at 2:09:17 AM
Do or did you have any moisture problems on your existing concrete slab before? If yes, then it is very important to break the capillary action by filling in the proper amount of crushed stone ! Don't use sand for this !!!
Thereafter (on top of your crushed stone) you could use a layer of sand to cover the sharp edges of the stone. On top of this layer sand you should place a layer of a Poly Vapor Barrier to prevent the rigid foam from soaking water. Most rigid foams are not "clost celled" and will wick water if exposed to it. In this case your insulation would be going to zero and the foam would act like a sponge !!!
It is not a bad idea to install another double layer of the Poly Vapor Barrier on top of the rigid foam a) to prevent the foam from going wet during the concrete install and b) this layer will act as a cleavage membrane so the concrete slab can "move" a bit. Around the perimeter you should install a 10 mm thick foam strip a bit higher than the thickness of your concrete slab. This is very important for the necessary movement of your slab because it will always a bit expand and also a bit shrink.
Now you can install your concrete slab with a thickness of at least 4 inches! Keep in mind you'll need expansion joints about every 10 to 12 feet. Don't forget to rebar with 10mm bars (don't use mesh because it is not appropriate) and install the bars in a raster of 4 to 6 inches by 4 to 6 inches. The "bar-grid" must be located about 2/3 deep from the finished surface. That means you have to put at least 1 inch to 1 1/2 inch thick "spacers" (don't use wood as spacers !!!) Underneath your grid to keep it from the ground (the rigid foam) when you pour the concrete in. It is not good at all if the "bar-grid" is laying on the ground while pouring the concrete in and thereafter lift the grid up a bit like some people do it, because it would be not properly embedded at the right place inside the concrete slab !
If you don't need insulation underneath your slab, it is also possible to pour the concrete right on top of your first "sand-bed". This layer of sand will also act like a cleavage membrane. And instead of installing a Poly Vapor Barrier you could use an additive to mix the concrete which would make the concrete water tight.