Why is thicker insulation used in an attic than in the walls of a house?

Heat loss by convection occurs when the air is contact with an attic whose temperature is higher than that of its surrounding walls. The warm air in the in the attic is less dense than the surrounding cool air, so it rises because of buoyant forces. The surrounding cooler air falls to take the place of the rising warmer air, and a convective circulation is set up. The transfer of heat will be slowed down by using thicker insulation in an attic. Since most of the heat loss is through the attic that is why thicker insulation is used in an attic than in the walls in a house.