Underground the best is Foamglas (do a Google) but I doubt that you could buy it and the cost would probably deter you. Next best would be Armaflex or Rubatex which you might be able to buy from an HVAC supply. I think that Grainger has Rubatex in limited sizes.
The important thing to remember in underground installations is that you MUST keep the insulation dry (fiberglass is a big NO). I would use nothing less than a one-inch thick wall. You might need a sheet metal or plastic covering anywhere that rodents might gnaw on the insulation, I know they will gnaw on any rubber or foam type of insulation.