Seasonal Storage of Moisture in Roof Sheathing

Classic research into attic moisture problems tended to concentrate on the static prediction of instantaneous temperatures at the underside of the roof sheathing, which was regarded as an inert medium. However, recent work has demonstrated the existence of daily and seasonal cycles in attic moisture parameters. Over the course of a day, the attic air humidity may vary by a factor of three, and during the course of a winter there is storage of perhaps 45 kg (100 lb) of water in the roof sheathing and roofing trusses. On a daily basis the moisture flow is quite significant, of the order of 2 kg per hour (5 lb per hour); this is far greater than the moisture generation rate in a house, which is typically 0.45 kg per hour (1 1 per hour). The daily cycles suggest that as the roof sheathing is warmed by incident solar radiation, water is driven off and removed by the ventilation air. A simple method to predict the seasonal variation of wood moisture content has been developed by considering the hour~by-hour transport of water into and out of the wood surfaces of the attic. To validate the model, hour-by-hour measurements of wood resistance, attic and outside dew-point and meteorological variables were made over a four-month period on an unoccupied house in Oroville, CA. The roof sheathing moisture content was found to vary from approximately 14% in December to 7% in early April. Measurements are compared with predictions.