Sally: I like a lot of the quick fix ideas you have received on this post. And, I have avoided the obvious because I figured someone would come up with the "optional" answer of your problem.
The exhause fan is not properly installed.
You probably would have already looked into this issue before hitting this site. However, if this fan is in fact just putting moist air into a joist space, and is not at "least" being exhausted into your attic where your attic venting system can take it away, you could have other problems down the road. If you have had it looked at and all is ok, go forward with the fix-it ideas noted by others, if you are not sure it is installed properly, call up a heating contractor or a good fix-it up guy.