Radiant slab heat with tuck-under garage

[premis]

I'm planning a new house build, I plan to have an L-shaped garage with one section 24x22 and the other section 26x40. I'll have a tuck-under garage (garage under a garage) under the 26x40 section.

I'd like to at least put pex in the floor so I have the option of heating the upper garage in the future. I know the normal process is 6mil vapor barrier on the dirt, then 2" of rigid foam board, then pex on that. Do I do the same thing on top of the under-garage? Would I need to put a vapor barrier down on top of the pre-cast? I'm assuming I'd want 2" rigid foam board so I'm not heating the ceiling of my under garage, correct?

For anyone not familiar with a tuck-under, here is how they do it. -->

 

[Bondo]

Would I need to put a vapor barrier down on top of the pre-cast? I'm assuming I'd want 2" rigid foam board so I'm not heating the ceiling of my under garage, correct?

Ayuh,..... The vapor barrier would be under the lower floor, not the precast,......

I'd want the foam, 'n rebar/ mesh over the precast though,....

 

[Quick240]

My garage is like this. For the upper garage they did a liquid waterproofing across the slabs after they were grouted and before they poured the topping layer.

 

[larry4406]

Why not heat the ceiling of the under garage? Heat both spaces via the same slab?