I used to have an AC unit in a window in my upstairs, but it was too hot for it, I ended up getting a "swamp cooler", an evaporative device that you pour water in and it runs like a regular fan with windows open. It works well in my climate, might not work well in high humidity, and it is very inexpensive to use. The downside is that it is large and that you have to pour water into it throughout the day. There are built-in units available. I got the idea to have one from a friend whose whole house in Northern California was cooled with this type of system built in. It was an old farmhouse - in 100 degree weather even the upstairs felt good. Having the windows open is nice, too. Although it might not work security wise for those that need to lock up all windows most of the time.