spray foam insulation is ideal for insulating crawlspace, cathedral ceiling, underside of floor of an extension, basements and many more hard to do areas.

2lbs density spray foam insulation adheres to practically anything and creates air barrier at 1″ thick and a vapour at 2″ thickness

With R6 per inch , proper adhesion, air and vapour barrier  included, where else are you going to get insulation equal or above

If 2lbs density polyurethane spray  foam gets wet you don’t need to remove it and it doesn’t promote mold

Spray foam insulation might be more expensive, but it’s three times the job.