After doing some thermal investigation, I have restored a bunch of late '50s steel casement windows for use on an addition. I had found a place, Innerglass, that made interior, spring loading, storm windows that sounded like they would take my casements from beautiful to practical.

However, recently I have read about "magnetic bellow" design window inserts, so I am looking for advice.

I live in central Illinois. Lots of the casements will be facing south. I know these windows arent the smart choice, but I really like them.

Any advice on how to make these windows the best they can be, thermally, would be greatly appreciated.