Ben,
You have several choices in separating the generated heat from the heat loss into the ground below. Even those floors which are radiant concrete slab on grade get a positive heat exchange by putting a plastic sheet moisture barrier on sand covered with 4" of rigid foam on top of which goes your tubing pylons tied into the rebar mesh for strength. . If the idea of a concrete floor (can be made to look like stone if you wish)does not appeal then a conventional joist system (works with fabricated joists too) with two inches of rigid foam and a mylar outer skin goes underneath the joists or tji's.
blow in about 4-6" of low expansion foam. your inlet tubing boards go on top (with or without subfloor underneath) . Prior to mounting your subfloor/tubing boards rough in your plumbing on top of the blown in insulation. The stadler-vega plumbng system is easy to install and you have no glue fumes to inhale. (google em up).
Put either a hardwood (nail carefully) or glueless laminate floor on top of your tubing and it is a snap to keep it at 78 or so degrees year round.
Yes I have a list of preferred vendors which I could email to you if desired. No I am not being compensated by any of them.
deralte