Keep in mind that heat always moves to cold.
You are heating the room, the heat disappears into the ceiling, then the walls and finally the floor.
Wherever you place your heating, most heat is emitted in straight lines and then moves by convection.
Think of a Thermos flask the contents are warm, the heat is retained inside the Thermos by a vacuum, the only heat loss is via the thin walls where they enclose the stopper.
To keep the heat inside your room, you need to insulate and the best way is to line the ceiling, walls and floor with sheets of polystyrene, glued to the room side of the ceiling, walls and floor. This keeps the heat in the room.
If you place the insulation below the floor, the heat from the room, warms the floor, the joists and is moved by conduction to the cold air outside the room.
By having a fully floating floor, you save heat have warm feet and save money.
You insulate by covering the existing floor with sheets of polystyrene, then you lay a new floor, of t&g, glued floor boards or OSB boards over the polystyrene making sure that the new floor does not touch any part of the existing home. This keeps the heat in the room. Refit the base boards making sure they do not touch the new floor.