What Is PDRC? Passive Daytime Radiative Cooling Explained

Passive daytime radiative cooling (PDRC) is a real, peer-reviewed physics phenomenon, not a marketing term. CryoPaint commercializes the same physics that has been demonstrated in Nature and Science in a sprayable waterborne coating. Here is what PDRC actually is, what it can do, and what it cannot.

The two-line definition

PDRC works by combining two optical properties on a single surface:

1. High reflectance of incoming sunlight, so the surface absorbs very little solar energy.
2. High thermal emission tuned to the 8 to 13 micrometer (µm) infrared band, so the surface radiates its own heat through the atmosphere directly to outer space.

When the heat radiated out exceeds the heat absorbed from sunlight, the surface ends up cooler than the air around it. That net energy loss is the cooling effect.

Why the 8 to 13 µm window matters

Earth's atmosphere is mostly opaque to thermal radiation, which is why heat tends to be trapped near the ground. Inside a specific band between roughly 8 and 13 µm, the atmosphere is transparent. Thermal energy emitted in that band passes through the air column and radiates out to space, which sits at about 3 K (around -454°F).

A surface tuned to emit inside this window has a direct thermal connection to the coldest practical heat sink in the solar system. A surface that emits broadly across the infrared spectrum (most ordinary paints and coatings) sends some of its heat to space and some right back into the warm atmosphere, which limits cooling.

What the peer-reviewed work has shown

The headline result that opened the field was published by Raman, Anoma, Zhu, Rephaeli, and Fan at Stanford in Nature in 2014. They reported sub-air-temperature cooling under direct sunlight using a multilayer photonic structure (Nature 515, 540 to 544; DOI 10.1038/nature13883).

In 2018, Mandal and colleagues at Columbia published a paint-like polymer coating in Science that achieved roughly 96% solar reflectance and strong infrared emission (Science 362, 315 to 319; DOI 10.1126/science.aat9513). That paper is the one that moved PDRC from the optics lab toward something you could paint on with normal application equipment.

Both papers, and the dozens that followed, are publicly accessible.

What CryoPaint does with the physics

CryoPaint is a waterborne, low-VOC PDRC coating with:

• 94% solar reflectance across the visible, UV, and near-infrared.
• 90% 8–13 µm emissivity, meaning in the wavelengths that bypass the atmosphere, it radiates its own heat to space at 90% of the maximum rate possible.
• Up to 15°F below air temperature on a flat sky-facing surface under good conditions.

That last number is a ceiling, not a guarantee. The next section covers when you get close to it and when you do not.

What PDRC actually requires to work

The physics is fixed; the conditions are not. To get useful sub-air-temperature cooling, the surface needs:

• A clear view of the sky. Heat radiated in the 8 to 13 µm window has to actually reach space, so clouds, heavy tree cover, glass overhangs, and steep walls all reduce the effect.
• Low humidity. Water vapor absorbs in the infrared. Above roughly 60% relative humidity, the cooling effect drops noticeably.
• Still air. Wind is a first-order factor, not a footnote: convection drives the surface temperature back toward the air temperature. Blowing on hot coffee cools it; blowing on cold coffee warms it. Breezes and gusts are fine, but sustained wind above 5 mph starts degrading performance, and it keeps degrading as speed rises. Corrugation ridges, parapets, and wind shadows help hold a still layer of air on the surface.
• A relatively flat orientation. Performance falls off above about 30 degrees of tilt from horizontal.
• Little or no insulation between the heat source and the paint. PDRC pulls heat out through the surface. Heavy insulation blocks heat from reaching the paint in the first place.

If any of these are off, the paint still reflects 94% of sunlight, so it still beats standard exteriors. It just will not run the full sub-air-temperature trick at peak power.

What PDRC is not

PDRC is not a perpetual motion machine, not a refrigeration cycle, and not an energy source. It is a passive heat transfer pathway from one body (your roof, your truck cab, your container) to another body (deep space) that happens to be very cold. The Second Law of Thermodynamics is honored at every step. Heat flows from hotter (the surface) to colder (space) through a wavelength band where the atmosphere stays out of the way.

It also does not directly chill the surrounding air. It cools the surface you put it on. Whatever sits under that surface (cabin air, attic, container interior) then cools by ordinary conduction and convection from underneath the painted surface.

Bottom line

PDRC is a real, published, replicable cooling pathway that uses the cold of space as a heat sink. CryoPaint turns it into a coating you can buy by the gallon and apply with an airless sprayer. The performance ceiling is up to 15°F below air temperature on the right surface under the right conditions. The floor (when sky view is blocked) is still as good as the best cool-roof white paint, which is itself useful.