When warm indoor air comes in contact with these cool surfaces, moisture condenses and creates water spots. As temperatures change and warm, these moist spots may become the source of mold growth, which is the black stain you see on your walls and ceilings. The solution is to stop air from penetrating your insulation. Cold air entering the soffit vents on the windward side of the house blows through the attic insulation on its way into the attic. In batt insulated attics, cold air may be driven into small gaps along and below the batts.
Wind driven cold air also can intrude in insulation beneath siding and through sheathing.
Corners can be especially troublesome, with wind intruding through gaps in sheathing and corner framing. Some attics have little space to work in and being able to improvise is important. Using a newer fiberglass batt, encased in wind resistant wrap, may be in order. Fold the batt in half and stuff it, fold first, into the joint where the roof meets the top of the outside wall. As you stuff the batt in, position a baffle or chute to direct air above the insulation.