These are great fans--I bought one for the upstairs hall, and one for the downstairs dining room of my colonial home.  WIth both of them running on exhaust, the whole house is cooled off within an hour on a cool summer night.  There is a bit of understanding needed with the dimensions though.  I had no choice but to mount the fan on  the inner window frame for both windows, because the extensions do not reach far enough to mount on the outside of the window frame.  The windows cannot be shut with the fan mounted, because they have a "lip" at the bottom, that overhangs a bit and runs the length of the window, to make opening and closing easier.  The lip interferes with the window closing while the fan is in place, so the fan must be removed from the mount if heavy rain is expected (Hurricane Irene, for instance!)  Even so, the fan is easily removed, once the screws are in place.