A lot of sunlight comes into your non-northern windows over the course of a hot day. This has two negative effects. First, the sunlight makes the room temperature hotter, and second, the bright light makes the room seem hotter (a psychological effect). It’s a fact that if two rooms are the same exact temperature, but one has a lot of sunlight and the other is dark, people prefer the darker room.
Solar screens reduce the amount of sunlight coming into a room by up to 90 percent. It costs around $2 per year to cover a typical window, and the effect is dramatic. You get the most heat reduction in your house when you install sunscreens on the biggest south-facing windows.
You can tack sunscreens up with ordinary thumbtacks, and they look good from both inside and out if you take care to cut the screen corners flush with the window frame. You can make a retractor mechanism for a few dollars. This device allows you to retract the screen when you don’t want it to cover the window. (For the same reason a room seems cooler in the summer with a screen, the room will seem warmer in the winter without the screen.)
Chapter 25: Ten Cheap Solar Projects and Devices 365