Bathroom exhaust fan leaks

My bathroom exhaust fan leak through the light. It will accumulate a fair amount of water, maybe a half gallon, in the exhaust pipe running from the ceiling to the pipe going out of the roof.

This only happens in the winter, and specially when it's raining (not storm, heavy rains or wind) or snow is on the roof.

The bathroom is 60 square feet. I bought a 80 cfm fan. I put insulated venting tubing running for the top of the fan and up the pvc pipe going out of the roof. It's still leaking.

I am 99% sure it is not leaking in through the roof. I looked at the flashing and the rubber boot this fall and it seemed fine. There is also no sign of water in the attic and the exhaust tube is not wet on the outside. And as mentioned, water accumulates inside the exhaust tubing.

I live up north where it gets very cold in the winter, like below zero for a week straight. We have one bathroom, and four people shower in that one room, and this is over the span of about 1 1/2 to 2 hours. The exhaust fan vents up into the attic, then runs about 4 to 5 feet horizontally across the attic floor the does a 90 degree and up through the PVC about another 5 feet out of the roof.

I am pretty sure the problem is either of a combination of two things. As I said it runs about 5 feet across the attic floor than does a 90 to the PVC and goes up about 5 feet. The attic is very well insulated, meaning it is VERY cold up there in the winter. I am thinking the length of the run and fact that it is horizontal so long then does a 90 degree going up, it is allowing the water to condensate and accumulate, and then eventually drip back out.

OR

I am 99% sure there is no flapper valve on the pipe coming out of the roof. It's just a PVC pipe, makes a 180 and comes back down towards the roof. All PVC. I am wondering if cold air is coming in and causing or contributing to the cooling problem.

I have attached 2 pictures to demonstrate what I am talking about

 

Run the bath fan longer to dry up the condensation.

 

We leave it running from when I get out of the shower at 6:30 until about 10 am. The last shower is usually over by about 9:15

 

what is the RH in your house and what part of the country are you located?

 

I think we need to narrow down when it leaks. If rain or snow causes it to leak, I would tend to say it sounds like an ice dam and that the pipe boot (roof flashing) is leaking. Ice dams cause water to back up under the shingles, causing leaks.

The gasket on the roof boot flashing (around the pipe) can also fail.

If it leaks when the roof is clear of snow, no rain... it could be condensation.

 

Not sure of the RH. We are well insulated through. We have winters with weeks with temps below zero and I have never had a gas bill (natural gas heat) over $80. I am in Northwest PA. Erie, The city that got almost 100 inches of snow around Christmas.

The attic is significantly colder than the rest of house in the winter and hotter in the summer.

Sorry, I am still trying to determine a specific pattern. It definitely only happens when it is cold outside. It will happen in rain, definitely when snow is on the roof. But it doesn't appear to be leaking through the roof itslef. Nothing in the attic where the pipe comes through the roof is wet. I will get up and inspect the flashing and rubber boot as soon as the weather allows