It happens only in summer. It's not happening because of the use of the shower, it must be something to do with the outside heat and humidity meeting the cold air in our basement. We don't use the basement a lot so I wonder if the 'make-up' air is part of the problem. When we bought the house, we discovered that the previous owner installed a non-insulated duct so we replaced that and the fan at the same time. oh - and we are in central Indiana

When you consider that people at the location can't see the problem,it's almost wishful thinking that anyone 2k miles away can. With expectation more guessing without benefit of investigation will be wrong. Does vent have a functioning damper? If basement is air conditioned,over sized return air can suck outside air in through vent unless vent has state of art damper. You have to be my eyes and ears while doing some tests and observations. First while air conditioner is not running then again while it is running. Make sure no door nor window is open in basement then carefully feel for a temperature difference and/or air movement at bath vent opening where you have seen water dripping. Again while ac is running and while it is not running. Crack door that lead's from basement to outdoors or living area 4" while holding your face close as possible to crack. Is air rushing into basement through crack? Report test results.

HA! Your wishful thinking comment is kind of how I felt about posting my question! I was hoping that I would get lucky and someone would have an idea of what to try next. I will show your response to my husband and we'll try it!

The basement stairs are open. There is no door. Does that make a difference in trying this? If I understand you correctly, you are talking about doors or windows to the outside.

I went down there yesterday (before I read this) and stared at the ceiling for a few minutes hoping for some revelation to hit me. lol The only things that I thought were worth mentioning - the ceiling is a little low because that's where ductwork and pipes run through the basement. The other thing I noticed was that the hvac vent is pretty close to the bath fan. I'll see if I can get a decent picture and post it.

One other thing I might mention...when we bought the house, the drywall on the ceiling would get wet. After we replaced the duct with an insulated one, that is no longer a problem. The condensation stays within the duct and the fan/light.

I don’t think there is a damper at all so I am quite hopeful that you have solved our problem from 2k miles away!

Sounds like the warm humid exterior air is getting into the duct. Check the exterior wall outlet, that is where there should be a damper/flap. Often installers use the cheapest ones they can get and the dampers do not seal properly or the can stick open. Either fix it or replace with a higher quality one. If that does not resolve the issue, get a programable timer and install it on the fan switch. Program this to run the fan for say 15 minutes three times a day, that should expel any condensation and prevent it from accumulating to the point that it will start dripping out of the fan fixture.

Are you sure that vent even runs to the exterior or is just venting into the attic which is something I found in a clients house with their dryer .People do weird things .

Not a pro - but just a guess.

Is that vent metal? Try replacing it with one that is plastic.

Might be why it only happens in the summer. Vent gets very cold

from the ac - warm moisture hits it and then drips. Worth a try.

What is the temperature and humidity in the basement? If you don't know then here is a gauge on Amazon that is inexpensive.

I would not be surprised if the basement humidity is above 60%. If that is the case condensation can form on a cold metal surface. I have lived in houses where the basement humidity was so high water would condensate on the cold water pipes.

"The basement stairs are open. There is no door. Does that make a difference in trying this? If I understand you correctly, you are talking about doors or windows to the outside."

I'm talking about any door or window leading out of basement to outdoors or main living quarters. As feared when we began this conversation,I'm already above my pay grade if I failed to make that clear while telling you how to run tests. It doesn't matter anyhow because the problem couldn't be 100% attributed to a bath fan. I know this because an uninsulated bath vent pipe wouldn't be enough to cause a wet drywall ceiling. Had you told me about the ceiling from the onset,I would have taken a totally different avenue. To solve this,you need boots on the ground.

klem1 - The basement staircase is open - there's no way to seal it off. Since we replaced the vent duct with insulated - the ceiling never gets wet. It stays in the duct and fan/light. I think you might be right about the damper. We are going to try replacing the one on the outside of the house with something better.

Mike - we ordered a dehumidifier earlier this week. It arrived yesterday and we started using it right away! I don't know what the final number was that it climbed to, but it was 65% within a couple of minutes. So even if that doesn't solve the condensation problem it still needed done!

Angel - when we bought the house it was just a plain metal one with no insulation. We did replace it with an insulated one and a part of our problem was solved because the drywall no longer gets wet. Unfortunately, the condensation still collects and comes through the fan/light.

Kitchen Abode - yes the one on the outside of the house is super cheap. We are going to try installing a good one there. If that seems to work, we'll replace the fan/light. I will tell my husband about your recommendation!

Patricia - yes, it does run all the way to the outside. It's in the basement - so thank goodness it doesn't go to the attic! lol

Thank you all for your help. This thing has been such a nuisance. My husband is responsible for keeping 7 fire stations in shape. You all have no idea how nice it is to help him figure out this problem.

A big reason condensation tends to form on vents/air diffusers (especially in humid climates) is because so many of them are typically made from metal. Choosing a product that isn't made from metal prevents the issue from happening in the first place. Feel free to check out our blog post that explains in more detail: https://blog.inviair.com/blog/diffuser-condensation-is-a-serious-problem.-heres-what-you-can-do-to-avoid-it