Passive Daytime Radiative Cooling (Really cool roofs)


Passive Daytime Radiative Cooling Groundbreaking materials have the potential to provide life-altering cooling to millions who lack it. How do they work? And how can they be scaled?
This year’s scorching northern hemisphere may be in the past, but, we cannot let it recede from view. With every possibility that the summer of 2024 — and each summer beyond that — will be worse, we need to make real progress on cooling technologies and solutions now.

At Third Derivative and RMI, we’re particularly excited about a class of advanced materials that provide a passive daytime radiative cooling (PDRC) effect, greatly reducing indoor temperatures — and their potential lifesaving and emissions reduction benefits.

PDRCs hold great promise for areas most impacted by extreme heat — where electricity is often scarce and where many families cannot afford air conditioning. Deployed at scale in the form of cool roofs across informal (self-built) settlements and low-income housing in India alone, PDRC materials could save 317,000 lives and over 68 million tons of carbon dioxide equivalent (tCO2e) in cooling-related emissions between now and 2030 — and far more when scaled to other countries.

What’s at stake?

Without concerted action, 8.3 billion people globally — 74 percent of the projected global population in 2100 — could be exposed to deadly heat conditions (more than 20 days during the year in which ambient air temperature and humidity cross survivability thresholds when body core temperatures exceed 37°C [98.6°F]), up from 30 percent today. The vast majority will be in low-income countries, with 1.1 billion in India alone. Even with major emissions reductions, 48 percent of the global population will be at risk.

In many of the world’s hottest countries, the majority live without air-conditioning — and the ability to escape life-threatening heat. Many of these deaths will occur in informal or self-built settlements, home to over 1 billion people worldwide.

Power outages make the problem worse. Demand for electricity typically peaks during heat waves as people need more and more mechanical cooling to stay comfortable. That strain on the electricity grid can lead to blackouts — and heat-related mortality risk more than doubles when blackouts occur during heat waves.

At the same time, as the world gets hotter, incomes rise and countries urbanize, exploding demand for mechanical cooling that will worsen global warming. Under a business-as-usual scenario, residential AC could consume 7,700 terawatt-hours (TWh) of electricity and account for over 160 billion tons of CO2e by 2050. For perspective, that is over three times greater than the emissions reductions anticipated by 2050 from the US Inflation Reduction Act, the biggest piece of US climate legislation in history.

Put simply, we cannot air condition our way out of this problem. We need alternatives that are more equitable, effective, and energy efficient.

What is PDRC? Is it just painting my roof white?

Not quite. Passive daytime radiative cooling products can cool even during the daytime, even when exposed to direct sunlight, and without any energy input (it’s passive, not active). They take full advantage of the physical properties of radiation — more on that in a moment. PDRC solutions have been developed in many forms, including coatings, thin films, fabrics, cellulosic microfibers, porous structures (like delignified wood), and aerogels.

While painting roofs white to cool buildings has been standard practice in hot climates like the Mediterranean for generations, PDRC-equipped roofs take things one step further. PDRC materials don’t just reflect the sun’s rays back, they are both highly reflective and highly emissive. They emit more heat than they absorb by taking advantage of longwave infrared thermal radiation — in a nutshell, they can bypass Earth’s “greenhouse” and send the sun’s heat back into space.

This means that PDRC solutions are able to radiate heat even with the sun beating down on them and can reduce surface temperature to at or below ambient outdoor temperatures. Their almost magical-seeming, counter-intuitive properties mean that a PDRC material is cooler when placed outside in the sun than it would be inside in the shade.

The impacts of this can be substantial. For example, a sheet metal or asbestos roof typical of many informal settlements can heat up to 88°C (190°F) in direct sunlight when ambient air temperatures are around 38°C (100°F). A PDRC solution can reduce roof surface temperatures to at or below ambient temperatures — a potential 30°C–50°C (54°F–90°F) difference.

This can translate to indoor temperatures that are 5°C–10°C (9°F–18°F) cooler on a hot day (i.e., to temperatures far below those seen in a typical informal settlement home).