When I first started looking at building my chicken coop, the first thing I did was go out on the net and look for pictures. There are all kinds of examples out there, but I needed to go cheap. Cheap as in building the window out of scrap material. I see no reason to buy a window, when one can be made from left over scraps from building the coop.
Besides ventilation, the screened in vent windows allow the chickens to be viewed without opening the doors. At ni time, if you want to check on the chickens, walk up to the coop, look in through the screened in windows to check on the chickens.
In the following article I will try to describe how to install a screened in window for a chicken coop. If some steps are left out, I apologize. But hopefully this article can give you the general idea.