A. A considerable quantity of moisture is reaching the north attic from below. Once the moisture is in the attic, it condenses on the coldest surface — usually the roof sheathing. Most condensation occurs at night, when the roof is coldest. The problem occurs only on the north side because the sun evaporates the condensation on the south side on a daily basis. The moist air is then vented through the soffit and ridge vents. On the north side, which is not sufficiently warmed during the day, the frost continues to build, until a really warm day comes along and the entire frost coating melts, producing the problem you describe.
During the first winter after construction, a major source of moisture can be the construction materials themselves. With a block foundation, there can be at least a ton of moisture lost from the mortar, wood, paint, and drywall taping compound in the house. In a house with a poured concrete basement, the amount may be several tons. The problem...
to read the full article.