[contemporary343]

Most materials, aside from metals, of sufficient thickness have high high infrared emissivity. All white paints, for example, already did radiate heat upwards through the atmospheric window. These more reflective films are no more emissive than previous paints (they're all quite emissive to begin with!)

The only difference, optically speaking, really is that they are better solar reflectors.

[dgacmu]

The difference is that the wavelengths in which they reflect and emit are different. Where you emit, you absorb. White paint has a lower solar reflectance because it emits and absorbs across a broader band. The better PRDC coatings absorb/emit more tightly in the atmospheric IR window.