insulation?most R per inch.

[robertglines1]

I have about a inch to put insulation under slideout floor..poor pre planning has left me with limited space!I also would prefer not to put inside on floor because it would raise floor height.I have thermax (foil faced foam board). can anyone suggest something better? area 30inch by 15ft is exposed to weather.  Bob

[artvonne]

  I thought pink foam had the highest R value for anything that came in a flat panel. I dont know what the R value is for spray in foam, but something tells me that wouldnt work in your situation?