It sounds like a good Idea
“Spray foam is a good moisture barrier with a low perm rating“, False. Closed cell spray foam has a perm rating of 2.6 at a thickness of 1?. In order to get to a perm rating of less than 1, the spray foam will need to be a minimum of 3.5? thick. This type of insulation has a fairly high R-value at 6.0 per inch. This means that you will need to buy an R-21 worth of spray foam in order to get a perm rating under 1.0. This product is priced by the square foot per inch of thickness.
The other major issues with spray foam are; if it is sprayed on your floor joists or rim (band) joist you will not be able to inspect or repair your flooring system, it is widely known that spray foam can trap moisture and invites wood boring insects, and the building code requires an inspection gap of 3? on the foundation wall for termites.
Spray foam may be a good alternative for the parts of your home that are above grade, but it is not a superior product for a crawl space regardless of the salesman’s insistence that it is an “air barrier” or that it is a “moisture barrier”.
Photo courtesy of Justin Nickelsen from Nickelsen Home Inspections LLC