The Gassho-style houses in the historic villages of Shirakawa-go and Gokayama are among the most famous examples of Japanese vernacular architecture. The houses have thatched roofs that are notable not only for their design but also for their passive performance. In particular, the houses of Shirakawa Village are been well adapted to the local terrain, climate, and culture. However, no studies have considered the terrain around Shirakawa Village, particularly the topographic features of the Shohgawa River Valley, though they are likely has an effect on the houses.
In surveys conducted to date, there are many issues to address. For example, no simulations or measurements have considered the effect of topography on the wind environment, and details about heating are not yet clear. Here, the advantageous characteristics of the houses are investigated from the perspective of environmental engineering through actual measurements and computational fluid dynamics (CFD) simulations.
First, the wind environment of Shirakawa Village was evaluated by CFD analysis using a complex topography model. Visualization of the results showed that the wind flows from the north and follows the contours of the land. The wind speed increases around the center of basin and decreases in curved and narrow area. The wind speed is also decreased by dense houses. In sum, the simulation shows that Ogimachi is a low wind environment.
Second, the thermal environment in the houses in summer was investigated. The thick thatched roof prevented sharp increases in temperature and maintained a comfortable environment. The daily temperature range in the attic surrounded by the thatched roof was narrower than that on the first floor, which is framed by board walls. Within the context of sericulture, the room air temperature and wind speed in the attic represented an appropriate environment for silkworm breeding.
Third, the thermal environment of the house in winter was investigated. The effect of radiant heat from the heater was able to reach only a limited area, and the room temperature inside the houses was low. Also, snow guard fence improved the heat insulation capacity of the house, diminishing heat loss. Warm air diffused upward by convection, and the temperature tended to rise over a wide area on the second floor. Thermal sensation near the heater was improved by the increase in radiation temperature although it did not reach a comfortable level. The environment in much of the house was cold but tolerable, as the Gassho-style houses were adapted for the climate in Shirakawa Village to suit both summer and winter.
About a quarter of Shirakawa Village, covering an area larger than a city block, was targeted as the analysis region for validation. Analysis results for the wind environment of the uneven terrain were used as the boundary conditions in the analysis of the houses. In this study, the Reynolds-averaged Navier–Stokes equations were used as a turbulence model and the simulation was run using structured grid, but the use of large-eddy simulation and an unstructured grid is being considered for future research. The accuracy of the analysis remains an issue, though this method can be used to assess the influence of geography and climate in the surrounding area. The results of this research are expected to be useful for introducing passive technology into architectural design.
