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Old 01-06-2017, 11:39 PM   #1
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Bathroom Fan Duct Leaking at Joints


Since getting cold here in Chicago, whenever I turn on the fan (particularly after showers), water drips down from the outside of the bathroom exhaust fan. I opened up my bathroom ceiling surrounding the bathroom vent fan after seeing dripping from around the outside, not the inside of the bathroom exhaust fan. I assumed it was simply condensation dripping down the inside of the duct work. I was wrong.

Metal ducts extend vertically approximately three feet from the fan to the roof. The attic is not insulated. There are two joints connecting vertical pieces. When I turned on the fan, I watched as water began to bead and drip from each of the two joints within about 1 minute. The dripping continued until I turned off the fan and then it slowly tapered off. The joints seem to have been simply "sealed" with standard cold weather hvac tape.

I tried insulating the ducts but it seemed to have no effect on the condensation and in fact seemed to almost exacerbate the dripping problem.

I just don't know how to proceed. Do I try to reseal the joints? Do I replace the fan and duct? Do I re-vent horizontally? Adding to the problem is that our contractor vented the fan from the abutting bathroom through this vent as well. That run is a horizontal flexible duct and is about 6 feet. It is covered with blow in insulation. I see no dripping from either fan when that bathroom fan is on and in a way that concerns me because I wonder if that moisture is even making it through the horizontal duct over to the vertical run where it is supposed to be extracted and blown outside.

I have attached a couple of pics. Any suggestions/advice would be much appreciated!
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Bathroom Fan Duct Leaking at Joints-img_20170106_162229276.jpg   Bathroom Fan Duct Leaking at Joints-img_20170106_165158070.jpg  

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Old 01-06-2017, 11:59 PM   #2
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Re: Bathroom Fan Duct Leaking at Joints


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The ducting above the insulation will condense bathroom moisture to drip from the inside because it is not insulated in an unconditioned attic. Also the flex duct is collecting excess moisture due to more than twice the surface area inside the pipe as smooth ducting and it creates turbulence to collect even more moisture as the air is bouncing off the inside walls way more.

Ideally; run the smooth metal duct to a gable and install a termination hood with self closing damper to stop incoming ambient air- even though the fan has one. Start with a 45* elbow to get above joists, add another ell to angle down toward hood, tape all joints, HVAC foil tape- including elbow un-used ones. Encase the ducting in some FG flex used for HVAC that has the vapor barrier and insulation. 4" metal fits inside the 6" flex. Pitch the duct run toward the hood to drain any moisture there rather than in bathroom. Run the other bath with its own termination hood/ducting/insulation, keep 3' away from other house openings- window/door, etc.

Gary

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Old 01-07-2017, 03:34 AM   #3
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Re: Bathroom Fan Duct Leaking at Joints


You need the moisture in the house. Only use the fan May - October when the attic is warm enough to endure the extra moisture. Keep a can of Glade Cinnamon and Roses on the toilet for those smelly times.
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Old 01-07-2017, 06:40 AM   #4
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Re: Bathroom Fan Duct Leaking at Joints


If you have moisture only at the taped joints, that means warm moist air from the bath is escaping and condensing in the colder attic air. Re tape and put band clamps on the joints to proper seal the joints .
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Old 01-07-2017, 01:39 PM   #5
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Thank you for the suggestions. If possible, I may try to first find a way to seal the seams and see whether the leaking continues. I have read much about the us of mastics in sealing the joints. Would this be recommended in my situation?
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Old 01-08-2017, 04:58 PM   #6
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Re: Bathroom Fan Duct Leaking at Joints


How is it terminated on the roof? Can you post an outside pic?
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Old 01-10-2017, 11:49 AM   #7
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Re: Bathroom Fan Duct Leaking at Joints


You can seal them with some joint fixing solutions.

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