Most of the exterior walls of wooden houses in Japan have a vented air layer between the exterior cladding and the insulation. This vented air layer is designed to dehumidify the exterior walls by discharging humidity to the outside and allowing outdoor air to enter, thereby decreasing the risk of condensation on the exterior wall during winter. It is assumed that the source of this moisture is the indoor air, and that the outdoor air is drier. However, the outdoor air is often highly humid during the rainy season and may become a source of moisture. The vented air layer also allows rain water to drain away. Any rain water penetrating through the exterior cladding is drained away through the vented air layer. However, rain can also enter the vented air space through the air inlets. Since it takes a certain amount of time for all the rain water to drain away, water may accumulate in the vented air layer and produce high humidity in the exterior wall. In order to evaluate the effectiveness of an exterior wall with a vented air layer, its hygrothermal characteristics should be investigated, taking the effect of rain water into consideration. This paper describes a case of moisture damage where stain appeared on the outer surface of the plywood wall in a wooden residential building. Experiments were carried out in climate chambers to clarify the conditions that are causing stain. Hygrothermal conditions in the vented air layer were simulated using heat, air, and moisture model, and the causes of staining were investigated.
Most exterior walls of wooden residential buildings in Japan have a vented air space between the exterior cladding and the insulation.This air space is designed to dehumidify exterior walls by discharging humidity to the outside and taking outdoor air into the walls, in order to decrease the risk of condensation on exterior walls in winter. Many studies have been carried out on vented air space, and the most effective thickness for dehumidification has been determined. However, most of these studies have only investigated the performance of vented air spaces under laboratory conditions, and the characteristics of vented air spaces in the field have not been clarified. Our previous study clarified the characteristics of ventilation volume through the vented air space based on field measurements. This study continues that work by reporting on the hygro-thermal characteristics of vented air space. The field measurement and numerical simulation have been carried out on the vented air space and it was pointed out that absolute humidity of the vented air space is lower on the average than the outdoor air due to absorption and desorption of the exterior cladding and the daily variation in ventilation volume through the vented air space.
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