Understanding of Heat Transfer;

Heat flows naturally from a warmer to a cooler space.
In the winter, this heat flow moves directly from all heated living spaces to adjacent unheated attics, garages, and basements, or to the outdoors; or indirectly through interior ceilings, walls, and floors--wherever there is a difference in temperature.
During the summer, heat flows from outdoors to the interior.

To maintain comfort, the heat lost in winter must be replaced by your heating system and the heat gained in summer must be removed by your air   conditioner.

Insulated ceilings, walls, and floors decrease this heat flow by providing an effective resistance to the flow of heat.
  
Heat ALWAYS flows from the warmer side to the cooler side by one or more of the following ways.

Heat Transfer Types;

Conduction - Conduction is the transfer of heat through a solid object. When    one part of an object is heated, the molecules within it begin to move faster and   more vigorously, when these molecules hit other molecules within the object   they cause heat to be transferred through the entire object.  

Convection - Convection is the transfer of heat by the movement of a fluid     (water, air, etc.) Inside of a wall cavity, air removes heat from a warm interior    wall, and then circulates to the colder exterior wall where it loses the heat.
   
Radiation - Any object will radiate heat to cooler objects around it by giving off    "heat waves". This is a direct transfer of heat from one object to another, without   heating the air in between. The Earth receives heat from the Sun or a heater stove supplies heat to its surroundings in this same process.  

We have commonly used (mass) insulation such as Fiberglas, Cellulose,   Styrofoam, etc. to slow down and resist R and U value, the heat transfer by way   of conduction only; they do nothing for the heat transfer by way of radiation.


 

Mass insulation products work by trying to ?trap ?the heat in air pockets contained between the fibbers in the product. Air is a good insulator against conduction but cannot stop radiant heat!

Once the insulation becomes saturated with all the heat, it is then transferred on through the wall or ceiling. Besides, of this, this conduction type of heat transfer is between 5 % to 8 % of the total heat transfer that leaves between 92 % and 95 % of the total heat transfer to be dealt with, mostly by your air conditioner and heater.