Residential Building Retrofits

One of my research areas focuses on evaluating retrofit strategies to improve the energy efficiency and thermal comfort of residential buildings in India. My work in this area has evolved considerably over the years, from phase change materials to cool roofs and, more recently, to the hygrothermal performance of external wall insulation.

Phase Change Materials (PCMs)

Initially (when I was a young Assistant Professor 😇), I was very excited about the use of PCMs in the building envelope to improve the energy efficiency of new and existing buildings. PCMs operate on the principle of latent-heat thermal energy storage. During periods of high heat gains (daytime), they undergo phase transformation (typically solid to liquid) and store the excess thermal energy, which is released back during periods of low heat gains (nighttime). Through this process, they seem to reduce cooling energy consumption and peak cooling loads, and delay peak loads to later hours. However, my research [Rai, 2021] showed that, regardless of the amount of latent heat stored by the PCM layer, the daily heat gains and cooling loads were equal for wall configurations having equal thermal resistances. Thus, PCM integration was found to be ineffective at reducing daily heat gains or cooling load requirements, although it appears suitable for peak load management. Another paper from my research [Sharma and Rai, 2020] compared PCM-integrated and insulated envelopes for reducing the summer cooling needs of Delhi homes. We found that insulation is not only much cheaper than PCMs but also outperforms them in reducing envelope heat gains and peak cooling loads. So, I am not so sure about the PCM technology anymore. 😔

Moving Towards Practical Solutions: Cool Roofs 

So, after my brief (was it unsuccessful? 🤔) stint with PCMs, my research shifted towards more practical and cost-effective envelope retrofit strategies. Our study [Ukey and Rai, 2024] focused on evaluating retrofit measures such as cool roofs and external insulation systems for detached single-story houses in India’s hot semi-arid climate. Among the retrofit strategies we evaluated, cool roofs emerged as the most attractive solution because they delivered thermal comfort improvements comparable to roof insulation while being considerably cheaper and easier to implement. Of course, no solution is perfect. The long-term durability and maintenance requirements of cool roof coatings under real outdoor conditions still need to be better understood. Oh well... research must go on! 😊

The Next Challenge: Hygrothermal Performance of External Insulation

While cool roofs appear to be an attractive retrofit solution for detached houses, their benefits are limited for high-rise residential buildings since apartments, except those on the top floor, don't have roofs! 🤔 In such buildings, external wall insulation could be a practical retrofit strategy for improving energy efficiency and thermal comfort. However, insulating the external walls is not as straightforward as it may seem😔. While insulation reduces heat transfer through the wall, it also affects moisture movement within the wall. If not designed appropriately, this may lead to moisture accumulation, interstitial condensation, mould growth, and even long-term deterioration of the wall.

To better understand these issues, my group is currently conducting field experiments on the hygrothermal (a fancy word for combined heat and moisture) performance of externally-insulated brick masonry walls under Indian climatic conditions. The photograph below shows our two full-scale test rooms at IIT Kanpur. One room has its original uninsulated brick masonry wall, while the other has been retrofitted with an external insulation (expanded polystyrene). By continuously monitoring the outdoor weather, indoor conditions, and temperature and humidity inside the wall, we aim to understand how insulation affects heat and moisture transport and whether it introduces any long-term moisture or mould growth risks. 🤔

Ultimately, our goal is to develop scientifically validated retrofit guidelines that improve building energy efficiency without compromising the long-term durability of the building envelope. Stay tuned for more updates! 😊