I am doing a garage conversion and have a depth of 200mm to bring it up to the same level as the existing house. I want to put in a floating chipboard floor. The easiest and cheapest way, with the highest insulation value, would seem to be fill the space up with polystyrene/Celotex sheets. Is this depth of polystyrene between the existing concrete floor and chipboard likely to settle at all? I know it works ok with at least 50mm but what about 175mm with chipboard on top? Anyone done this? Dave S