I have a house built on a crawl space (Vented) with wood floors throughout the 1st level. Problem is heating/cooling losses from floor (uninsulated crawl space). Question is can I install foam board to bottom of floor joists for insulation? Reason for foam board type insulation is due to access and ease of installation. Thank You

 

I understand why you are thinking of using the foam board insulation, but the "R" value just isn't there. I would probably put fiberglass batts BETWEEN the joists and secure it with insulation ties, nylon mesh, etc.