When you need a window that slides up & down, the Double Hung Interior Storm by Innerglass is your choice. It only needs 1 1/8"depth in the opening for the tracks to be installed at the left, right, and top. There is no track needed at the bottom. The seal is made with Q-Lon weather-strip on the lower sash. The tracks are in 2 parts. A "C" channel is applied with a single face foam tape into 3 sides of the opening. The double tracks are inserted into the C channel.
This arrangement allows the inner track to be shimmed to allow for "out of square" openings. The sashes are spring-loaded on the left side and are easily inserted into the left track and then into the right track. This feature makes the sashes easily removable for cleaning. Innerglass will match the meeting rail height of the existing window so that the look will be unchanged.