Floor level and under floor heating

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the builder of my conservatory has laid the floor height level with inside. This I think will compromise the insulation potential of our proposed ufh. We were having 10 or 12mm cement board on top of the chipboard timber suspended floor and then levelling compound and tile but tiler said we only have space for 6 or 8 mm board. Can someone advise whether I might be better off getting proper ufh insulated boards that are 8mm to get more effective heat through the tiles or doesn't the depth of board make much difference. Tia