We insulate attics to keep heat from escaping out of the living space and to keep the attic cold and dry. Most homeowners understand that. However, insulation is only half of the protection that a good attic needs. Insulation doesn't stop air from leaking and taking heat and moisture with it. A home performs well only when both heat and air are controlled.
Conduction occurs when heat flows from warm materials to colder materials.
Convection occurs when warm, moist air rises and leaks out of the living space and carries heat with it. Typical attic insulation dramatically slows the conduction that is the primary problem. However, insulation does not plug air leaks and prevent convection. This not only wastes money and energy, but the moving air also creates drafts and cold spots in the home. Air leakage into the attic can create other problems.