I installed triple pane residential sliders in place of my outdated windows. They not only do a good job with thermal transfer but the elimination of outside noise is astonishing.
If you want an inexpensive way to insulate the window openings, carefully cut 1"->2" extruded polystyrene (blue board foam) to friction fit the openings. Put a strip of tape on an edge for easy removal. You can at least put the foam board in the openings at night and easily remove them in the day. It will make a big difference.