New research reveals that maximising SRI in humid climates requires balancing reflectance against nocturnal radiant cooling.
Cool roof science is advancing beyond the simple equation of ‘white = cool’ to a more nuanced understanding of the interaction between solar reflectance, thermal emissivity, and nocturnal radiant cooling in different climate zones. New computational modelling integrating whole-building energy simulation with rooftop surface physics is showing that for humid tropical climates, optimising cool roof specification requires balancing daytime solar heat gain reduction against nocturnal radiant cooling — which can be counterproductive in warm coastal cities where overnight temperatures remain high.
The implication for Indian manufacturers is that a single SRI number is insufficient to specify a cool roof coating for the diversity of Indian climates — from the humid coastal cities to the semi-arid interior to the cold Himalayan region. Climate-zone-specific coating specifications are needed, with different emissivity targets for coastal versus continental climate zones.
Industry Implications and Market Context
This research supports the case for a more sophisticated cool roof rating system for India — similar to the CRRC (Cool Roof Rating Council) system in the US — that provides climate-zone-specific performance ratings rather than a single SRI metric.
What Happens Next?
AEEE and BEE are reviewing cool roof specification frameworks for Indian building codes. A climate-zone-specific rating system is under development, with finalisation expected in 2026.
Frequently Asked Questions
What is thermal emissivity in cool roof coatings?
Thermal emissivity measures how efficiently a surface radiates absorbed heat as infrared radiation. High emissivity (≥0.90) means the coating efficiently re-radiates heat; low emissivity means it retains it. For cool roofs, high emissivity is generally desirable but can cause overcooling in cold climates.
Why does India need climate-zone-specific cool roof standards?
India spans five BEE climate zones — composite, hot-dry, warm-humid, temperate, and cold. A coating optimised for Mumbai’s warm-humid climate (high reflectance, moderate emissivity) may not perform optimally in Delhi’s composite climate or Shimla’s cold climate, requiring different SRI and emissivity targets per zone.
Where can I find more information?
AEEE, BEE, and the CRRC (Cool Roof Rating Council) publish climate-specific cool roof performance data. CSIR-CBRI in Roorkee conducts building envelope research for Indian climate zones.
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