Steve:
That's how I did my garage for "R" value. A commercial spray foam firm comes in and gets it done in a matter of an hour for a job of that size. Vapor barrier is not needed in that circumstance but as Jim says be sure about your electric wiring because once that stuff hardens its encased everything behind it. Great for R value and critters can't get behind there and cozy in it like they can with fiberglass batting.