Who would have thought that an innocuous product invented 4,000 years ago in Mesopotamia would add to the heat island effect in Mumbai? Glass: it’s here, there, and everywhere, not just in our homes, but in our city too. Where space is at such a premium, high rises are becoming the norm. And increasingly, these skyscrapers have facades of glass.

But glass is not the only culprit. Thermal resilience should inform the way we design our buildings. However, it often takes a backseat, as developers prioritise faster construction methods and maximising the number of apartments over long-term sustainability.

Few of the existing rules in the Development Control and Promotion Regulations (DCPR) for Mumbai help build thermal resilience, but most of them are not mandatory. What are some of the amendments needed to minimise heat exposure?

Glass buildings: heat traps?

Studies show that reflective glass walls of buildings have a significant impact on the surrounding temperatures. Sameer More, a civil engineer who has worked on the construction of buildings with glass facades, says that these are inspired from American architecture. They work well there due to the cold climate. But glass is not an ideal material for the hot and humid conditions of Mumbai. 

He explains that fully transparent glass facades not only heat up the outside, but the inside too. They simulate the greenhouse effect, trapping heat inside. In a cascading effect, a higher load on the air conditioning system thus releases more heat into the surroundings. Energy requirements of the building increase too.

Read more: Heat waves a real and present threat, Mumbai must speed up climate action

On the other hand, reflective or tinted glass absorbs some light, allows a part of light to enter in and reflects part of it. This heats up the inside and causes a glare on the outside, increasing the external  temperature. Plus the glass itself heats up. 

Architect Aparna Dwivedi, who has a doctorate in climate studies, says that builders prefer glass as it allows for quicker construction and demolition. She carried out a study of vegetation outside glass buildings using thermal cameras. She found that the heat emanating from the buildings does not allow the trees to survive. 

Alternative measures

Harshad Bhatia, urban designer and architect, agrees with her. “Glass facades should not face east or west in the summer because most of the year the sun is moving from east to west from the southern side. And the northern facade is where we get the north light. That means you don’t have direct sunlight but you have light for you to work inside.”

The only criterion mentioned in the DCPR is that the material used for the construction of the corridors, stairways and facades should be fire resistant. But it does not put restrictions on the kind of material to be used. Not only are commercial and office buildings using glass facades, but residential buildings and even schools too are opting for them nowadays.

Harshad says that it would be ideal to use natural materials for construction or even human-made materials like bricks which make use of natural material. But he also acknowledges that these materials come with their own limitations. Consumption of construction material is very high today. The supply of such material may not be able to keep pace with the demand and bricks in particular may not be suitable for high rises.

Aparna suggests that the walls should be thick, or barring that they should have cavities which will not allow heat in. This, she says, will reduce the burden on cooling systems.

Read more: Vulnerability matrix helps protect those at risk from heat stress: Climate researcher Ujjvala Krishna

Need for open spaces

Architect Shriya Bhatia emphasises that thermal resilience in buildings depends on two key factors: the building envelope, which acts as a barrier between indoor and outdoor spaces, and the external environment, which influences heat exposure and retention.

She says, “There should be development control regulations for the outdoor environment.”

The DCPR’s regulation 27 does specify how much of a plot has to be maintained as recreational grounds when a building is constructed, depending on the size of the plot:

The regulations further specify that five trees will be grown per 100 square metres of the recreational grounds. Shriya believes that in addition to mentioning the number of trees, the DCPR should also give directions on the kind of vegetation and trees societies can grow in these open spaces, as this will directly impact the water requirement.

One caveat of this regulation is that if the total area of the recreational grounds exceeds 4,000 square metres, certain structures can be constructed on it. These structures could include electric substations, pump houses, pavilions, gymnasia, and swimming pools.

Avoiding concretisation

To ensure water percolation, 60% of the recreational space is to be directly on the ground and not on a podium. This area is to be kept free of paved surfaces. It allows a podium to be built on the rest of the 40% of the recreational grounds.

But, a BMC official informed us that currently these numbers are not applicable for new building proposals since this regulation has been challenged in the court of law. The petitioner has asked that 100% of the recreational grounds be soil (Mother Earth), with no podium at all. Pending the decision of the court, at present, 100% of recreational grounds are to be maintained on mother earth according to the BMC official.

Podiums introduce concretised surfaces into the recreational grounds. Concrete surfaces absorb heat and radiate it back into the environment. Moreover, surface temperature is a significant driver of atmospheric temperature. Aparna  suggests instead that these open spaces are kept completely green, so that water percolates into them. This, she says, will cool the external environment when water evaporates from the ground.

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Design for heat resilience

The external environment of the building includes both horizontal surfaces like open spaces, parks, the terrace etc and vertical surfaces like the walls of the building. Terraces and roof tops absorb a lot of heat. In summer months, the residents of the flat under the terrace bear the brunt of this. In fact, people prefer not to buy such flats due to the heating of the terrace as well as risk of leakage. Rooftop gardens are a heat mitigation measure. The DCPR, under the chapter ‘General Building Requirements’, does permit this, provided effective water proofing has been carried out.

While designing the building, it is important to measure the heat generated from both horizontal and vertical surfaces, keeping in mind the path of the sun. In addition to this, the material used on the external surface of the building, such as paints and finishes, also affects the temperature. Harshad says that the sun path must be factored in while deciding the material used on a particular side of the building. He says, “But these policies are not in place. Right now it’s as per the designer, developer and the orientation of the plot.” He adds that facade designs have to be done at a holistic, comprehensive city level.

Promoting better ventilation

Shriya also observes other changes in designs in the newer constructions. Where older houses had ventilators over windows, new designs do away with them. Moreover, the height of rooms from floor to ceiling is also reducing considerably. All this is contributing to the heat inside homes.

She observes that rooms in new constructions have less volume. “Volume also helps to minimise heat. Because of the height, hot air rises, and then you have ventilators for the hot air to escape.” The DCPR has set the minimum height at nine feet. Though the maximum height can go up to 13.77 feet, builders choose the minimum possible height so as to maximise the number of houses they can build.

The BMC official we spoke to said that there are certain regulations in the DCPR which are geared to make buildings heat resilient. But some of them are not mandatory. For instance, regulation 70 of the DCPR states that the builder may opt to get a certification from the Indian Green Building code (IGBC)/Energy Conservation Building Conservation Code (ECBC). He added that at the present moment amendments for thermal resilience to the DCPR are not on the horizon.

As the climate crisis gets exacerbated, our houses and places of work need to adapt to cope with it, whether it is extreme heat or even flooding. Moreover, as Harshad points out, technology is rapidly evolving too. The DCPR must address these issues and make amendments to the rules to tackle heat stress.

Also read:

• Under the scorching sun: Heat stress takes a toll on healthcare workers in Chennai
• Inclusivity is pivotal to measures against heat, says urban planner Jaya Dhindaw
• Chennai braces for extreme heat, prepares measures to combat rising temperatures