Plants, like humans, wilt in the heat, they need shade, water, and cooling air circulation. Few will survive in a greenhouse with a temperature of over 80°F/27ºC.
The easiest way to achieve adequate ventilation is to simply open roof vents, side vents (louvres), and the greenhouse door. Humidity should also be maintained by regulalry 'damping down' or wetting hard surfaces.
So when buying a greenhouse, make sure there are enough vents (both roof and louvres) and be careful not to underestimate the number of vents even a small greenhouse requires as these have more glazing in relation to floor area than larger greenhouses.
Roof and Side Vents
All greenhouses should be fitted with one or more roof vents. For every 50 sq ft of floor area you should have 10 sq ft of roof ventilation, this will provide a regular cooling change of air within the greenhouse.
The addition of side vents or louvres will assist air circulation and opening the door will also be very helpful.
Whether to leave vents fully or partially open overnight will depend on how much the temperature drops, and if the weather is changeable you may have to leave vents partially open.
If the vents are for any reason not easy to open, and to save you time and trouble, 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 and hence plant growth, shading needs to be kept to a minimum, just enough to regulate the temperature below 80°F (27°C). If you grow succulents, shading is not required.
Shading can consist of internal or external blinds or less expensive mesh which is pegged into place. Positioned outside, the shading will prevent heat being generated by passing through the glass but can be difficult to fit and maintain.
Internal shading is not so effective since it does not prevent sunlight passing through the greenhouse glass but can be more easily adjusted and maintained. Also bear in mind that shades can also be useful in the winter to prevent night time heat loss.
An alternative would be shading paint such as Bayer Coolglass that is mixed with water and applied by a brush or sprayer. More can be added as the summer progresses and it is easily washed off in the autumn, although traces can remain on acrylic and polycarbonate glazing.