The Influence of Passive Ventilation on the Interstitial Hygrothermal Environment of a Straw Bale Wall

Abstract

The greatest challenges for straw bale construction in Japan are moisture and straw′s susceptibility to microbial decay. In order to mitigate excessive moisture, some straw bale architects and builders in Japan have utilized passive ventilation. This paper examines the influence of passive ventilation on the interstitial hygrothermal environment of the straw bale walls of a building in Chiba Prefecture, Japan, known as Furyu. Furyu′s walls consist of straw bales surrounding a conventional timber frame. Underneath the bale wall is a ventilation chamber vented to the outside. The bale wall is exposed to the ventilation chamber below and air passes freely between the outside and sub-wall ventilation chamber. Twenty-three temperature and relative humidity sensors monitor the hygrothermal environment of Furyu. The study found that the relative humidity of the ventilation cavity is strongly influenced by the outdoor environment. The lower bale wall is in turn strongly influenced by the ventilation cavity. The results suggest that passive ventilation is not an effective means to control interstitial moisture in straw bale walls. Rather, passive ventilation appears to be a source of interstitial moisture during warm summer months.