Hygrothermal Conditions in Attic Spaces of Wooden Houses with Eave Ventilation During Winter in a Mild Climate Region in Japan

Abstract

A mock-up study was conducted with a one-story test house (3.64 m by 3.64 m) with eave ventilation in a suburb of Tokyo to examine the hygrothermal conditions in an attic space. A series of measurements were conducted to clarify the effects that the vent opening area and vented wall cavity had on the hygrothermal conditions. Additionally, a hygrothermal simulation was conducted based on the results of the experiment to estimate the amount of airflow within the attic space. The main findings from the experiment and simulation results are as follows:Considering that 20 % of the upward airflow through the vented wall cavity does not contribute to ventilating the attic space, the eave space should be incorporated into the hygrothermal model in order to predict the proper size of an attic ventilation opening.The airflow (advection) between the insulation mats to the attic space was found to be 2–20 m3/h (equivalent to 0.2–2.0 times/h air exchange rate in the attic space). The average vapor flow by advection was approximately 2.5 g/m2h at night (0:00 to 6:00), and the average attic absolute humidity increased by 0.2 g/kgDA during this period.The amount of moisture absorbed and/or desorbed by the wooden materials in a daily cycle is almost the same as that absorbed by the northern and southern sheathing boards, and thus had a significant effect. It should be considered when calculating the temperature and humidity in an attic space.