Thats a brilliant post! The thicker the heat storage system, the more heat it will store and the more heat it will transfer into the room. The heat trapped in the gap between the false ceiling and the RCC slab is bound to find its way into the room. The cooler you make the room by running your AC, the more heat will transfer into the room. The trick is to reduce DELTA T whcih is the difference between the temperature of your RCC slab and temperature of your room. That can be only done by properly insulating the RCC slab with Polyurethane foam or Extruded Polystirine which is jumbolon. The required thickness of the insulation material to minimize heat transfer needs to be calculated through formulas. Words like Enough or Sufficient or Quite effective are all relative terms. Yes it is expensive to buy Polyurethane foam or Jumbolon, but you will be doing that only once in your lifetime. Energy bill, electricty and gas, you would be paying all the year round, through out your lifetime. And that saying "sastaa roie bar bar, mengha roi eik bar" actually applies here.