I live in Washington, DC. The second floor of my home is uncomfortably warm and some of that has to do with radiant heat from the attic. There is no room for additional insulation—I need the floorboards because I use the attic for storage—so I thought of adding a gable fan. How powerful of a fan do I need? 800 CFM? 1200 CFM? More? My attic is 40' by 30' and peaked.