http://www.engineeringtoolbox.com/he...nks-d_286.html is worth a look, then. With bare steel it looks like 1" of foam might be a good value. Too bad it doesn't show numbers for polymer walls.

Can you really get a worthwhile layer of insulation on a pool bottom for reasonable cost? I'd be worried about rigid foam breaking, and flexible foam shifting or developing water pockets. This guy (http://ambivalentengineer.blogspot.c...ated-pool.html) was ok with it, though.