Floor reconstruction in old buildings with insulation under the floor slab

Hello, I want to completely renew the floor on the ground floor of my old building (strip foundation with fieldstone) and install underfloor heating. From the top edge of the finished floor, I have removed the old floor to a depth of about 35 cm, so that only sandy soil remains in the rooms. I would now like to first lay a foil on the sand. Is it advisable to already lay an insulation board and pour the new approximately 15 cm thick non-load-bearing floor slab on it, or can I omit the insulation there?

Regards

The insulation under the floor slab only makes sense if it is continuous, otherwise the insulation above is more sensible. Since you cannot insulate under your strip foundation, put the bitumen foil / coating / waterproofing on the floor slab and then the FB insulation and then the screed.