UNStudio designs Kyklos building with a focus on achieving a low carbon footprint

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The Kyklos building in Belval, the last building to shape the Central Square district, will be designed by UNStudio and local partner HYP Architects. Belval, a former industrial site in Luxembourg, has been successfully transformed into an urban center that now accommodates a university, a technological center, residential and office spaces, hotels, and retail.

The design team’s goal was to propose a building with the smallest possible carbon footprint from the outset. UNStudio is currently developing a fully-fledged sustainable design framework to give its designers full control of the embodied carbon impact of its designs, among other applications. The ‘Carbon Builder’ is part of this set of tools, allowing designers to study numerous options to intricately reduce their projects’ carbon footprint.


The team made the correct decisions early in the concept design process for the Kyklos building proposal, thanks to the early prototype of the Carbon Builder. It was possible to reduce the carbon footprint of the design concept for this building by around 80% compared to a traditional office building in Luxembourg by evaluating the embodied carbon. The carbon performance of the project has improved significantly by using steel with 100% recycled content and working with best-practice concrete mixtures. Given the function-use demands, a hybrid steel-concrete structure was chosen over other construction methods, resulting in better long-term carbon performance. The early best possible roadmap displays the ultimate possibility of achieving 115kg of CO2 equivalent/m2, compared to 580 kg/m2 for a typical office building in Luxembourg.

Ben van Berkel said, “Our new carbon tools have shown us that timber is not always the best option and that highly sustainable buildings do not all have to look the same. We can now demonstrate that interesting geometry and high levels of sustainability are not mutually exclusive.”


The last building in Belval’s large development is called Kyklos, which is the Greek word for ‘circle‘. It is strategically located at the development center next to Place des Bassins. The design of the emblematic Place des Bassins connects two basins, which were once a part of the former steelworks site on which the district is developed, in the form of a singular ‘infinity loop’. This loop represents the alliance of the industrial site and its revitalization as an urban space.

The building is seen as the third loop in the equation, acting as a sustainable connection between urban development and its future. The design of the building aims to integrate it with the urban environment and follow a holistic, sustainable approach. This includes all aspects of the building, from its functional organization to its construction techniques and materials.

The design takes into account the master plan, the main pedestrian path, and the existing main square to create a building that complements its surroundings. The result is a combination of the square’s circular geometry with the surrounding buildings’ orthogonal geometry. The Kyklos building offers 7,600m2 of total surface area above ground and a basement of 2,250m2, providing eight floors of office space above a mixed-use lower level. The design incorporates numerous strategies to ensure high levels of transparency and daylight. One corner of the building is setback on the lower levels, creating an overhang that further protects from overheating.

Project Info

UNStudio team: Ben van Berkel, Arjan Dingste with Stella Nikolakaki, Bruno Peris Vila and Cristobal Ignacio Burgos Sanhueza, Laura Lopez Iglesias, Regiane Fernandes de Oliveira, Taliia Nurutdinova, Matthias Kooijman
Local Architect: HYP Architects
Visualizations: by Play-Time Barcelona
Client: Atenor, Ahrs Group, Agora Luxembourg
Location: Belval, Luxembourg
Building surface: 7,600 m2 GFA _3600m2 below ground
Building site: 1200 m2
: Office with mixed-use plinth
Status: Winning competition entry

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