Maruhon Bathhouse

Abstract We present a renovation project for a bathhouse at a well‐established 400‐year‐old ryokan (Japanese‐style hotel) in Sawatari Onsen, Gunma Prefecture. We strove to create a naturally ventilated quiet space—without a loud whirring fan—using the air flowing along a curved slab in the bath space. We also sought to highlight the transparency of the onsen (hot springs) water by diffusing light throughout the space. In the rest area, the curved slab forms a large backrest, where patrons relax and enjoy the view outside. The curved form of this slab was created through studies on seating comfort by making mockups and through the confirmation of the light environment using 3D computer graphics (3DCG). Furthermore, we confirmed through computational fluid dynamics analysis that the buoyancy‐driven ventilation effect was caused by the heat of the hot spring in the bath space and this effect ensures the sufficient air exchange rate.