How do I install a vapor barrier and insulation in an existing floor?
Super late to the discussion, but for those who are still using this as a reference, the important thing to note is that open cell spray foam insulation is not a vapor barrier. Use closed cell foam if you are intending to create a vapor barrier. Another solution that may be more cost effective would be to cut foil-faced EPS and place to underside of substrate (make sure it's a tight fit, foil face down). Tape edges to joist to ensure a seal and then add insulation. This isn't a 100% vapor seal, but better than no barrier at all. You could also look into liquid applied vapor barriers.