Typically, the biggest attic leaks are found where the walls meet the attic floor, duct runs, above drop ceilings and overhangs, in knee-walls, and uncovered whole house attic fans. Look for dirty or discolored insulation which indicates that air is moving through the insulation material (and filtering out the dirt/dust). There are lots of other holes in the attic floor that should also be sealed, such as: recessed can lights, plumbing stacks, electrical wires, attic hatches or doors, and around chimneys or flues. Visit Seal and Insulate with ENERGY STAR for more information.