Since plants suffer in a greenhouse that is too hot, and young leaves can be scorched in bright sunlight, use some form of shading during summer. A cheap and effective method is to apply a coat of shading wash to the outside of the greenhouse in early summer (image 1). You can then wash it off in late summer when the heat of the sun is less intense. Sheets of fine mesh are also available. These can be attached to the outside of the greenhouse, although they are less effective than shading washes (image 2).
Even in winter, greenhouses need ventilation to remove damp air, but in summer it is vital to ventilate well to control the temperature. Vents in the roof allow hot air to escape, while low side vents admit fresh, cooler air from outside; open both to achieve good circulation, but reduce ventilation during cool nights.
In spring, greenhouse staging can be packed with trays and pots of germinating seeds and young plants waiting to be moved outside. When they are planted out, however, the staging often takes up valuable space. Shelves that fold down to make way for the main greenhouse crops are a useful solution to this problem.