In slab hydronics – garage and snow melt system – GreenBuildingAdvisor

I planned to put pex in the slab of my new garage, and in enough of the driveway to get snow melted from the garage to the front door. This is climate zone 8. The plan was to do either eps or xps rigid boards on top of a layer of gravel, a vapor barrier in the garage only, and then the slab. The concrete guys i have spoken to said they typically put down a mesh, tie the pex to the mesh, then rebar grid over. 

One of the contractors said the rigid insulation under the slab causes the concrete to crack and cure abnormally. And they are worried about the vehicle weight on a slab with rigid insulation under. He suggested bubble insulation (which i think has almost zero effectiveness) or spray foam the ground. I would do the spray foam layer, but the location is remote, and finding a contractor likely will be difficult. And i am not sure spraying foam would resolve concern on the curing issues.  

Opinions on the sub slab insulation is appreciated, particularly related to the contractor’s concerns. And while on topic, any opinions of pex type (pex-al-pex, or normal O2 barrier)?

Tracy