We have lived in our home 22 years and have have been battling mold in our master bath the entire time. We are getting ready to paint and noticed our exhaust is really rusted. Through this process I have learned our fan is only rated for 80 cfm and the room should have one about 300 since the room is almost 3000 cubic feet. What will need to be done other than making the housing larger to accommodate a larger fan? I believe our duct is only 3"... Any ideas on the best way to handle this?