries out.

enter image description here

The vent cap does appear to have a damper, although it likely isn't water tight. So some amount of water might get by in heavy rain. It wasn't raining at the time of the recent dripping, but there has been a lot of rain in recent weeks. Small amounts getting past the damper over time could collect in the troughs in the accordion duct until there's enough to flow (but that wouldn't explain why it would wait for a day after the rain to start flowing).

There are four identical vent caps on the same section of roof, exposed to the same rain. One farther to the right is for a fan that gets far more shower use. Only the one in the picture gets icicles and water running from the cap, and drips from the bathroom exhaust fan. I can't think of a water source that would affect only the one fan, and in both warm and cold weather.

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    Is the mylar duct insulated? If not, the cold air in your attic could be causing condensation build-up on the walls of the duct. As the humid bathroom air flows into the duct, this will collect condensation and then drip down.
    – Shimon Rura
    Oct 19, 2017 at 2:49
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    The vent should also definitely have a baffle, so it's only open when your fan is blowing air out. If that's missing, you'll have warm humid air flowing out even when the fan's off, which could be condensing on the vent cover outside and creating those icicles.
    – Shimon Rura
    Oct 19, 2017 at 2:53
  • @ShimonRura, I don't know if the duct is insulated. It's a pain in the butt to get to that part of the attic. I've only seen the end that connects to the fan and that portion is not insulated. My recollection is that the bathroom fan does not have a flapper. I would need to get a closer look at the vent to see if there's an internal one (there doesn't appear to be one directly behind the screening). If I can locate the vent online, it might indicate whether it contains one. Thanks for the leads to pursue.
    – fixer1234
    Oct 19, 2017 at 3:16
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    This is why exhaust vents should only rise slightly in the attic and then run downhill to an eave vent. The ductwork should be covered by the attic insulation. You can try refitting this but if you have family members that take long hot showers (wife) then the problem will most likely persist until you reroute the ductwork. 😞
    – ArchonOSX
    Oct 19, 2017 at 10:11
  • "Only water source I can imagine is rain"... well, was it raining at the time? Were there windy conditions?
    – Harper - Reinstate Monica
    Oct 19, 2017 at 15:10

2 Answers 2

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Replace the mylar ducts in your attic with insulated mylar ducts. Run it longer than you need to create dips in which condensation can collect and dry out.

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  • This was pretty much the conclusion I came to. The source of the water can be internal or external, or both, but the vent cap and bathroom fans aren't different in this one case, so the solution has to be the duct.
    – fixer1234
    Oct 24, 2017 at 23:54
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If installing an insulated duct does not get rid of the leakage problem you might try changing the exhaust fan with one with a greater efficiency to push out the warm moist air out faster. The CFM rating should not be less than the square footage of your bathroom ceiling. To be sure your fan blower is adequate, I would use a fan with a rating 20% greater than the recommended CFM for your bathroom size. Also pay attention to the sone level. Get one with a rating of 2 or less if at all possible. Finally, if you don't want the fan to operate longer than necessary replace fan wall switch with a timer switch.

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