HydroSKIN: Façade for Urban Rainwater Retention and Evaporative Cooling

The rising climatic impacts and the limitation of natural resources force to drastically rethink the largest urban surfaces: those of the building skins. Nowadays, façades are predominantly designed to protect the interior. What if the immense area of building envelopes would contribute to more urban sustainability and climate resilience? Extreme heat and heavy rainfall events affect our cities with considerable personal injuries and billions of material damage on buildings and infrastructure. While social developments lead to increasing urban densification, surface sealing, and the construction of high-rise buildings, the effects of climate change urgently require the creation of more „spongy“ surfaces for rainwater retention and evaporation without occupying any urban ground surface. HydroSKIN represents a revolutionary façade concept for rainwater harvesting and evaporative cooling. The lightweight textile skin absorbs the wind-driven rainwater hitting the building façade. The use of harvested rainwater inside the building e.g., for toilet flushing, plant irrigation and wash machine operation aims to a reduction of fresh water by up to 45 % accompanied by a significant decrease of energy consumption. In heat periods water is released by HydroSKIN to cool the interior and exterior environment by evaporation. The aim of this artificial lightweight retention surface is a drastic reduction of urban inundation and heat risks by relieving the sewage infrastructure and providing natural microclimate regulation with a minimal amount of embedded mass, energy, and CO2 emissions. The multi-layer design of HydroSKIN can be individually customised to the respective climate conditions, user requirements and design guidelines. The single layers are combined into an easy dismountable and fully recyclable system that is characterized by a minimal weight per unit area of only about 1 kg/m² in dry state and 5 kg/m² in a wet state thus being adaptable to both new and existing buildings. Optical investigations of the water droplet impact behavior of on textile surfaces indicated an absorption of almost the entire amount of wind-driven rain by the textile outer layer. Empirical evaluations of the cooling effect showed a temperature reduction at the textile surface of about 8-12 K, accompanied by an associated cool downdraft into the urban space below. High‐rise buildings are characterised by a high carbon footprint with no or marginal qualities on urban microclimate. As Fazlur Khan once pointed out with „Premium for Height“, the material, energy and CO2 consumption increases significantly with the building height due to the rising wind loads acting on the façade. What if these were not seen as an obstacle but as a potential to mitigate urban heat islands and inundations? Above a building height of 29 m more wind-driven rain strikes the building façade then an equal roof or ground area. Cooling capacity also increases with the wind impact on the building skin. Considering this potential as a new “Benefit for Height” we foresee a completely new era of climate-adaptive and climate‐resilient high‐rise buildings and cities with a low carbon but climate-positive footprint.