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If you are asking "should you use insulation below a radiant slab", the answer is a resounding "yes". The ground is a heat sink which, below frost-line typically is the annual average temperature for your area; in our case, Boston, that is about 55 degrees. Although it eventually warms up, that is on your dime. At least R-10 around the perimeter and at least R-5, if not R-10 also, underneath it all.
Your area and local codes will dictate what the thicknesses are, regardless. But even if they say R-5 on the perimeter, I would go R-10 if you need heat at all.
As for the sequence of construction, depending on your area and soil conditions, pea gravel, a vapor barrier sheeting, sometimes more pea gravel as ballast, insulation, welded wire mesh (to which the tubing often is tied), then the pour. The R-10 perimeter insulation acts as an expansion joint too.
There are systems of plastic screw chairs which allow you to clip the tubing to the insulation directly, not to mention that there are a good half-dozen variations of what was mentioned.
Might I suggest the RPA's "Radiant Basics" and "Radiant Precision" books (written by John Siegenthaler) as a good place to start? I refer to those books regularly among others.