Window measurements should be taken before buying the air conditioner, and when installing the air conditioner, it might be necessary to remove the window so the unit can fit. After the unit is in place, all air leaks should be plugged, and the window should be secured with boards.
It's especially important to measure the window ahead of time because, basement window sizes often differ from standard home windows sizes.
To plug up any space between the window frame and the unit, boards can be placed under the unit so that it rests on the outer sill. The air conditioner should then be titled a bit to ensure proper drainage outside. Even with this shelf in place, there still might be openings along the side or top. These can be filled with Styrofoam or plywood. If there is a sliding window, tape can be used to produce a seal along the sliding board so that the window can still be opened.
When an air conditioner is installed in a basement window, home security is also an issue because the window locks no longer function. To keep a stranger from prying open the window, a board or stick should be firmly wedged between the upper window frame and the top of the sash.
The weight of a window air conditioner is another consideration. Some portable air conditioners weigh as much as 100 pounds. A unit that's too heavy might easily get dropped during installation into a basement window. Also, an air conditioner should not be installed if the wood on the sill is rotting, because the sill might collapse from the weight of the unit.
Some older windows air conditioners with external brackets require holes to be drilled in the house siding.
When installing a newer model with supports that fit into the window opening, the supports should be set up according to the manufacturer's directions.