Moisture enters an attic both from the house and from the ventilation air. It has been assumed that when the roof sheathing temperature drops below the attic air dew point, condensation occurs on the roof sheathing. If this were true, then increased attic insulation levels would require increased attic ventilation rates. Results from an experimental study are presented, which show that in fact the roof sheathing is in dynamic equilibrium with moisture in the attic air, and that several hundred pounds of water can be stored in the attic wood without ill effects. A model of this process is presented and used to predict hour-by-hour and seasonal moisture levels. Applications of the model are discussed.