Natural ventilation allows for an inexpensive means of cooling. With the use of atrium roof vents the clear opening to the sky quickly allows the hot air to escape similar to a chimney.  In conjunction with side inlet vents and shade curtains temperatures can be less than outside ambient temperature. The inlet vents can be the guillotine vents with their clear opening or the old style rack and pinion. The quiet environment of natural ventilation is a great shopping experience in a retail garden center as compared to noisy fans. Less expensive cloth curtains will give you an economical natural ventilation option in the roof or the walls.