UNStudio Kyklos Building in Luxembourg: How It Cuts Embodied Carbon by 80%

The Kyklos building by UNStudio, designed with Luxembourg practice HYP Architects, is a competition-winning office project in Belval, Luxembourg, that aims to cut embodied carbon by around 80% compared with a typical Luxembourg office building. Rather than relying on timber alone, the design uses a carefully optimised hybrid steel-concrete structure, recycled steel and refined concrete mixes, tested with a digital tool called Carbon Builder. This article explains how the Kyklos building approaches low-carbon design, why embodied carbon matters, and what Pakistani developers, engineers and contractors can apply in a country where concrete and steel dominate construction.

Belval: From Steelworks to New Town

Belval, in the south of Luxembourg, is one of Europe’s best-known regeneration projects. A former steelworks site has been transformed into a mixed district of university buildings, offices, housing and public space, with some of the old industrial structures retained as landmarks. The Place des Bassins is a central square in this district, designed around a series of connected basins that recall the site’s industrial past.

Kyklos, from the Greek word for circle, is intended as the final piece that completes the Place des Bassins. The name also reflects the idea of a closing loop: the building is meant to represent the next stage of Belval’s development, with a stronger focus on sustainability and circular use of materials.

Why Embodied Carbon Matters

For years, green building efforts focused mainly on operational energy: the electricity and fuel used for cooling, heating and lighting once a building is occupied. As buildings become more efficient to run, a larger share of their lifetime carbon comes from embodied carbon, which is the emissions produced in making, transporting and installing materials such as cement, steel, glass and aluminium.

UNStudio points out that, for a building designed to be net zero in operation, the materials can account for the majority of its total carbon footprint over time, which the architects put at about 90%. In other words, an efficient building with a heavy, carbon-intensive structure can still have a large climate impact before it opens.

“Timber Alone Is Not the Answer”

Mass timber is often promoted as the default low-carbon structure. UNStudio’s position on Kyklos is more nuanced: the architects have said that timber alone is not the answer. Timber can perform well, but its benefits depend on sourcing, transport distances, fire protection and the amount of concrete and steel still needed for foundations, cores and connections.

For Kyklos, the team calculated the carbon footprint of different options and concluded that a hybrid steel-concrete structure offered the best long-term carbon performance for this building. This is an important lesson: the right answer comes from measuring options, not from assuming one material is always best.

The Carbon Builder Tool

UNStudio and HYP Architects developed a design tool called Carbon Builder to examine and improve the carbon impact of their projects. It allows designers to compare structural systems, materials and quantities early in the design process, when changes are still cheap to make.

On Kyklos, Carbon Builder played a significant role in shaping the design. The architects report that it helped achieve an embodied carbon reduction of around 80% compared with a standard Luxembourg office building.

How the reduction is achieved

• Recycled steel. The project specifies 100% recycled steel, which typically has a much lower carbon footprint than steel made from iron ore.
• Refined concrete mixes. Concrete mixes are optimised to reduce cement content, since cement production is the main source of concrete’s emissions.
• Efficient structure. A hybrid frame uses each material where it performs best, avoiding over-design.
• Early measurement. Carbon is calculated alongside cost and floor area from the start, rather than checked at the end.

Health and Wellbeing Targets

Low carbon is only part of the brief. Kyklos aims for BREEAM Outstanding, the highest rating in a widely used sustainability assessment, and WELL Platinum, which focuses on occupant health, including air quality, daylight, water, acoustics and thermal comfort. Good indoor air and ventilation design commonly draws on ASHRAE standards. Targeting both certifications signals that the building is meant to be good for people as well as for the climate.

How Embodied Carbon Is Measured

Figures such as 115 kg CO₂e/m² come from a life cycle assessment, a structured calculation of the emissions linked to each material in a building. The basic process is similar wherever it is used:

1. Take off quantities. The volume of concrete, weight of steel, area of glass and other quantities are measured from the design, much like preparing a BOQ.
2. Apply carbon factors. Each material is multiplied by an emissions factor, ideally from a product-specific environmental declaration rather than a generic average.
3. Add transport and construction. Emissions from delivering materials and site activities are included.
4. Consider the life cycle. Replacement of finishes and services over the building’s life, and end-of-life demolition or reuse, can also be counted.
5. Divide by floor area. The total is expressed per square metre so that different buildings can be compared.

Because the first step is so close to quantity surveying, contractors who already prepare accurate BOQs are well placed to start tracking embodied carbon. The main gap in many markets, including Pakistan, is reliable local carbon data for products such as cement, steel and bricks.

Typical Office vs Kyklos Approach

Strengths and Questions

• Strength: a measured, evidence-based approach rather than a single-material solution.
• Strength: embodied carbon treated as a design driver from the start.
• Question: stated figures are design targets; actual results depend on procurement and construction.
• Question: recycled steel and low-cement concrete must be available locally at the required quality.

Lessons for Pakistan

Construction in Pakistan relies heavily on reinforced concrete, brick and steel. Cement and steel are both carbon-intensive, and they are also among the largest cost items in any BOQ. Reducing the quantity of these materials therefore helps both the climate and the budget.

Any change to structural systems or mixes must still meet the Building Code of Pakistan (PEC), including seismic requirements, and should be designed by qualified engineers.

ACCO applies value engineering on office building construction in Lahore and commercial plaza construction in Gulberg, where structure is a large share of cost. Our grey structure construction cost guide explains how concrete and steel quantities affect the budget, and every ACCO project is priced through an itemised BOQ. See our services for design and build options.

Frequently Asked Questions

Conclusion

The Kyklos building shows that serious carbon reductions are possible with familiar materials when designers measure carefully and optimise early. UNStudio and HYP Architects did not chase a single fashionable material; they compared options and chose the structure with the best long-term performance. For Pakistan, where concrete and steel will remain the main structural materials for years, the most practical lesson is to design structures efficiently, specify mixes carefully and reduce waste, which saves both carbon and money.