When a terracotta pot is filled with water, it permeates the porous surface of the clay vessel. As the temperature outside rises and the water trapped inside these pores evaporates, the process draws out the latent heat from the water inside the pot. Through this evaporative process, the pot cools down after losing its latent heat — cooling the water inside simultaneously.

For centuries, Indians without access to active cooling methods have been using this phenomenon to cool water in terracotta vessels called ‘matka’ in many Indic languages. These terracotta vessels have a long history of being used in South Asia to cool water in the sweltering hot summer months — with the earliest fragments found in the region dating back to the Indus Valley civilisation, more than 3,000 years ago.

Terracotta pots were not the only traditional cooling method used by people living in warm, tropical regions. Indians have used traditional cooling methods like sun-shading; lattice screens; arched, domed, and vaulted ceilings; and evaporative cooling solutions like step-wells or ‘baori’ to create cooler microclimates in their homes.

In the field of architecture, such passive cooling methods are considered “vernacular architecture” — a term with colonial connotations used to describe traditional or indigenous architecture that is designed and built using local materials and resources, and adapted to the local culture and climate. These traditional building solutions tend to work well in their respective contexts, as they have undergone and withstood centuries, if not millennia, of experimentation and innovation using techniques and materials available locally.

Although globalisation and the democratisation of technologies in the age of capitalism have brought more comfort and new opportunities to humanity, it has also led to the homogenisation of products and designs as well as a dependence on global supply chains for tools, materials, and components. In response to increasing global warming and supply chain disruptions caused by climate change, however, the possibility of passive cooling solutions for buildings is currently having a resurgence — with an effort to recover ancient technologies used throughout history in regions that have always had to deal with hot climates.

In India, contemporary architects are using traditional passive cooling methods like sloping roofs, internal open courtyards, lattice screens or ‘jaali’, and evaporative terracotta cooling walls to cool homes and offices during unprecedented heatwaves.

In New Delhi, where temperatures soared to 52.3°C in the summer of 2024, architects like Monish Siripurapu — founder and principal architect of CoolAnt, part of Ant Studios — are using the evaporative cooling ability of terracotta in innovative ways.

CoolAnt’s terracotta cooling towers called Beehive — made from hundreds of handcrafted terracotta cones and cylinders stacked in a honeycomb pattern fitted inside a stainless steel frame. The cooling towers reverse the evaporative cooling effect of the matka to cool the air around the terracotta cones and cylinders by pumping recycled water over the stacks. As the water evaporates from inside the Beehive’s many terracotta cones and cylinders, it cools the air that passes through it. Since their first tower for a client in New Delhi, the studio has created over 30 such cooling towers in schools, public buildings, airports, and offices across India.

Ant Studio is far from the only architectural company using the ancient material as a cooling solution. Avinash Ankalge — architect and one of the co-founders of A Threshold, an architectural firm based in Bengaluru — has been experimenting with recycled terracotta tiles to build passive cooling screens and façades for buildings.

A Threshold’s designs use a cooling method called ‘mutual shading principle’ to cast shadows over a wall so that the sun’s glare doesn’t heat it. The same passive cooling method was used extensively in homes, havelis, and palaces in Rajasthan — especially in Jaipur and Jaisalmer.

Beyond terracotta, another traditional cooling method Indian architects are reviving in their designs is the ‘jaali’ lattice screens — a signature feature of later Mughal architecture. The term ‘jaali’, which means net, is used widely in Islamic architecture in Central and South Asia. Cut and carved from marble or red sandstone in ornamental geometric patterns, jaali became a popular architectural feature in Mughal India between 16th and 18th centuries CE.

These lattice screens use the Venturi effect to cool air. When air flows at a higher speed as it circulates through a narrower passage, it creates negative pressure, causing a partial vacuum that propels the fluid. Jaali is typically installed as an external lattice screen to buildings, and its cross section usually shows a larger opening on the outside and a smaller opening on the inside. As the wind passes through the jaali, it creates a difference in pressure between the interior and exterior of the building, and cools the air as it is compressed and released.

As the world becomes warmer and artificial cooling becomes more necessary because of global warming, but also unsustainable because of high energy costs and larger carbon footprints, it is not an overstatement to say that cooling interiors will be one of the foremost architectural challenges of the future. However, looking back to these traditional passive cooling methods without using electricity and with local materials and traditional construction methods may help us chart a sustainable path for a warming future.

Learn more about the state of Indian architecture here.

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