A Method to Predict the Hour-by-Hour Humidity Ratio of Attic Air

Long-term measurements of attic wood moisture content have shown that at least 50 kg of water may be stored in roof sheathing members over the course of the winter, to be released in the spring. Overlaid on this seasonal cycle is a daily variation driven by temperature changes in the attic. A dynamic model is required to predict the resulting attic air humidity ratio. Hourly moisture flows in a typical attic—from ventilation and from the wood—are examined, and a simple first-principles dynamic mathematical model to relate them is developed. The model predicts the hour-by-hour attic air humidity ratio. The results are compared with measured data. The implications for attic ventilation rates at different levels of attic insulation are discussed.