[911Rob]

I'm not a salesman for these systems and I've only installed the "in slab" types, both interior and exterior.

But I can't help but think that a system installed in between the joists, after the fact would be as efficient as having the loops under the flooring, above the subfloors? You're heating a whole bunch of space below the subfloor for no reason?

As for what is critical.... working against leaks, now and in the future. Continuous piping is a must.