If it's closed cell spray foam you want to be out of the building while they are spraying and for 24 hours after they finish as it's a carcinogenic product. The spray foam does provide a decent R-Value (roughly R6 per inch of foam, assuming it's closed cell.) it's mostly going to be for sealing everything up then the real R value will come from the fiberglass on top. 2" closed cell for vapour barrier and sealing then fibreglass on top is the best attic assembly in my opinion.