If the top plate shrinks and pulls away from the drywall, cold air will enter the partition and then the living space through openings such as electrical boxes. Prevent this by caulking the top plate or laying poly beneath the insulation batts in the ceiling. The same problem can also occur where a partition wall joins an exterior wall. Cold wall-ceiling intersection at eaves may have mildew from condensation when insulation doesn't extend deeply enough into the eaves. Fix by extending batt far enough to completely cover the living space, but make sure to leave ventilation space below the roof sheathing.
Attic bypasses occur when no seal or insulation is placed over the folding attic stair.
A gap below the kneewall in the second story is a common area of air leaks. Unless the joist space is blocked below the kneewall, cold air enters the joist space. Block with rigid foam sealed with caulk. Leaks around plumbing vents and wiring is a frequent problem. Fix by caulking or foaming around openings in the top plate or using rubber boots that slip around pipes or chases.