We have a ranch--basement, living level and attic--built in 1968. We bought it in 2002. Decided to use attic for some storage and noticed what looks like black mold on the roof beams. We are doing a mold test now. Were told to put in some sort of attic fan. Is a gable mount better than a roof mount? And how do we figure size of fan needed? House measures about 25ft. wide by 45ft. long. Thanks for ANY help you can offer!!