In January 2021, Déchelette Architecture was selected through a competitive process to design a mixed-use building at 17, rue des Quatre-Cheminées in Boulogne-Billancourt, France, a neighborhood adjacent to Paris. The brief called for the creation of a four-story building containing eight social housing units and a ground-floor shop, constructed with a focus on low-carbon, bio-sourced materials. Located next to the Billancourt market, the site offered the opportunity to test new construction methods in a dense, urban setting.
To support this ambition, the City of Boulogne-Billancourt transferred the land to the client for the symbolic sum of one euro, enabling greater investment in sustainable design experimentation. Déchelette Architecture was responsible for the entire project, from design through to construction oversight, including guiding site teams in the use of unfamiliar materials like raw earth.
Urban composition and passive design
The five-level building, including a green roof, is organized around a central circulation core. At ground level, it is divided into two independent entities: a commercial unit and the residential entrance. The building’s compact rectangular plan optimizes energy efficiency, and its dual-aspect apartments ensure natural cross-ventilation and high-quality daylighting.
Each floor contains two nearly identical apartments. Living rooms are located along the southwest-facing street front, taking advantage of sunlight throughout the day, while bedrooms face southeast toward the quieter garden and are extended with balconies. The stairwell receives generous natural light from its garden-facing elevation, and the rear of the site is landscaped to integrate with the urban fabric, offering visual permeability from the street.
This layout supports thermal regulation and natural airflow while promoting visual and physical connections between public, private, and communal outdoor spaces.
A monolithic facade of rammed earth
A defining feature of the project is its self-supporting façade made from prefabricated rammed earth—an unprecedented approach for collective housing in France. The raw earth panels span four levels, rising 12 meters in height and 10 meters in width, with a wall thickness of 50 centimeters. A total of 80 square meters of façade area was constructed, comprising around 40 cubic meters of earth.
The earth was sourced from the region around Lyon and shaped into blocks by Terrio, a specialist firm located in Saint-Priest. Using individual molds and a pneumatic rammer mounted on a gantry, Terrio dried the blocks in their workshop before delivering them to the site. The construction company STM-LBTP was trained by Terrio to carry out the assembly. Three masons completed the installation in one month, laying 12–15 square meters of rammed earth wall per day.
The monolithic facade rests on a base of massive stone, offering durability, thermal mass, and a grounded visual identity. Compared to traditional concrete facades, the rammed earth construction emits only 23 kgCO₂eq per square meter—substantially lower than concrete, which averages 250 kgCO₂eq per square meter. This significant reduction contributes to the project's overall carbon performance.
Wood structure and acoustic insulation
The structural system is composed primarily of wood. In line with cradle-to-cradle principles, the architects minimized the use of concrete, restricting it to the ground-floor slab and the circulation core. All framing and load-bearing elements above are made from timber. For acoustic comfort, high-performance wood wool panels were incorporated within the partitions, combining insulation with sound absorption and contributing to interior thermal stability.
The rear facade, facing the landscaped garden, is clad in wood. Its natural, rural character was selected to foster a visual and material dialogue with the outdoor space. This design decision aligns with the project’s commitment to creating a humane and contextually sensitive environment while remaining within a strict environmental budget.
Off-site prefabrication and dry construction
The construction process prioritized off-site prefabrication for efficiency, quality control, and environmental management. All major components—including the concrete stair core, the timber frame, and the rammed earth blocks—were fabricated before being delivered to the site. Only the ground-floor slab was poured in situ.
This prefabrication strategy accelerated the construction timeline, particularly for the rammed earth components, which saved an estimated five months. The speed and simplicity of the dry assembly process allowed for a clean, quiet building site with minimal water usage, reduced material waste, and lower noise pollution—benefits for both workers and the surrounding neighborhood.
Environmental performance and certification
The building’s low-carbon construction was validated by its performance indicators. With a post-construction emissions calculation (PCE) of 793 kgCO₂eq per square meter, the project exceeded its original low-carbon objectives. These achievements were recognized with an HQE (Haute Qualité Environnementale) Level 3 certification—the highest rating under the French environmental quality standard.
The carbon reductions were achieved not only through the choice of materials but also through the building’s form, orientation, and spatial configuration. The dual-aspect layout, high daylight access, and minimized reliance on mechanical systems support both comfort and energy efficiency across the building's lifespan.
