Originally Posted by Crash
but wouldn't specifically say what problems.
Sometimes people decide things based on "instinct".
They may be right, but I'd rather give a reason based on consensus or good practice or logic or physics or sumpin'. Figuring out why your instincts are telling you something is not so easy.
You could search "building science" and building code sites to see what they usually recommend, and see what recommendations are "conspicuous by their absence".
Good answers for you may depend on your local climate and your local geology.
You could make having a slab a non-negotiable constraint for this project and have several contractors quote a price. Depending on the quotes it might be worth it to ask a civil engineer for a consult on this.
The thermal inertia and thermal conductivity of your slab would be constant, but the summer/winter ground temperature would change the performance of your radiant system unless the slab is insulated from the ground.