Requirements of Airtightness for Moisture Control in Attic based on Whole Building HAM Model

Attic moisture control is one of the key issues addressed by the hygrothermal design of wood-frame houses. In general, exfiltration and natural ventilation are recognized as the main factors in determining the hygrothermal behaviour of an attic. Thus, hygrothermal analysis using a network flow model should be performed in order to predict the hygrothermal behaviour of an attic. In this paper, sensitive analyses for predicting hygrothermal behaviour were implemented to study the design value for attic moisture control. A whole building HAM analysis model was developed for these sensitive analyses, and over 30 cases taking both the equivalent opening area of the attic and the airtightness of the ceiling into account were examined. Results of the analyses indicated that the current guidelines regarding the equivalent opening area and airtightness are required in cold climate regions of Japan. However, it was also found that these guidelines may be relaxed in regions with milder climates. Additionally, it should be noted that the design value for attic moisture control was derived from the calculation results.