In illustrations: Here’s how we can design homes to stay cool in India’s increasingly hot summers

Imagine a residential complex in a city such as Indore or Surat in the year 2030. If it looks like the high-rises being built in these cities today, it will be a multi-storeyed structure towering over the city, filled with modular homes with thin concrete walls. People living in the higher storeys will tend to keep their windows shut despite the heat, because the wind will be so powerful that it will make their doors slam. Those living on the topmost floor will be uncomfortable because they will be heated from all sides and from above.

The views will be sweeping, and large French windows will show them off: the city spread out beneath, barring the occasional spindly tower at eye-level. The homes will get hotter by the year. Thin concrete and glass are no guard against the subcontinent’s punishing summers. Each home will require air conditioning in order to be liveable, and those air conditioners will exude more heat into the city.

An estimate by McKinsey projects a 40% probability of at least one heatwave occurring in these urban areas between 2025 and 2035. Temperatures in the summer in Indore have already gone as high as 44 degrees Celsius and Surat, despite being a coastal city, saw temperatures rise six degrees above normal this year. By 2030, places like Indore and Surat – which are already Smart Cities – will possibly become tier 1 cities, and in the decades after, megacities.

“India is no stranger to heat. It has a long history, a public memory and practices for dealing with it,” wrote Chandni Singh, a climate change researcher at the Indian Institute for Human Settlements, Bengaluru, in The New York Times.

What if some of those traditional practices to adapt to heat, could be applied to the homes we build now, for the people who migrate to these cities in hope of building better lives? What could those homes look like?

Mongabay-India spoke to several architects and urban design experts to identify some of the ways in which residential buildings, in particular, can be made cooler – a set of techniques called “passive design” because they do not require the expending of energy for cooling.

The illustration of a possible design for such a residential building is based on existing research and inputs from experts.

The apartment complex, with buildings designed to be cooler, will be designed for thermal comfort, a somewhat subjective measure, because people’s idea of comfort differs based on their own preferences, the context they are in, and their life experiences.

“Thermal comfort is the comfort hours which you get inside your structure. This is without air conditioning or any mechanical cooling system,” says Rajneesh Sareen, Programme Director, Sustainable Buildings and Habitat Programme at the Centre for Science and Environment in New Delhi. What humans think is comfortable though, has remained largely unchanged, Sareen says. “Your materials and other science may have changed, but your understanding of wind and the breeze [too windy] remains the same. Your body understands those in a similar fashion.”

The three main passive design elements that go into creating thermal comfort are shading, ventilation and insulation. Shading blocks direct heat from entering the home, while still allowing light. Ventilation allows air to flow through the home, ideally allowing – cooler – outside air to replace inside air at an optimal rate of 10 changes an hour. Insulation keeps the home from absorbing too much heat, while still staying comfortably warm when it’s cool outside.

In the Indian subcontinent, which lies in the Northern Hemisphere, the sun travels directly overhead in the summer, and falls at an angle from the south in the winter.

“In the summers the sun on the southern side is very high, so even if you have a simple chajja [overhang for shade], it prevents the sun from hitting the glass in the window. That’s enough for that. And then in the winters, the sun angle is low on the south. In the winters you require the sun, so that same chajja is not going to obstruct the sun in the winter. This is especially true of places in north India, and that works out pretty well,” says Saswati Chetia, an architect and associate director at Greentech Knowledge Solutions, who specialises in energy efficient building design.

The Eastern sun, where it rises, is mild and provides light, so Sareen suggests keeping living and working spaces on that side, while cutting the Western sun as much as possible with shading and insulated walls.

Ventilation is tricky because it requires the flow of wind through the house, but only when it is cooler. You also do not want too strong a wind. To achieve optimal air flow, window design becomes crucial. French windows allow large amounts of light in, but they are inefficient from a ventilation point of view, because at any given time, only half the entire window area can ever be opened. Windows that open inward or outward, on the other hand, allow more than 90% of the window to be opened.

Chetia also points out another flaw with French windows: They contain too much glass, which is a poor reflector of heat. Large windows turn homes into greenhouses, effectively trapping in heat that you then need an air conditioner to cool down.

She suggests switching to older fashioned windows that were half glass – to let in light – and half wood, or another material that blocks both heat and light. If the view is compelling enough to still merit a French window, a heat reflective coating on the glass or a shade on the window becomes advisable.

Ventilation is also a major reason for limiting the height of the building to no more than four storeys. Studies of wind flows in cities show that higher storeys experience uncomfortably high wind speeds. Tall buildings laid out in rows also create wind tunnels within the city, altering the patterns by which air flows through the city, and affecting life on the ground.

One issue with using outside air for ventilation in larger cities is air pollution. As Chetia points out, “In places like Delhi, we really need to work on getting our outside air right, so that we can use it. It’s a resource that is there, and you are just polluting that resource and you’re not able to use it. And for other cities, it’s that don’t pollute your air to that extent. If you are not able to use it, it will be disastrous.”

Insulation is the third part of designing for thermal comfort and this is determined by the materials that are used to construct homes. Modern residential complexes are made with reinforced cement concrete that is moulded concrete set around a steel frame for strength.

Reinforced cement concrete walls are great for multistorey construction because each individual wall is strong enough to be load bearing. This means there is no need for special beams or a frame. Reinforced cement concrete walls can also be very thin, which is a boon for higher constructions, because the walls are light.

Making moulds for reinforced cement concrete walls is expensive and the cost can only be justified if the moulds are used over and over, in modular constructions in multistorey residential complexes. Using such modular walls saves on labour and increases the pace of construction, which is why reinforced concrete cement finds increasing favour in city construction.

It also means there is more floor area since the walls are thinner – as thin as 15 cm – and flats are sold based on floor area. But thin walls also mean faster heat transfer from outside to inside. Concrete is a poor insulator, with twice the thermal conductivity of clay bricks, and almost thrice that of environmentally friendly but less strong materials like Autoclaved Aerated Concrete.

In the homes of the future, thicker, or better insulated walls might be the way to go. In older buildings like the Benares Hindu University building in Varanasi, the walls are up to 24 inches thick, allowing in even lesser heat during the day, but trapping in heat to release during cool nights. But that doesn’t mean modern homes need such thick walls. There are plenty of compromises that can be struck along the way.

“So instead of trying to do a very thick material wall, what we try to do is use hollow blocks somewhere or use double walls which is a cavity wall. There are two layers of thin walls with this air gap in between, which helps in reducing the heat gain. So instead of building an 18 inch or 24 inch wall, you might be able to achieve the same comfort in 12 inches itself,” suggests Anurag Tamhankar, a senior architect at Biome Environmental Solutions, a design firm with a focus on ecology.

This still requires a compromise though: if walls are thicker, floor area will be less, which means that the carpet area of the house becomes less, and that is the measure on the basis of which homes are sold.

“Most of the time people think it’s very expensive to work on [passive design],” says Sareen. “Then you have to tell them that it’s not expensive. Even the design, which is 2% of total cost of the project, can eventually give you a lot of gains.” Many of the solutions, like building orientation or careful window design do not add to the cost of construction, but still provide a measure of thermal comfort.

Whether these designs are scalable, is yet not established. Since real estate costs are high in urban areas, builders try to build high to maximise land usage, indicate experts. But passive design could be adapted in tier 2 cities which are still expanding, where land costs are still not entirely prohibitive and there is, perhaps, still time to reimagine how people live.

There are several design ideas that have evolved in the Indian subcontinent that merit consideration when building the homes of the future. Such ideas can be divided by class and chronology, says Amita Baviskar, sociologist and Professor of Environmental Studies and Sociology and Anthropology at Ashoka University.

In dense areas of Delhi like the walled city of Shahjahanabad, she explains that it was a combination of trying to deflect the direct rays of the sun while allowing for ventilation. The devices used would be thick walls, porches, verandahs, chajjas, roshandaans [high ventilators] above the doors, and central courtyards. “Most houses will be ground floor, one, two storeys, with an open terrace on top. And that open terrace again became the place to catch breezes, sleep out on summer nights, catch the sun in the winters and so on.”

What the British brought was a style that she classes broadly as tropical architecture: “It’s colonialism meeting local climate, which was based on a very privileged access to a lot of space.” So such homes had large gardens, verandahs and extended overhanging roofs.

Baviskar also notes that there are similarities between the worker-housing chawls of Mumbai and the homes in Shahjahanabad in the shared balconies, the corridors, and the architecture of the shared courtyard.

Mughal architecture incorporated water channels and fountains in indoor spaces, which enhanced evaporation and thus, cooling. Tamilian and Keralite homes often have wrought iron gates and verandahs before the home, allowing for a transitional space between outside and inside that screens heat. Local building techniques like mud plastering and thatched roofs in Rajasthan would also reduce sun exposure.

While it is useful to look to vernacular techniques for inspiration, Tamhankar points out that not all such ideas are practical anymore. “Many of the techniques which were there, they are not probably suited for our current lifestyle,” he says, pointing out that mud walls require re-plastering every year.

There are other practical issues: the lack of skilled labour and raw materials for such vernacular design techniques, says Sareen. Chetia also points out that brick laying, for instance, requires a mason, and then someone to supervise the mason, all of which increases the labour cost. Prefabricated concrete brings this cost down. Still, the design cost is a small percentage of the project cost and smart design choices can result in considerable savings on cooling bills over time.

Newer but established ideas like roof gardens and solar panels on the roof are also important. The solar panels screen plants on the roof from direct sun, and can power at least common areas in the apartment complex, resulting in further energy savings.

Cold comfort?

Only 8% of households in India had air conditioners in 2017 according to the India Cooling Action Plan, but that number is set to grow rapidly. The International Energy Agency estimates that Indians own 67 million airconditioner units in 2022. By 2032, they project that this number will go up to 318 million units.

Much of this airconditioning will be devoted to cooling homes in urban areas, which heat up faster than rural areas. Such a high demand for cooling will drive up electricity demand, resulting in what the International Energy Agency calls a “cold crunch”.

The solutions for this are of course multi-fold: air conditioners should become more energy efficient, urban planning should be improved in order to avoid the Urban Heat Island effect, and the homes of the future need to be built for greater thermal comfort. That does not mean that many such homes won’t still require air conditioning, but their design will mean that the air conditioners can run for fewer hours, and at higher temperatures than otherwise.

While a dense and compact typology increases efficiency in terms of usage of land and distribution of services, a high-rise building may underperform on sustainability, affordability and adaptive comfort, notes a policy brief by Centre for Science and Environment. “Studies show that the taller a building, the higher are the emissions per square metre due to higher cement and steel load per unit area,” it notes.

“Go beyond [ground + four storeys] and you are increasing your embodied carbon, because you require more concrete density to go higher, you require a lift, you require fire systems and all the extra stuff,” says Chetia, which is why she suggests keeping buildings high density, but low rise. Emissions are automatically reduced when buildings do not require air conditioners to run continuously.

With inputs on illustration from Sugeet Grover, Programme Manager at Centre for Science and Environment, New Delhi, and Saswati Chetia, an architect and associate director at Greentech Knowledge Solutions.

This article was first published on Mongabay.