One idea for improving the efficiency of our original double hung windows is to add an interior storm. After picking up a couple furring strips at the big box store, I mitered 45 degree angles and did a dry fit in one of the kitchen windows. I would probably cut a groove on the inside edge for a large piece of Plexiglas. Of course the wood will be stained to match and I'm planning on using felt weather stripping around the outside edge. The tricky parts I'm still undecided on are the method for attaching the storm to the window frame and how best to connect the storm frame at the corners. Any suggestions?