I've been reading this forum off and on for years--great stuff! I thought I'd share a neat little leaky window solution for seldom used rooms. I have a large victorian era house in the northeast with original windows. After much research, I've decided that replacing the windows with quality modern units is not nearly as cost effective as the marketing hype would leave you to believe. Besides, the old windows are period correct and that's important to me. I've had the windows recaulked and sealed and they are far tighter than before. Further experimenting, I've installed custom storm window insets in a a couple rooms, and have attached simple bubble wrap packaging material in another room. The bubble wrap works as well as the storm windows--maybe better. Of course, the storm windows look better, but the bubble wrap turns out to be a very inexpensive way of increasing window effeciency. The air pockets must have insulative properties, and the plastic (which is sealed all the way around the window) stops drafts.
My next step is to try a couple Window Quilts and see if their claims pan out. They look promising, and the technical parts of their website (http://windowquilt.com) really summarized the whole issue of window efficiency and the role they play in room comfort in an easy to understand manner.
Finally, and this one's a little out there, I'm thinking of putting solar collectors on two large south facing windows to see what kind of free heat they can generate. Does anyone have experience with these? What designs work best?