Greenhouses have to be designed to not merely capture solar energy but also adequately ventilate excess heat if temperatures get too hot.
You should always make sure you have adequate vents in the roof of your greenhouse as heat rises and you want that heat to escape.
Greenhouse roof vents work really well as they are located where heat gathers naturally at the peak 
of the roof.
If vents in the roof aren’t possible, make sure to at least have a windowpane to two you may open.
Ideally you need to have cross ventilation so two windows on two sides of the greenhouse work best.
However you should not leave the vents or windows open at nighttime as you’ll lose any heat gain and cool your greenhouse too much.
So you close the vents or windows during the night and what occurs the next morning: you forget! Today your greenhouse overheats and if you’re unlucky, you lose some plants.
Or you forget to close the vents at nighttime as well as your plant life suffer if the night time is too cool.
Here is a whole lot even more information on this subject Greenhouse Auto Vent