I'm stuck. My greenhouse will be 15 x 22.5 saltbox with south side 9 ft, peak at 12 ft and north wall at 7 ft. I figure that gives me about 3500 to 3800 cubic foot. What cfm do I need to pull via an exhaust fan? There will be vents in the lower wall area of the northwest corner for cool air entry, just cannot decide what kind of fan and how big or if I need two of them.