ᐅ Floor Reconstruction in an Older Building with Insulation Below the Slab

Hello, I want to completely renew the ground floor in my old building (strip foundation with rubble stone) and install underfloor heating. From the top edge of the finished floor, I have removed the old floor about 35cm (14 inches) deep, so only sand remains in the rooms. I would like to first lay a membrane on the sand. Is it advisable to already place an insulation board on top and then pour the new non-structural floor slab, approximately 15cm (6 inches) thick, or can I skip the insulation there?

Regards

Insulating under the concrete slab only makes sense if the insulation is continuous; otherwise, it is more practical to insulate above the slab. Since you cannot insulate under your strip foundation, apply a bitumen membrane/coating/sealant on the concrete slab, then place the floor insulation on top, followed by the screed.