For passive or naturally ventilated greenhouses, the roof vent area should be 20% of the floor area and located on the leeward side of the greenhouse. The lower side vents should be 10% of the floor area.

Thanks for the replies. Very useful information.

@xentar_gw: The shutters we're looking at work like the wax ones you described. It looks like CFM ratings only apply to mechanical ventilation - would this be necessary in my climate, where there is a regular breeze?

@oakhill: That seems like a good guideline. How universally does it apply?

The 20%/10% rule works anywhere the greenhouse needs to be cooled. More vent opening does not improve cooling substantially. Keep in mind that even with these openings, the greenhouse can still be 3-5 C warmer than outside. Some sort of mechanical cooling- mist system, etc., and/or shade cloth, would be needed to keep the GH the same or cooler than outside temps.

To me, greenhouse ventilation is about preventing the greenhouse from turning your plants into brown ash, not really cooling it down further than the outside temperatures. They are ovens, and I've had my greenhouse up to 50 degrees Fahrenheit warmer than the outside temperatures, on a sunny, winter day, in partial shade, and without any type of heat and vents closed. Without that ventilation, your 32c days could potentially be 60c inside of the greenhouse. Because of this very situation, I only use my greenhouse in the winter.

@oakhill: 3-5ºC warmer could, under some conditions, be too warm. By mechanical cooling, do you mean ventilation as xentar_gw describes?

@xentar_gw: That's interesting. We're using it to extend the summer growing season + to grow more exotic plants in summer.

Dorfkrug,

Electric fans, in addition to the proper vent area, may cool the GH somewhat more, but an evaporative/mist system, perhaps with shade cloth as well, is usually required to get the GH even a few degrees below outside temps on sunny days.

Oh yeah, of course, that makes sense now that I think about it. So we'd need something else for the extreme days. Thanks for your feedback!