Shutters for better home thermal efficiency?
Shutters would benefit during the heating season in at least two ways 1) stop most long-wave heat going out and 2) create an air insulation layer of some sort. During the cooling season, it will at least keep radiated heat out.
I guess the trouble is the cost, as well as the massive expanse (number, width) of windows in some houses. There's really no place in my current house to use shutters except in a handful of room. Windows generally have to be inswing too.
There was one Italian household near where I used to live that had the roll-up external shutters (another country where shutters are popular). Not sure if this was used for security, privacy or for insulation purposes.
Curtains and cellular blinds are said to be an option but humidity in my parts means lots of window wiping every day (yes, even in the 0.19 u-factor coated triple-glazed doors I just had installed).