I have built a wooden frame with an emergency blanket on it that I lift onto the window when needed. It lets some light in, but no thermal radiation. The silver side on the outside reflects heat, while the golden side lets thermal radiation pass through to the outside. The problem with aluminium foil was that it also kept heat in, and also it was annoyingly noisy when ventilating the room.

Still doesn’t stop the surrounding walls from soaking up heat.