Explained: As Indoor Heat Rises, Can India Turn To Green Roofs?

With climate change, temperatures are rising in many parts of India. Even as the India Meteorological Department has declared the onset of the 2023 monsoons over most parts of the country, its progress has been sluggish.

In order to mitigate the impacts of indoor heat, and to allow people living or working inside to continue to do so without extreme discomfort, India will have to build or retrofit its physical infrastructure to withstand the heat. One of the solutions mooted to mitigate indoor heat is a ‘green roof’ or a ‘living’ roof.

What are green roofs and what are some of the challenges in adapting them to the Indian context? Our #TIL explainer tells you more.

What are green roofs

A green roof, or rooftop garden, is a vegetative layer grown on a rooftop. Green roofs provide shade, remove heat from the air, and reduce temperatures of the roof surface and surrounding air. Green roof temperatures can be 30-40°F (up to 4.4 degrees Celsius) lower than those of conventional roofs. In addition, green roofs can reduce building energy use by 0.7% compared to conventional roofs and reduce peak electricity demand, according to the United States Environmental Protection Agency (EPA).

In India, roughly 8% of households were air-conditioned as of March 2018. This is estimated to rise to 50% by 2050, according to a report by the The Energy and Resources Institute. This would not only lead to an increase in hydrofluorocarbons leaking from AC units, but would also translate into a significant increase in energy needs, as IndiaSpend reported in November 2021.

Green roofs can be traced as far back as the Roman Empire, wherein they grew trees on top of buildings. During the 19th and 20th century, rooftops in major cities of the United States were greened to replace the rising land costs of building parks. This 2014 paper pegged Germany as the world leader in green roof technologies, where more than 10% houses had installed green roofs.

According to a market analysis report by Grand View Research, an India-and US-based market research and consulting company, the global green roof market size was valued at $1.1 billion in 2019 and is expected to grow at a compound annual growth rate of 17% from 2020 to 2027. Europe led the global market in 2019 and Asia Pacific is also expected to witness notable growth by 2027, it states.

A paper, by two researchers from Nagpur, published in the International Journal of Renewable Energy Research in 2016 investigated the impact of green roofs in India. The paper found that experimentally, compared to an exposed roof, the room air and interior surface temperature of rooms under the green roof were reduced by a maximum of 17% and 22% respectively. Compared to a bare roof, the room air temperature of a green roof was reduced by a maximum of 4.4°C, the research concluded.

There are two main types of green roofs: extensive and intensive. Extensive green roofs have a thin substrate layer [surface upon which the vegetation grows] with low level planting, typically sedum [a type of plant] or lawn, and can be very lightweight in structure. Intensive green roofs have a deeper substrate layer to allow deeper rooting plants such as shrubs and trees to survive.

Extensive systems offer the most cost-effective solution, and are preferred for retrofitting onto existing buildings, as the structural capacity of the roof will not have to be increased to take the extra weight of intensive planting, one paper, presented at a 2014 conference on Passive and Low Energy Architecture, by researchers from Raipur, had noted.

Intensive green roofs impose a heavy load on the roof, thus needing structural design at the initial stage, or retrofitting in case of existing buildings. This system generally requires more care, including irrigation.

Architect Salil Riswadkar of the Pune-based Riswadkar Associates has executed a few green roof projects through his firm. He is positive that green roofs can work in India with some pre-planning.

“We did some basic landscaping and vegetation in some residential and commercial projects, for example, a terrace of 300-400 square feet,” said Riswadkar. “We observed that the temperatures in the rooms below were much lower than the adjoining rooms, depending upon the vegetation, soil type and drainage system. For example, in one of our residential projects, the room beneath was cooler by around 2 degrees Celsius. The reduction in power consumption was a welcome byproduct,” he said.

Challenges

Over the years, many cities and states in India have developed their own Heat Action Plans (HAPs). These provide a detailed standard operating procedure on what is to be done before, during and after a heatwave event, and what could be the short-term and long-term measures to tackle heatwaves.

The Delhi-based non-profit Centre for Policy Research (CPR) analysed 37 heat action plans as part of their report titled ‘How is India Adapting to Heatwaves’, released in March 2023. The researchers found that 10 HAPs had mentioned green roofs as one of the solutions.

For example, Himachal Pradesh’s heat action plan has recommended promoting cool roof initiatives such as painting the roof white, creating green roofs and walls, and planting trees in neighbourhoods to keep them cool, as departmental responsibilities.

The Gujarat heat action plan has a separate section on green roofs, but it states that due to higher costs and need for water, they are likely not a cost-effective solution for heat reduction in low-income communities in India.

Experts IndiaSpend spoke to also cited the higher cost of green roofs as a challenge.

The US EPA compared the costs of extensive green roofs and intensive green roofs. The costs of constructing green roofs depended on the components, including the growing medium, type of roofing membrane, quantity of plants and drainage system.

The US EPA estimated that the initial costs varied from $0.93 (around Rs 76) per square metre for a simple extensive roof to $270 per square metre (around Rs 22,000) for an intensive roof. Maintenance costs for either extensive or intensive green roofs range from $8 to $11 (around Rs 650-900) per square metre.

To compare that to the prices in India, Riswadkar said that the cost of a green roof project his firm had executed which fell in the ‘extensive’ category was around Rs 7500 per square meter.

Specialised systems for green roof layers like waterproofing, drainage layer, and root barriers need to be installed, which will add further costs, another paper, by the researchers from Raipur, published in the International Journal of Scientific Engineering and Research in 2019 had noted.

“Even in HAPs, cool roofs are recommended more strongly than green roofs because green roofs are expensive and require a lot of water, maintenance,” said Aditya Valiathan Pillai, fellow with the Initiative for Climate, Energy and Environment (ICEE) at CPR and author of the report mentioned above.

“Green roofs may not be scalable for low-rent neighbourhoods. For a poorer population to have those sort of rooftops and water availability when they barely have enough water for themselves… So it’s limited to urban rooftops with fairly large surface area, including corporate/government buildings. Green roofs reduce the cooling burden but increase the water burden. So, it won’t be mainstream India’s heat response.”

The ‘cool roofs’ that Pillai was referring to above are made of material or coating that reflects sunlight and heat away from the structure, reducing roof temperatures. This makes indoors cooler, increases comfort and reduces the amount of air conditioning needed during hot days. Cool roofs are considered a simple and cost-effective solution as they reflect sunlight and absorb less heat. Depending on the setting, cool roofs can help keep indoor temperatures lower by 2°C to 5°C as compared to traditional roofs.

Tejas Chavhan, Director at Green Spaces, a consulting company for green and sustainable development, believes that green roofs can work in India if there are subject experts available and if maintenance contracts can be given.

“It needs irrigation, trimming and I see a big gap [in service providers]. If there is a company or service which can help in year-round maintenance, it will definitely work. There are people who want to do it, but don’t have the know-how to maintain it,” said Chavhan.

Riswadkar is also positive about the benefits and feasibility of green roofs, but in new constructions only.

“Green roofs can work if planned during the construction stage because the soil will add a dead load on the building,” Riswadkar said. “So, it has to be accounted for at the planning stage itself. Retrofitting does not work. Doing it on old buildings is definitely not recommended because they were not planned to carry this dead load. Also, green roofs require a proper drainage system to make sure there is no leakage. That kind of drainage system can only be put in at the planning stage itself.”

(Mira Dasgupta, intern with IndiaSpend, contributed to this report)

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