Dear sirs,
The whole idea of enclosing the green house is, not only for increasing the heat but also for giving the Green House Effect to the plants,farming inside.
The plants breaths air in the atmosphere throughout the day and night. In the day, they consume Carbon dioxide and omit Oxygen. they consume oxygen and omits carbon dioxide in the night time.
The plants require high quantity of carbon dioxide in the next day, from morning ,for its photosynthesis process for food preparation. In green house, the throughout night omitted carbon dioxide will be stored as the side curtains will not allows it to go out. The carbo dioxide is a heavy gas and cumulates in the bottom portion of the green house.
This is my known knowledge and experts may correct it ,if any mistake, as it is a technical authority subject.
That is the reason, the poly houses are closed all around it and they become Green Houses.
with best wishes, g.p.rao, farmer