Aerogel Windows

Instead of using glass in windows, which wastes precious thermal energy, use aerogel, a material nearly identical in compostion but with unusual properties.

For those of you who don't know, aerogel, created by NASA, is the world's least dense solid. It is pure silicon dioxide and sand, just as is glass, but is a thousand times less dense than glass because it is 99.8 percent air. It is sometimes called "solid smoke," for its cloudy translucent color and super-light weight. Surprisingly, this seemingly brittle substance is durable and easily survives launch and space environments.

The most relevant thing, though, is that aerogel provides 39 times more insulating than the best fiberglass insulation. This could lower heating costs since you wouldn't have all the indoor heat escaping through the glass windows, which provide little insulation (unless you live somewhere warm, in which case it would be the other way around).

I'm aware that they have double-paned glass that provides decent insulation so this might only be for a niche market (Antartica, Siberia, Alaska, etc.).