When you put a hot thing near a cold thing, they exchange heat and the temperatures even out.
The heat moves via light - infrared light usually. Cooler things emit less light overall, and the light they do emit tends to have a longer wavelength (thats why the embers of a fire might glow red, but a hotter fire might be orange or yellow).
For each color of light that something might emit or absorb, there is a 'shinyness' (technically known as reflectance spectra). A shiny surface reflects light, which means it will take longer to heat up, or longer to cool down.
By making a material which is shiny in the redder (ie. longer) wavelengths, yet not shiny in the shorter wavelengths, you can make a material which heats up fast when put next to a very hot item (like the sun), but doesn't cool down fast when put next to a very cold object (like the night sky). That would make a great paint for a building in a cold climate! This would be a visually 'black' material to our eyes, although you could design it to be any human color if you trade some performance.
Likewise, one can do the reverse. Reflective of short wavelengths, and non-reflective of long wavelengths. That will help it stay cool in the sun, and lose as much heat as possible at night to the sky. The ideal material for this would be visually white, but again, you can trade a little performance for any other color.
It is also possible to have panels which physically flip to reveal another coating either between summer and winter, or between day and night to get even better results - although obviously with a complexity cost. Shutters in some european countries are an early version of this idea.
Note - the above is a simple explanation. For the full explanation, see [1]
Can a coating on the outside of a material really have an appreciable impact on heat transfer from inside to outside, at night? I get that a coating can block or admit light or radiation coming from somewhere else, but surely if the material itself is hot then thermal conduction within the solid will make the coating itself hot, and shed heat, too? Wouldn't you need some sort of "heat shield" offset from the outer walls for your approach to work at night, if at all? Or if not, why are we still covering homes with big chunks of styrofoam, polyurethane and mineral wool?
Among architects, I'm seeing a lot more hype about high thermal mass, e.g. sand-lime bricks and woodfiber insulation, to design houses that respond very slowly to outside temperatures.
There are benefits to all these approaches, and combining them all gives best results.
But specifically on coatings, it's true that conduction and convection also play a role, and radiation is only part of the picture. However, it is theoretically possible to design a coating that can cool a material by 45C (~80F) compared to ambient at night, and practically a cooling of 4C (8F) has been achieved [1]. 4C is pretty substantial, considering it stacks on top of the effects of insulation, high thermal mass, and efficient heating/cooling appliances.
Remember that in the daytime the effect is far more substantial - the difference between the best and worst thermal coatings could easily be 100C - when people say 'that rock is so hot I could fry an egg on it', it's simply due to the absorption properties of the rock under sunlight.
> why are we still covering homes with big chunks of styrofoam
The energy loss through foam depends on the temperature on each side of it. So if you can use a fancy coating to keep the outside of your house cool/hot in the day/night, then the energy losses through the styrofoam are reduced, and eventually your heating/cooling bill is reduced.
> Wouldn't you need some sort of "heat shield" offset from the outer walls for your approach to work at night, if at all? Or if not, why are we still covering homes with big chunks of styrofoam, polyurethane and mineral wool?
If a home is well-built, the wall assembly already includes a physical gap between the structural sheathing or exterior insulation (if present) and whatever cladding materials you're using.[0] That gap allows for airflow and drainage behind your cladding, so that the water that gets behind it--and it will--can drain out and everything can dry.
That gap would help create something of a thermal break (the cladding isn't attached directly to the house but to furring strips with a very low surface area), but it would still heat up the air in the gap itself. In that case, reflective paints would probably still be beneficial.
That said, I think their greatest benefit would probably be in roofing. Metal roofs, for example, also require an air gap[1] behind they can get extremely hot.
As mentioned, non-reflective surfaces are good emitters as well as adsorbers. So the argument, AFAIUI, is that a surface which is non-reflective in the infrared will radiate away more heat during the night. Crucially including heat that has been adsorbed in other wavelengths and then stored in the material during the day.
