I need an air current in this room, even when the sun shines directly into it.
I was thinking covering the hopper window with either paper or reflective foil and leaving it a bit open, but I don’t know if this is a stupid idea.
A closed window means an unlivable room.
Reflecting the sunlight away will reduce the heating you get from the sun. As far as opening a window or not, open if outside is cooler, else close.
If it’s pretty windy then it may be worth opening the window if the temperature is similar.