To be frank, I'm not sure if replacing the single pane with double pane is: A - something I can afford at the moment and B - something I can DIY (which adds to point A).

I have a feeling that the windows are a large part of my electricity usage (due to AC constantly running), I have them "foiled" and really tempted just to board them up. Before I go to that extreme, I was thinking maybe there's something I can attach to the outside that would essentially add that 2nd pane, or at least block some sun from hitting the glass.

suggestions?