The foil won't be much help as it reflects light. Underfloor heating works initially through conduction, from the wire/pipe into the finish layer. This then emits heat through transfer to air and a little light wave energy.
Therefore to get the most heat into the room, You need a good dense mass above the element, this will give better heat transfer to the room and the thicker the insulation mass under the element, it will loose less heat downwards.
Sorry for going all technical!
Therefore to get the most heat into the room, You need a good dense mass above the element, this will give better heat transfer to the room and the thicker the insulation mass under the element, it will loose less heat downwards.
Sorry for going all technical!