For proper attic ventilation, the Home Ventilation Institute recommends a system that provides at least 10 air exchanges an hour. Use the following formula to determine the fan capacity (cubic

feet per minute, CFM) needed to provide this minimum airflow. Attic Square Feet x 0.7 = CFM

Note: For roofs pitched 7/12 or higher, you may want to add 20% more CFM capacity to handle the larger volume of attic space; 30% more for roofs 11/12 and higher.