Aerogel is insanely expensive. It has an R-value of 10 per inch and the handy property of costing about $1 per R-value per square foot. So a piece of aerogel 1" thick by 4' x 8' would cost $320; the equivalent piece of 2" thick XPS foam board (also R-10) costs about $50 these days. So with aerogel you're paying a 500% to 600% premium to have your insulation be half as thick as XPS would be - and to essentially have no compression resistance at all (vs. the 20+ PSI of foam board).