I edited my previous post with an example that i robbed from here:
http://portal.ncdenr.org/c/document_...&groupId=38322
The topic of underfloor insulation doesn't have a lot of coverage, mainly because engineers regard it the same as the boundary between the ceiling and the attic.
Another good article on air gap u-value:
http://blogs.mentor.com/robinbornoff...ation-part-iv/
If ur really interested, you can backtrack to the 3 previous posts from there