I – Choosing Wall Materials For Cooler Homes Using Computational Methods

Image: Sanjaynagar Slum Redevelopment Project, Ahmednagar, Maharashtra
Image Credit: Community Design Agency

The following article was originally written by a Gubbi Labs Staff Writer and published here.

Eco-efficient wall materials, such as aerated autoclaved concrete blocks, enhance indoor comfort by significantly reducing temperatures in naturally ventilated houses.

With rapid urbanisation and a growing population, India faces a pressing need for affordable and sustainable housing. The Pradhan Mantri Awas Yojana (PMAY) by the Govt. of India aims to address the country’s housing shortage by providing millions of low-income homes to urban migrants and the poor. The Govt. of India has also set up the Global Housing Technology Challenge to provide innovative solutions to large-scale housing issues. One of the key issues of mass-housing in a country like India would be maintaining comfortable temperatures or thermal comfort indoors.

Researchers from the Indian Institute of Technology Bombay (IIT Bombay) and Community Design Agency, Mumbai have developed a new method that can help choose wall materials that maintain comfortable indoor temperatures in naturally ventilated homes. They used a numerical and simulation-based technique called Computational Fluid Dynamics (CFD) to explore the relationship between wall materials, variations in airflow, and thermal comfort. The researchers chose local and eco-efficient options such as burnt clay bricks and AAC blocks that reduce environmental emissions and transportation costs. Their findings can improve living conditions and occupant well-being even for low-income housing in India. The study was published in the journal Energy and Buildings.

Thermal comfort directly impacts the health, well-being, and productivity of individuals. India’s tropical climate, with its extreme heat and humidity, makes living conditions tougher without proper airflow. The effects of global climate change, such as severe heat waves and urban heat island phenomenon, further exacerbate these conditions. “Construction materials play a major role in defining the quality and livability of the building, and it is important to make sure that mass housing projects adopt the right materials that provide comfortable living at a reasonable cost,” says Prof. Albert Thomas of IIT Bombay, who led the study.

Building envelopes, which include roofs, walls, floors, windows, doors, and foundations, act as barriers between the indoors and outdoors and significantly affect heat transfer. Wall materials, which typically account for over 40% of building envelopes, are crucial in determining indoor temperatures by absorbing, storing, and emitting heat.

“This study is particularly relevant since low-income housing in India is still predominantly naturally ventilated spaces with little to no access to HVAC (Heating, Ventilation, and Air Conditioning) systems, unlike the Western world. Through this study, we have attempted to use a real-life case study and modelled the same to understand the impact of material selection on thermal comfort in indoor spaces,” remarks Dr. Vandana Padmanabhan, an author of the study and Materials and Technology Lead at Community Design Agency.

The study proposed a framework that simulated indoor temperatures by considering different wall materials and analysed the airflow patterns within naturally ventilated housing units using CFD modelling techniques. While concrete bricks might be the most economically viable, they do not provide the best thermal insulation and are environmentally hazardous. The researchers chose aerated autoclaved concrete blocks (AAC), compressed stabilised earth blocks, burnt clay bricks, and compressed fly ash blocks for the simulation. Airflow patterns included conditions such as “all windows open and doors closed” and “all doors and windows closed” and similar building operating conditions. Using CFD modelling, the study mapped the temperature distribution and airflow within the housing units in these scenarios.

The researchers found that AAC blocks outperformed other locally available materials, such as compressed stabilised earth blocks and compressed fly ash blocks, in insulating the indoors. They could maintain cooler temperatures owing to their low thermal conductivity and specific heat capacity, which reduces heat transfer. The selection of wall materials depends on factors such as local availability, construction methods, and structural needs. “AAC blocks have lower compressive strength than some of the other wall materials. Solutions to this include reinforcing AAC with additional structural supports or combining it with other materials to maintain structural integrity while benefiting from its thermal insulation properties,” explains Tripti Singh Rajput, the lead author of the study and a PhD student at IIT Bombay.

To encourage universal adoption of AAC blocks, enabling mass production and wider availability can help. Prof. Thomas shares that “given the number of buildings to be built in India in the next few decades, if a policy change can be brought out for a wider adoption of materials such as AAC blocks for government-driven low-income housing and mass housing projects, it will result in mass production and hence, lead to the reduction of the overall cost”.

The study also highlighted the differences in global and Indian building standards and guidelines. Compressed stabilised earth blocks and compressed fly ash blocks did not meet the recommended values for thermal performance as per building design guidelines for acceptable thermal conditions, such as the EcoNiwas Samhita and ASHRAE 55. However, according to India-specific standards suggested by the Indian Model for Adaptive Comfort in – Naturally Ventilated (IMAC-NV) and Residential (IMAC-R) models, the indoor temperatures of all the tested wall materials were found to be within the comfortable range under different operating scenarios.

The thermal comfort metric used in the study was based on a solution that assumes that the different parameters, like temperature and airflow, are stable. Upgrading to a solution that considers varying states of different parameters and includes more dynamic and precise personal and environmental parameters of thermal comfort could further strengthen the framework’s robustness.

Talking about the future of the research, Prof. Thomas says, “More research can be done on generalising the results across different building types and climatic conditions, which will help in easy decision-making and policy formulation across different building types and scenarios.”

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The original article titled Analyzing The Thermal Performance Of Walling Systems In Low-Income Housing Through Computational Fluid Dynamics Approach was written by IIT Bombay’s researchers and was published in the journal Energy and Buildings, Volume 319.

The original paper can be found here.

 

II – Green Roofs Can Effectively Reduce Floods In Dense Urban Areas, Finds Study

Image Credit: CHUTTERSNAP on Unsplash

The following article was originally written by Ms. Arati Halbe and published here.

Planting small trees on roofs of buildings in dense urban areas can reduce flood volume and runoff

In the past decade, floods in big and densely populated cities have become more frequent. They are increasingly causing damage to property and infrastructure, and loss of life. As buildings, pavements and roads made of concrete or tar increase, water-absorbent areas on the ground reduce. After heavy rainfall, water flows rapidly in large volumes and quickly accumulates in low-lying areas. Risks of epidemic breakout and infections after the floods recede are higher due to the dense population over a large area. Any damage to centres of commercial and national importance situated in cities can also potentially affect a larger population.

Floods in cities are very different from rural floods in their nature, causes and effects. They need different analysis and mitigation methods. Cleaning stormwater drains, adding stormwater silos that can act as local storage and avoid runoff, interlinking lakes and installing water pumps to remove accumulated water are some measures that the administration is implementing centrally. However, these methods involve large infrastructure changes and are expensive. Implementing small-scale distributed measures such as rainwater harvesting, rain gardens and green roofs that help control runoffs closer to their source is more sustainable.

Small-scale and sustainable measures cost less than large infrastructure changes. However, it is important to study their effectiveness and quantify their benefits. In one such effort, Tushar Bose, faculty, CEPT University, Ahmedabad and Prof Pradip Kalbar and Prof Arpita Mondal at the Indian Institute of Technology Bombay (IIT Bombay) evaluated the performance of ‘green roofs’ in reducing floods in dense urban areas. The study was published in the Journal of Environmental Management. The project received funding from the Science and Education Research Board, Govt. of India.

Green roofs are created by planting trees on rooftops of buildings in a shallow layer of soil over a waterproof membrane and a drainage system. Green roofs can keep the building cooler in summer and absorb rainwater. Excess water can help slowly recharge the rainwater harvesting system, avoiding fast runoff. Installing green roofs needs additional expenditure and increases the weight load on the building. It also needs regular maintenance. So, the advantages of implementing green roofs must be carefully evaluated against their cost.

Previous studies assessing the effectiveness of combined strategies such as rain gardens, infiltration trenches and green roofs are available for Western countries. There are very few studies that assess the performance of only green roofs, especially in the Indian context. In India, not all buildings are suitable for green roof installation. For example, slums and some low-cost housing have roofs of metal or concrete sheets and thus are not suitable for green roof installation. “A significant contribution of the study is that it provides a realistic performance assessment in a highly dense urban area and quantifies the overestimation of runoff reduction. These overestimates arise from scenarios that consider all rooftops without evaluating the green roofing potential,” say the authors.

The researchers chose the Odhav area of Ahmedabad, Gujarat to study the effectiveness and performance of green roofs. They divided the total area of 100 hectares into nineteen sub-catchments to create the model. They identified buildings suitable for green roof installation. Buildings with roofs of metal or concrete sheets and industrial buildings are not considered suitable. They considered land use, local rainfall patterns, the terrain and natural pathways of the water drainage of each area to calculate the runoff and flood volume in the sub-catchments. They created a computer model that mimicked how water flowed in the area. Using this model, they calculated the runoff and flood volume assuming different scenarios of heavy rainfall events and various percentage values of green roof implementation.

The data that the researchers used included the model of the terrain of the area, soil type, stormwater network layout, and whether the land is unused or used for buildings, gardens or other purposes, obtained from the Ahmedabad Municipal Corporation and in-person surveys; and rainfall data from IMD

The IIT Bombay team considered scenarios where green roofs were installed in 25%, 50% and 75% of the suitable buildings. They calculated the flood volume reduction for a total of 36 scenarios, where they considered three green roof application rates (25%, 50% and 75%), four extreme rainfall event frequencies (extreme rainfall events once in 2, 5, 10 and 25 years), and three scenarios of extreme rainfall event duration (2, 3 and 4 hours). They also explored the minimum application rate needed for each of the 12 scenarios created by varying the extreme event frequency and duration. The researchers also calculated the uncertainties in the model predictions.

The study finds that using green roofs can reduce the flood volume by about 10-60% depending on the percentage of green roof application for higher than usual rainfall of one in two year frequency. However, the reduction is not linearly proportional to the application rate because flood volume also depends on the drainage network capacity, especially at high volumes of rainfall. The researchers observed that when less than 25% of buildings have green roofs, the flood volume and runoff reductions can be as low as 5%. When the green roofing is more, there is a collective effect over the whole area and flood volume is reduced. The study also quantifies the uncertainty in the prediction of flood volume reduction and finds that the rainfall intensity contributes maximum to the uncertainty of runoff reduction.

The findings of the study can help policymakers make informed decisions about city-specific green roof implementation.

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The original article titled Performance And Uncertainty Assessment Of Green Roofs For Urban Flood Reduction In A High-Density Catchment In Ahmedabad, India, was written by IIT Bombay’s researchers and was published in the Journal of Environmental management, Volume 365.

The original paper can be found here.

      

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