You don't usually insulate the PEX. You insulate between the slab and the ground and around the perimeter with a good closed-cell foam designed for the purpose. Then the heating tubes are usually installed about 1/3 of the thickness of the slab from the top.
The fact that you are asking that question here suggests that you need more help that we can give you, because we don't know the details of your situation. A good design requires enough information to do the design and analysis.
You need to get a good thermal design of the whole slab system. If you get it wrong you end up spending a lot of $energy$ to heat the earth. Then, when the energy costs become unbearable you are forced to abandon an expensive system and replace it with another expensive system that doesn't send a lot of BTUs toward the center of the earth where it is not needed.