waterfall in bath exhaust fan!

I was in the attic and noticed the flex duct connected to a bath fan (which is never used) was sagging with water. I sliced it open and found it almost completely restricted with ice and water and the inside covered in mold.

It was uninsulated flex thru a cold attic. The fan is never used so I am confused why condensation would occur at all?

Unfortunetly the fan terminates right under a roof vent, it doesn't have its own vent. I know not ideal but I have neglected to fix that since the fan is never used.

So in order to curb this water (again no idea why it accumulates since fan is unused) I replaced the flex duct with insulated flex duct. I added insulation around a short piece of rigid duct that is connected directly to the fan.


So why is this occurring?
What can I do to stop the condensing?

 

Even though you don't use the fan, the natural air flow, technically called stack effect, is pushing warm air past the less than effective damper inside the fan. That warm air is cooled and forms condensation, very nasty condensation as you discovered. If the shower is used in that room then the fan should also be used. If it will not be used at all, cover it in some manner to stop the air leakage.

Bud

 

Shower is used daily in that bath, never occurred to me to even bother with the fan.

will the condensation stop with the duct now insulated?

 

The condensation may be reduced, but it may just wait until it exits up by that roof vent.

There are several things here you will want to address. Providing a vent to the outside you already mentioned. If you are not experiencing mold on the walls or other high humidity problems in the house, then running the fan now isn't necessary. So covering it in some manner will for now stop the condensation.

Is this fan close to an outside wall?
Is the house dry, as in low humidity?
Type of house and age?

But, a dry house is an energy indicator which, if fixed, could reduce your heating costs.

Bud

 

Anyone else confused?
One post says bathrooms never used, next one says they use the shower and never bother with the fan.
Huge mistake not using that fan every time that shower is used!!!.
Vent on the roof covered with snow?

 

bud- no sidewall to vent. House in general is about 45%rh and is old and likely drafty into the attic.

So with the insulated duct in place I still noticed some condensation right at the outlet where I have about 6" of the core of the insulated duct exposed right at the roof vent (like you speculated bud).

I disconnected the insulated duct the next night and checked the next morning but this time it was bone dry at that location.

So should some condensation be considered normal? Obviously with a vertical vent you cant slope it to go outside. And if it had a dedicated roof jack, it would still condense right at the outlet correct?

Would fabricating a drip leg of sorts be a good idea in this situation?
How about a tight damper to close when the fan is not in use?

 

They do make better dampers (almost anything would be better than what I've seen inside those fans) as long as you locate it where it will be warm to avoid condensation and ice. They make Ball type dampers where the fan can float the ball, but natural air pressures cannot. I've seen them on the outside of a house for dryers, but not for bath fans. Need some searching. Took a look, and if you search for inline dampers I think you might see something you could add to your exhaust path to reduce the natural flow.

45% is high and especially for a leaky house. That humid air is leaking through many paths and forming the same condensation in places you can't see. That's why they recommend you seal everything, even though you have to turn around and provide fresh air. The intentional fresh air is through controlled paths and thus not damaging the structure of the house.

Bud