• The findings, published in Nano Materials Science, highlight how building temperatures can be reduced by more than 7°C while almost halving peak heat release rate in the case of fire.
• They are the result of an international research collaboration between Spain’s IMDEA Materials and the Hong Kong Polytechnic University.

An international research team has demonstrated how conventional radiative cooling coatings can be optimised to further reduce building surface temperatures, cutting energy consumption, while also improving fire safety. 

Radiative cooling coatings passively lower surface temperatures by reflecting most incoming sunlight, while at the same time emitting heat as infrared radiation (IR) back through the atmosphere. Because more heat leaves than enters, the surface becomes cooler than the surrounding air, helping to reduce indoor temperatures.

Such coatings rely on microscopic silicon dioxide (SiO₂) particles, the same material found in sand and glass, to scatter sunlight and emit heat efficiently. The particles are typically added to a polyurethane (PU) polymer resin to create coatings used on roofs and façades, lowering energy consumption and improving interior comfort.

New research, however, has taken this technology one step further. By engineering the microscopic structure of the particles into a dendritic, or tree-like shape, the team was able to create an improved multifunctional coating.

“Both experimental and simulation results show that the reflectivity of dendritic SiO₂ is much higher than that of solid, hollow or mesoporous SiO₂,” says Dr. Wei Cai, one of the publication’s authors and a Marie Sklodowska Curie Actions postdoctoral fellow at IMDEA Materials Institute.

Specifically, the PU/dendritic SiO₂ composites achieved solar reflectivity of 95.5% and IR emissivity of 94.5%. This, in turn, resulted in daytime temperature reductions of 2°C compared to existing PU coatings, and 7.3°C compared to ambient temperatures.

The coating’s improved performance comes from the dendritic structure creating more interfaces, which scatter sunlight more effectively and increase reflectivity. Additionally, the Si-O bonds exhibit high infrared emissivity within the atmospheric window.

These two characteristics endow the material with both high reflectivity and high infrared emissivity, further enhancing the radiative cooling performance and contributing to a reduction in the material’s temperature.

“This temperature decrease strongly confirms its potential in decreasing the energy consumption required for building cooling,” adds Dr. Cai.

At the same time, incorporating the dendritic SiO₂ spheres into the polymer coating was also demonstrated to significantly increase its fire-safety performance.

Most notably, the material’s Peak Heat Release Rate (PHRR) was reduced by 48.4%, lowering its maximum fire intensity by almost half. In a real-world scenario, this could slow fire spread and improve evacuation conditions.   

Effectively, the engineered particles serve to increase the viscosity of the polymer as it heats, trapping combustible gases, and forming a protective barrier that slows flame growth and reduces heat release.

This dual behaviour addresses a long-standing limitation of radiative cooling materials, which typically neglect fire safety in building applications.

“The results provide a new design strategy for building materials that combine energy efficiency and safety”, says Dr. Cai. “This potentially enables passive cooling coatings to be deployed in real buildings, particularly in hot urban environments where both overheating and fire risk are critical concerns”.

This project has received financial support from the European Union (SOLAR-MATER: Year-Round, Fire-Safe, and Sustainable Solar Management Materials; No. 101149333) and the National Natural Science Foundation of China (No. 22205228). Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Executive Agency (REA).