['97ventureowner]

I was presented with the same problem in my house when I wanted to add an air conditioner because most of the windows on my main level are "sliders". I ended up having to go to Sears to get an air conditioner that would work in my window,( cost $449 for a 12,000 btu unit 4 years ago. ) When I installed it in my window I ended up with a space about the same size as the a/c unit's height above the unit itself. I ended up taking a piece of plexiglass and putting a metal frame around it and installing it above the unit to take up the space that was left over. If you don't have as much space as I did, you could use insulating foam board, (I've seen it in blue,green,or pink colors,) that would do a good job of filling in the space while insulating the cool air from your a/c from the warm outside air. I also ended up buying long sections of foam sections that were 1"x1", and 2"x2" and about 2 to 3 ft. long and cutting them to fit in any cracks or open area around the a/c unit.They "smush" down very well to fit in the open area, then expand, sealing the area.