Insulation is specified just like every other mechanical application. How thick the slab is and the insulation to support it is determined by its intended purpose. To make it simple, insulation is a matter of percentages; the more you cover the less you lose in terms of comfort and efficiency.
Rigid insulation is specified by PSI just as concrete is specified. In the case of XPS the manufacturer writes the spec. so the educated designer or engineer (even a literate layman) may choose the correct rigid insulation for the application at hand. In this case it is slab-on-grade concrete slab with radiant floor (PEX tubing) attached for space heating.
The most important thing about pouring any slab is the substrate. "Verify that substrate is flat, sound, clean, and free of oil, grease, [objectionable air surface voids], [fins], [irregularities], [materials or substances that may impede adhesive bond]."
The common specification for a residential/light commercial slab is 25psi e.g. Owens Corning Formular 250 or equivalent. If you are really worried about "heavy" equipment and shun the help of professionals, Formular can be specified to 100psi, read commercial jet hangers...no worries.
You should also ignore those promoting blankets or foil for use as insulation below a radiant slab. Yes, you must insulate under a heated slab and yes the perimeter is the most important place to do it, as insulation is all about the temperature differential. When you heat a slab you make the difference between the inside and the outside temperature greater. Moving the tube further away from the outside wall is of little use as concrete has an equivalent R-value of R1 per foot.
Be careful taking advice from your peers.
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