Insulation, Radiant and Vapor Barriers - Insulating floor of raised room?

Hi all. My newly purchased home has an addition which is a single room that does not share the foundation with the rest of the house. It's just a small room raised up on posts about 2.5 feet off the ground. I suspect that it is going to get really cold come winter and am thinking of insulating the floor of it.

I will, of course, put a vapour barrier in first, what what is the best way to insulate it? I would probably use fiberglass, but I'm not sure how to best keep it in place. Also, if I do go with fiberglass, would I need to put something else under it to keep it away from the elements?

Thanks!

I would still put a 6mil poly down on the ground. Cover it with small gravel. Now put a R19 are more if it will fit. up in the joist paper side to the floor above. Now any cheap plywood, any siding. Or some have just used the fiberglass screen to cover on the under side of the room here and hold up the insulation. Do let all the 3 sides open all the time. Could box it in with just lattice work like.

ED ;)