Last winter, I got a call that water was leaking from the ceiling of a house that was less than 10 years old. Previously, the owner has my predecessor over to insulate the bath fan vents because the cold attic had caused condensation in same. Now the problem was worse than ever before and the customer was livid. I went through everything, and was at the end of my rope. This was until I asked the customer a question or two. There were two bathrooms, but only one vent was having this issue.
"How long of showers do you and your son take?" "He's in there quite a while, but we're only in for a few minutes," he replied a bit indignantly. I checked both fans and both were in working order. "How long do you guys run these after you're done?" I pried. "We've never run 'em," said the customer. I thought about this for a minute as I put the last cover back on. Even if the fans aren't running and the dampers to them are closed, there is still going to be warm, moist air travelling the ducts to the outside. Problem was that without the benefit of the fan, the damper outside was just going to hold that air and moisture inside, where it would condense on the inside and outside of the metal duct. The result was soggy drywall in the homeowner's bedroom.
Exhaust fans are in there for a reason, and I told the customer on those grounds we could no longer warranty the repairs I did. I told him he had to run the fans or risk damage to the ceiling, end of list. I assume he heeded my advice, because he never called back. Maranatha!