I'm building a 24" by 40" pole barn in centeral Maryland.  I'm planning on adding pex tubing into the slab, but I'm not sure how to layer the insulation and under lying layers.  None of the builders I've talked to know either.  Most just dump 4" of stone and 4" of concrete.  What's the best combination?

Thanks, Ron C

Ground conditions dictate substrate, but on good ground a half foot of sand, 2" of polystyrene (don't forget to cap the edges) and 1/2" barrier PEX on 12" centers a foot off the outside walls.

I would not use sand because it moves around too much. The same with clean stone. The lack of particle size variation keeps the particles moving and settling. 
The best thing to use is DGA (dense graded aggregate). It has particle sizes ranging from 3/4" down to fines, and no dirt or organics. Here in NJ, we also use what is called "State DOT mix 5" available from the quarry for road building projects.
Build up the sub-grade no more than 6" at a time and compact with a machine. Level the subgrade to +- 1/2". Insulate the slab edges with 2" foam, and under the slab with 2" or whatever you can afford. Then a 4 mil vapor barrier, tubing, steel, and concrete.

DGA is good stuff. Around here the vapor barrier has to be 6 mil and goes on the ground first. I thing the XPS is a fine vapor barrier; taped if you're afraid of vapor.

Thanks, that will help alot.  I was mixed about using crushed stone or sand.

Have any of you used a product like Ez Floor?  It's a "all in one" vapor barrier, insulation, tube support system.