The easiest way to ventilate a greenhouse is simply to open everything up. The number of doors and windows a greenhouse has is often determined by its size; for example, the smallest greenhouse kits have no windows. If purchasing a kit, choose one that has at least two windows to aid in ventilation. If building a hoop greenhouse, design wide entrances at both the front and back that can be held open. A stagnant, too hot environment will damage plants as surely as frost will.
Open windows on opposite walls to create a cross breeze. Attach solar-powered window openers for your convenience. Solar-powered vent and window openers ensure that fresh air will get into your greenhouse even when you are unavailable to open the windows manually.
Install fans to circulate air and ventilate the greenhouse. Perch a solar-powered fan in an open window on the greenhouse roof. Stand a portable fan in the doorway of your greenhouse for increased ventilation. You may also install small, clip-on fans in different corners of the greenhouse.
If you have a solar greenhouse with no electrical power, you will need to use solar-powered or battery-powered fans.
Mount an exhaust fan in a window in the greenhouse. An exhaust fan pulls hot, humid air out of the greenhouse, unlike other fans that circulate air. When used together, these two types of fans will quickly cool off and ventilate a greenhouse.
Monitor the temperature of the greenhouse. While ventilating the greenhouse, avoid bringing the temperature below 70 degrees F. It is good to keep a greenhouse thermometer in the greenhouse at all times and check it at regular intervals. With a little experimentation, you should be able to maintain the heat of a greenhouse between 70 and 90 degrees F.