You don't say what you are growing but most plant roots like temperatures of about 70F (+ or - 10 degrees) . Soil microbes start dying after 90F. My greenhouse is heated with solar hydronics but I have a concrete slab with the coils under the concrete. I had to add more tubes to one wall to keep my plants totally happy (its a lean-to so more coils to the house wall). I'm just trying to keep the air temperature above 55 - 60F. It seems to me by making the soil your heat sink, you may cook some roots to keep the plant canopies happy.
I hope you have a way for the foundation to drain, otherwise, you will have a swamp.
PS: You can 'Suggest a Change' to move this thread to the Greenhouse Forum or ask
@Calif_Sue to do it for you. Its great to hear from someone who is doing the same thing I am.