David (finallyME), this is hilarious!! I too emailed Aspen Aerogel yesterday but got back a fairly nebulous answer. I basically wanted to know where to find the products, and they told me. You asked a better question and got back a better answer.

Anyway, I've handled Aerogel as well as space shuttle tiles..lots of 'em...from the Columbia shuttle crash. Aerogel feels like crunchy perlite, only more brittle. If you closed your eyes and someone placed a 1" cube of solid aerogel in your hand, you wouldn't feel it....very light stuff. Space shuttle tiles 'heft' like blocks of wood, a little heavier than balsa, with a consistency more like fire brick. Both are made from the same thing...silica.
I'm not absolutely sure but it seems to me that "flexible'' aerogel would have to be granules embedded or infused in a fibrous matrix of some kind, thus, not pure aerogel. That would explain the weight. I suspect the insulation value would still out perform other battings. I'd have to see an insole to understand why it doesn't crush. Notice, there is no comparison between goose down and aerogel on the website.