Plants, like humans, wilt in the heat, they need shade, water, and effective air circulation. Few will survive in an enclosed greenhouse with a temperature of over 80°F/27ºC.
Circulating air has a cooling effect on plants, and the easiest way to achieve adequate ventilation is to simply open the greenhouse door together with any vents. Whether to leave them fully or partially open overnight will depend on how much the temperature drops.
When buying a greenhouse, make sure there are enough vents (both roof and side louvres) and be careful not to underestimate the number of vents even a small greenhouse requires.
If the vents are for any reason not easy to open, think about buying a greenhouse that already has automatic vent openers. Many manufacturers will offer them as an optional extra, or they can be fitted to an existing greenhouse.
Auto openers are also very useful if you are away from home for an extended period. On holiday you can relax, safe in the knowledge that the vents are opening each day and closing in the evening.
In the height of the summer, the greenhouse will also need shading. As this reduces light transmission, shading needs to be kept to a minimum, just enough to regulate the temperature.
Shading can consist of internal or external blinds or less expensive mesh. Placed outside, the shading will prevent heat being generated by passing through the glass but will be harder to fit and maintain.
Internal shading, including pegs to clip it in place, is available from Eden and will also fit other makes of a greenhouse that has a recess in the glazing bar.
An alternative would be shading paint such as Bayer Coolglass that is mixed with water and applied by a brush or sprayer. At the end of the summer, it is easily washed off.