I have a window fan for a large horizontal sliding window in a house with no central air. It's on the second floor and the room generally sits at about 78-80 degrees at night making sleeping incredibly uncomfortable with just floor fans to push the hot air around. So the window is about 4 feet tall and the fans extenders won't cover the entire window. Will running the fan to intake the cold air from the outside still work or will it just get blown in and immediately get sucked out from the uncovered area due to the pressure differential. If thats the case what can I do to prevent that.

 

just cover it with cardboard are something

 

1/4" plywood can act as extensions. You do need to block the whole area around the fan and you also need some way for the air to exhaust. Some will blow through bath fans or range hoods, but ideally another open window in the same or adjacent room.