Ten years later, Herstal City Hall is featured in "Green Building Envelopes"

The vegetal as a project material

In the face of climate change and the growing intensity of environmental and social challenges, architecture is being called upon to move beyond its traditional frameworks. Increasingly, buildings are conceived as living systems, capable of interacting with their surroundings, enhancing user comfort and contributing to urban resilience.

Through projects realised around the world, Green Building Envelopes demonstrates how organic resources can engage in dialogue with advanced construction technologies to shape cities that are more sustainable, more responsive and more liveable. Rooftop gardens, planted façades and vegetated interior spaces become fully-fledged design tools, contributing to air quality, acoustic insulation, thermal regulation and urban biodiversity.

Herstal city hall: a modular planted façade

In the Liège region, Herstal City Hall, designed by the office AAFH and completed in 2015, offers a compelling example of this approach to architecture as an active interface between the built environment, climate and city.

The project stands out for its innovative planted façade, which departs from the principle of a continuous green wall. Of the building’s 2,500 m² exterior envelope, approximately 1,000 m² are covered with vegetation, arranged in a chequerboard composition combining planted modules, glazed surfaces and solid panels.

An autonomous and adaptable system

One of the project’s defining principles lies in the modular and autonomous design of the planted façade. Each planted module is independent and equipped with its own water supply system located at the upper level, forming a closed irrigation loop. This autonomy facilitates maintenance, allows for the targeted replacement of modules and offers significant flexibility in the evolution of planting over time.

In total, 23,000 plants have been integrated into the façade. Their selection is based on precise criteria: water retention capacity, insulating performance and adaptation to the specific solar exposure of each module. For the botanical design, the architects collaborated with landscape designer Louis Benech, notably known for the renovation of the Tuileries Garden in Paris.

Beyond their landscape dimension, the planted modules play a full role in the building’s environmental performance. In summer, they contribute to a cooling effect, while in winter they enhance thermal insulation, much like green roofs. This natural regulation reduces energy demand while improving indoor comfort.