Q. Last winter, the tenants on the north side of a new duplex we built complained of heavy condensation in the attic. Water dripped on the north side of the attic floor from the underside of the roof sheathing, but nothing like this happened on the south side. The attic is very large, and it is divided by a fire wall. We installed balanced soffit and ridge vents, and all exhaust fans are vented outside. What is causing the condensation?

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 often disappears after the first winter.

Additional moisture may be coming from a damp crawlspace or basement, excessive use of a humidifier, or a gas-burning appliance, such as a water heater or furnace that is not venting properly. A plumbing wall or flue chase can provide a direct path to the attic from the basement if these passages are not sealed at the ceiling level.

You may be able to isolate the problem by "reading" the indoor humidity levels in the house on the windows. If there is condensation on double-glazed windows, the indoor humidity is too high and ventilation fans in the kitchen and bathrooms should be run continuously until the condensation goes away. If condensation occurs on the windows on only one side of the house, the problem may be caused by the use of a humidifier or improper venting of gas-burning appliances. If there is no condensation on the windows, the moisture is probably coming from the basement or crawlspace through the plumbing or flue chases.