Foam board

[thehandyman]

1980 24ft Caravelle
OK removed the old pink and gross insulation from the belly pan. Plan on putting in ridged foam board. Is it better to glue directly to the floor or add some wood strip first then the foam board so there is an air space?
I wonder who's idea it was to put the insulation on the frame then the floor over that. It act's like a wick on top of the frame if you have a leak.
Was surprised that there was only some surface rust on the frame. Some scraping and a coat of POR% 15 took care of it I hope.

[Belegedhel]

I'm not sure why you would want an air gap for foam board. Seems like that would be counter productive. I used a few layers of foam board glued and screwed to the underside of my sub-floor, and finished with a layer of reflective bubble wrap (reflectix). I did leave a healthy air gap on the underside of the radiant barrier (as recommended by the manufacturer).

[Lumatic]

What I have seen, but not tried, is use canned expanding foam around the edges of the foam panels.