I have a bathroom on the top floor of my house, It has a window but no exhaust fan. I noticed a problem with cracking paint. So I want to install a fan. I have two choices to mount the fan either on a exterior wall or ceiling but since the bathroom is part of a dormer, the ceiling only has a stud cavity to mount the fan; I also run into the same problem with the wall. Also the walls are plaster so no matter what this wont be easy. Do they sell fans that fit into that small space and also how do I run the duct pipe in such a small space?