HORTUS

Location:
Allschwil, Switzerland
Type of use:
Office
Year of construction:
2025
Size:
10.000 m²
Architect:
Herzog & de Meuron
Developer:
SENN

Necessity

The construction industry, with its enormous negative impact on the environment, faces significant demands when undertaking fundamental change. SENN therefore set itself the ambitious goal to build its next office building in such a way that it is radically sustainable. That meant to create a building that will repay its embodied energy within a generation. For this reason analyses and calculations focused not only on building materials but also on new forms of tenancy to e.g. ensure comfort and prevent dead space. To achieve this ambitious goals, interdisciplinary teams from SENN, Herzog & de Meuron, ZPF Ingenieure, Blumer Lehmann and the clay specialists at Lehm Ton Erde worked intensively together. The building now stands as a beacon of sustainability, and the process has generated valuable knowledge that will support the realization of highly ambitious sustainability goals in the future.

In order not to compromise the ambitious goal of offsetting the embodied (grey) energy within a single generation, HORTUS decided against integrating laboratories. This would have had significant implications for the construction and the materials used, as the region has an elevated earthquake risk and laboratories require stricter tolerances than a purely office building.

Affordability

HORTUS is very flexible and can respond to the diverse needs of its tenants as well as external visitors. Its ground floor is open to the public and provides a multifunctional space for the entire neighborhood. With an excellent restaurant, meeting center, and gym, it can attract people far beyond its immediate area. Affordability, even with the high quality, is absolutely within the range of its immediate neighbors.

Simplicity and Appropriateness

HORTUS strikes the perfect balance between high-tech solutions that promote sustainability (for example, with the specially developed ceiling lamps with sensors for sustainability-relevant building parameters) and minimalist, function-oriented technologies. The design perfectly reflects this: The office building, constructed entirely of clay and wood, requires no glue or nails. It was designed without a basement to avoid concrete. HORTUS' interior design prioritizes the use of recycled and natural materials wherever possible. The ceiling element (normally the most CO2-intensive component of a building) was developed in an intensive research process in collaboration with ZPF Ingenieure and Lehm Ton Erde. For HORTUS frames of mass-timber were filled with rammed earth. This combination delivers strong thermal and acoustic performance while using renewable, cradle-to-cradle materials. The ceiling elements were manufactured in a field factory located directly next to the construction site. The clay used was also excavated there.

Sufficiency and Efficiency

The first crucial step was to „flip“ the process of constructing a building to: Materials → architecture → systems (rather than: systems → architecture → materials). The main goal was to create an energy-positive building that pays back its embodied (grey) energy within a single generation. To achieve this, the design had to start from the material itself, its sustainability and efficiency, rather than selecting the most suitable material afterward for already established architectural plans. The architects therefore had to work with the material best suited to the specific location and start form there.

To meet these goals, an important aspect was also to use materials as sparingly as possible. Also the clay used for the ceiling elements was sourced directly on site and could be taken from other construction plots developed by SENN.

HORTUS is being certified according to the ‘Standard Nachhaltiges Bauen Schweiz (SNBS) 2.1 Platinum’- the highest level of this sustainability standard. In addition, criteria are deliberately exceeded to set a benchmark. The energy generated by the photovoltaic system at HORTUS (994 kWp, approx. 890,000 kWh/year) is used within an energy self-consumption association. Heating and cooling energy in HORTUS is 100% fossil-free. Heat and cooling are distributed through the highly efficient Rio-Therm recirculating convector system. All areas in HORTUS are mechanically ventilated. When a tilt-and-turn window is opened, sensors detect the improved CO₂ concentration, prompting the system to automatically reduce the mechanical air volume flow. All rainwater from the roof is collected in an underground 80 m³ rainwater tank, supplying the building’s toilet flushing systems.

Scalability

In the principle of circular construction all building components are catalogued in a way that enables future reuse. Each component has a specified future, so that re-use or recycling is possible. The design for disassembly approach means that, at end-of-life, local recycling or re-deployment is possible. Local firms or future projects can reuse materials rather than discard them.

The HORTUS project has led to the creation of two companies: LEHMIT and REMATTER. This allows the innovations developed through the project to be applied to other projects and scaled.

Beauty

Design follows radical Sustainability: clay and timber construction, as seen in the half-timbered houses of the border region of three countries, represent an traditional technology that had fallen into oblivion. HORTUS reminds its visitors, in a new way, of this familiar and enduring technology. The design does not treat sustainability as an additional technical concept, but as part of the building's identity and beauty. The clay used for the always-visible wood-and-clay slab was taken directly from the site. Also the green space of the vast and flowing park area leads directly into HORTUS’s entrance and the garden. This lush green courtyard is the heart of the building and was designed by landscape architect Piet Oudolf. In that sense, HORTUS aligns with Swiss culture through thoughtful, long-term design in harmony with nature.

Unique Principles of Success

1. Set a measurable sustainability target

The vision was to create a building that would pay back its embodied non-renewable energy within a single generation and become energy-positive after 30 years. The story behind this began in 2019, when SENN posed the central question of how a radically sustainable building could be realized. As a result, SENN commissioned Senn Technology to develop a conceptual idea. This led to a concept aimed at causing no or only minimal environmental impact during the construction, operation, and eventual deconstruction of the project. A particular focus was placed on amortizing the embodied energy within one generation, based on the following basic formula: Amortization period [a] = Embodied energy of construction [kWh] / (PV production energy [kWh/a] -Operational energy [kWh/a])

The goal of the HORTUS project is to minimize the environmental impact of the construction and to demonstrate that the construction industry can make a significant contribution to combating climate change. The building is intended to function as a powerful energy source immediately upon completion. With this ambition, the project requirement was defined: all non-renewable primary energy used in construction should be fully compensated within one generation.

At HORTUS, both the operational energy needs and the surplus energy are generated via the planned photovoltaic system, ensuring that the non-renewable primary energy is fully compensated within one generation.

2. Drive innovation through interdisciplinary development

To achieve this radical sustainability goal and turn utopia into reality, interdisciplinary teams from SENN, Herzog & de Meuron, ZPF Ingenieure, Blumer Lehmann and the clay specialists at Lehm Ton Erde worked intensively together. Developing the innovative ceiling, made from timber and rammed earth, took around seven months before it met the ambitious requirements, including strict fire safety regulations.

During the planning phase, SENN built extensive expertise across the three pillars of sustainability – ecology, economy, and sociology. From this knowledge, various innovative products and systems emerged, such as the Rematter ceiling system, Ecotool, BioValues, and developments in lighting technology.

3. Lifecycle thinking in materials and building systems

As a CO₂-intensive building material responsible for around 10% of global greenhouse gas emissions, concrete was omitted from HORTUS wherever possible. The only exception are the short piles driven into the ground on which the building stands. Unlike a full basement, their carbon footprint is negligible. Instead of concrete, local materials such as timber, rammed earth, and recycled paper were used. The ceiling elements of HORTUS account for only about 7.5% of the building’s total CO₂ emissions. In comparison, a conventional reinforced concrete flat slab would contribute roughly three times as much (around 25% of total emissions).

To achieve this ambitious goal, the planning processes were specifically adapted: materials, construction methods, and building systems were selected to require the least possible amount of non-renewable primary energy and to produce the lowest greenhouse gas emissions. This ensures that, on one hand, the environmental impact during construction was reduced to an absolute minimum, and on the other hand, the building’s operational energy consumption remains well below conventional limits. Any unavoidable non-renewable primary energy currently used in construction is offset by the building’s sustainable surplus energy.

Limitations

Even more second-hand material would have been desirable. However, there is not yet enough material available on the market for such a large wooden office building, and/or the timely preservation (and processing) of these materials remains challenging for construction. Furthermore, the industry is primarily geared towards steel and concrete. The expertise and number of skilled workers required for such precise and demanding timber constructions present a further challenge. If more projects in the future follow the principles of circular construction, even more ambitious sustainability goals could be achieved.

HORTUS

Join our movement

Participants

Supporters