There is definitely a reason that there are professionals that specialize in insulation installation.  If you attic is under insulated, you will find that your house heat will rise up through your ceiling and end up in your attic space.  Warm air in your attic space will definitely lead to a mold problem as a mix of warm air and cold roofing members will certainly generate condensation….and then mold.  If your attic is over insulated, you will find that the insulation extends out to the edges of your attic space closing off the lower ventilation.  It is an excellent idea to install baffles along the lower portion of your roof in the attic to allow for proper airflow (ventilation will be addressed later).