A wooden floor that curves up to become a wall allows light and air to flow through this bathhouse by Japanese studio Kubo Tsushima Architects.
Set within a 17th-century hotel complex in Japan's Gunma prefecture, Maruhon Ryokan was redesigned by Kubo Tsushima Architects as part of a long-term plan to attract tourists and promote sustainability in the area.
"The existing building has been extended and reconstructed many times, and has a complex geometry with connected-accommodations," architect Hidekai Kubo told Dezeen. "This bathhouse is on the node of the pass which connects two wings of accommodation."
From street level, the bathhouse appears to follow a traditional form complete with a gabled roof. But on the inside, a floor constructed from Japanese cedar curves up to create a wall that separates a bathroom and a designated rest space.
Using computational fluid dynamics – a type of mechanics that uses numerical analysis and algorithms to solve and analyse fluid flow – the architects designed the unusual interior to create buoyancy-driven ventilation.
"We applied the behaviour of ventilation to the design so that the natural wind from the bathroom window rises when heated by the spring water, and goes out from the roof window," Kubo explained.
The shape of the wall creates a light source, diffusing daylight flooding in through the open facade into the lower-level bathroom. LEDs illuminate the space at night.
In the downstairs bathroom, spring water flows from a black bamboo tap into a small pool that is submerged into the floor.
Along with providing the lighting and ventilation for the bathhouse, the curved wall also acts as a backrest for a large bench in the upper-level rest space.
"To construct the bench, we used Japanese cypress wood," said Kubo. "It has pleasant aroma, and it makes people relaxed who recline on the bench after bathing."
Kubo Tsushima Architects previously renovated a wedding chapel in Tokyo's Ebisu district – painting each windowsill in colours of the rainbow to "impart faint hues" as light filters through.