Jason Wells got to toy around with a few blocks of Aerogel, the fantastically light (and fantastically expensive) material made famous by its use as insulation in NASA spacecraft like the Mars Rover. Using just everyday materials from his house, he managed to test the futuristic product's strength, optical properties, reaction to different liquids and temperatures, and electrical conductivity. He concludes from the experiments that it should work really well as a fire retardant or insulation, as well as pulling moisture out of pretty much anything (including his finger!). Aerogel weighs only three times as much as air, but is even more effective than your everyday pink insulation. It may only be the world's coolest insulation material, but are you the world's coolest anything? Didn't think so. [Jason Wells via Crunchgear]
An empty wall full of air would be a good insulator if it wasn't for convection currents in that air, so you stuff the wall full of something that is just dense enough to prevent the air from moving around but the result is still mostly air, something like spun fibreglass. The fibreglass itself isn't that great an insulator, it's the trapped air that does most of the R-factor heavy lifting.