Insulation, Radiant and Vapor Barriers - Building new, builder suggests foam.....

Hello all. New home construction, two story concrete board siding, crawlspace, bonus room over the carport. Average ventilation around crawlspace. Excellent ventilation along soffit and with off ridge vents. Floor of crawlspace at this time is simply red Georgia clay. My builder is suggesting we place a plastic vapor barrier directly on the dirt then spray the poly foam into the floor joists, outer walls and attics. Tell me the good the bad and the ugly about foam or any suggestions. I want to put some fans underneath with thermostats or moisture operated controller.

Im having foam applied to my house next week. I converted a garage into a family room with a cathederal ceiling. There are two types of foam, open cell and closed cell. Open cell foam has a R value of 3.5 / inch and from what Ive been told is that it has the possibility of trapping moisture. The closed cell foam has an R value of 7 / inch and completly seals the cavity that its sprayed on. Both are vary expensive around $3.50-$4.50 per square foot. Shop around to get the best price.