Ventilation of the greenhouse is required to stop the temperature inside rising so high as to “cook” the plants. If you install a max/min thermometer in your greenhouse, you will be astonished to see how hot it gets on a Summer day. The temperature will easily reach up to 40 C even in Scotland. Above 25 C, plants become seriously stressed and can wilt or die. Plants use a lot of extra water at these temperatures and if they run short of water, they will die. As a rule of thumb, if the temperature in the greenhouse is too hot for you, it is too hot for the plants and extra ventilation is required.
Automatic ventilation devices are luckily available, that will open the ventilation windows when the temperature gets to a certain level in the morning, and close them again as it gets cooler in the evening. You can adjust the temperature that they will open and close at, by screwing the rod in or out. As they work by using the expansion and contraction of a metal rod, caused by the change in the temperature of the air inside the greenhouse, there is no need for an electrical connection. ( T P £20 )
While they are designed and made for roof opening ventilators, if you fit them to vertical windows, you may need to help them to close fully by fitting the device shown on the r.h.s. of the photo. It consists of a stone in a net, suspended from twine passing over a bent nail, and attached to the window. Yes, it may look a bit “Heath Robinson”, but it is cheap and it works. I am an Engineer!!