@Sue Sorry to come late to the party... Just wanted to to say that even with perfectly working exhaust fan, you'll likely still see a bit of moisture while you're taking a shower.

Make sure you run the fan for about 20 minutes (or until all the moisture is gone) after you're done.

If you had a fan strong enough to immediately suck out all the moisture from a shower, it wouldn't be very comfortable to be in the room.

Check out this answer for more information.

For a bathroom, you're looking for about 11-15 air changes per hour.

No answer has ever actually explained how to do old work? They all seem to just say, "use a fan box."

Debating to bounty this question, or should it be a new one?

... or I guess I could just answer the question in the title.