Insulation is the loose celled material used in various parts of a structure to help slow the transfer of heat into or out of your home.
It is rated by it's Resistance value or R-value. The higher the R-value, the more effective the insulation is at stopping the transfer of heat. The amount of insulation should be determined by it's R-value, not by any amount of inches.
According to Xcel Energy, as much as 70 percent of the energy you pay for is used to heat and cool your home. A properly insulated and air sealed home results in lower heating and cooling bills, can increase the value of your home in a selling market and provide overall comfort by eliminating drafts and deadening loud noises in and outside of the home.