Basic Ideas:
You will want to layer the foundations with sufficient insulation to hold your heat in your floor and not have it escape to the ground.
Under your floor you will want:
Another Option: is that you can insulate all the way down the the foundation
HUG Hydronics radically simplifies in-floor heating; from simplifying plumbing to being able to use multiple heat sources for dual fuel applications, including renewable such as solar and wood.