Just finished framing a single story 800 sq.ft. addition to an old farmhouse.  There’s been a big push around here for icynene insulation, a spray in low pressure foam that makes it sound like the best invention since the cordless drill.  Cost is around $1 per sq. ft. of sprayed area.  I especially like the idea of spraying the crawl space perimeter.  Is it really as good as they say?