I just read the whole thread - not usual for me- but am interested in what you all have to say on the subject. My most dramatic moment with quilts at the window was when we had window quilts installed in our home in Colorado along the front range. We had pretty good sunny days there, and loved our big windows, but at night, the cold came thru and chilled me to the bone. We got the window quilts (commercial) and I was amazed at how warm they kept the room.
In this house, I made my own version of them just using some old unused upholstery fabric, insulbrite, and an old sheet for backing . I just tried it out to see how it would work, and hung them from a simple spring tension rod just inside the mini blinds that were already in place. I wanted them to keep the morning sun out of my computer room in the summer, cause it was so hot! They really worked and also kept out the light for my grandchildrens naps, and I just pop them in and out using the spring tension rods. Kinda temporary and lazy, but effective.
Good luck with yours - it is a good idea!!