Cooling Paint You Can Actually Make

[NightHawkInLight] has been working on radiative sky paint. (Video, embedded below.) That’s a coating that radiates heat in the infrared spectrum at a wavelength that isn’t readily absorbed or reflected by the atmosphere. The result is a passive system that keeps materials a few degrees cooler in direct sunlight than an untreated piece in the shade. That sounds a bit like magic, but apparently the math checks out.

This isn’t the first time he’s demonstrated infrared cooling, but the previous demonstration used Barium Sulfate microspheres. The process of making those microspheres is a bit beyond the reach of the average garage tinkerer, so [NightHawk] has spent the last year working on a DIY-friendly recipe. And it looks like it’s a success, as the recipe is washing baking soda, de-icer, and citric acid, all combined in a blender. The result is microsphere crystals of calcium carbonate, but that was only half the battle. To really get effective cooling, the resulting paint needs a very high density of spheres. The trick here is to manage the size of the crystals, mixing large, medium, and super-tiny spheres to achieve maximum density. With the recipe given, the secret is nucleation sites. The more crystals start growing at once, the sooner the solution is sapped of suspended calcium carbonate, and the smaller the resulting crystals. Put simply, blend longer for smaller crystals.

So the procedure is to make three batches of microspheres, of the three different sizes, and then mix the dried powder in the ideal ratios for maximum density. This powder has to be suspended in something to make a paint, and the current recipe is an acrylic with suspended water particles. Mix water, acetone, and bits of Plexiglas. When the acetone evaporates off, the water is left trapped in the acrylic, making a bright, reflective coating. Mixed with the calcium carbonate, it’s both reflective and emissive on that magic IR frequency that passes right through the atmosphere. The only problem is that calling the result a paint is being a bit generous. It ends up about the consistency of cake icing, and [NightHawk] had some success applying the mixture with an oversized piping bag. There’s more to come, including instructions for turning the mixture into self-contained cooling panels, for a power-free air conditioning solution. Stay tuned!

49 thoughts on “Cooling Paint You Can Actually Make

  1. Interesting. However, he needs to measure the temperature by direct contact thermometers not by radiation thermometers, as they may be affected obviously by this peculiar emission spectrum paint. Maybe he did, couldn’t stand watching a 40’ video. Why can’t people write anymore…

      1. There are quite a few sites that are nothing but writing where the writers are getting paid..
        And in fact those are getting more and more commercial I fear.
        Not to mention that people make millions writing books.

        Of course you are correct that all that is not on But now that you learned there are other sites, well, the world is your oyster.

    1. It’s actually a pretty good video. If you had watched it, you’d know the thermals are taken of the same substrate of which the paint is on the other side; furthermore the samples are isolated somewhat from other forms of thermal transfer.
      I’m sure there is a website that grabs transcriptions from videos.

          1. Click the three dots to the lower right of a video, click on “show transcript,” scroll to highlight all of it, copy, paste. But there is no punctuation and they include a bazillion time hacks. Same here, but with the time hacks removed. Enter the video’s URL:


          2. No thanks. Most people just pour out a logorrhea that, once transcribed, is basically garbage. Articulated, structured text with illustrations is still the best material for transmission of knowledge, and no AI can do that, and it´s unlikely it will soon. There is a reason books still exist.

      1. Indeed. What was left unsaid was any difference in cooling capability there might be between the two “paints”. The test methods varied from this vid to the Tech Ingredients one so no comparison from them could be made.

    1. The video shows that when the acrylic has pores, much more of the sunlight is reflected from the surface than if it were clear, just like snow versus ice. Even though the microsphere additive is also white, it’s actually mainly there in order to emit the infrared light rather than to be the only reason the paint is light colored. Sunlight is much more intense than the infrared that’s passively emitted, so if you don’t reject as much as possible, you absorb more than you emit. And anything that makes the paint whiter can also make the necessary coating thickness lower because less sunlight penetrates far enough to be absorbed by the underlying material.

  2. Thank you for the level of detail in your presentation. As this is one of those incredibly rare instances where I actually have all of the chemicals as well as all of the equipment on hand and live in the tropics, Iʻm going to have to try this.

  3. How did is just a standard white paint? Biggest worry is keeping this stuff clean, where it’s most useful it tends to be dusty.

    (I too am hoping someone else watches this obscenely long video).

  4. Just had my swimming pool deck painted in a Sherwin Williams “Cool Feel” paint (comes in various colors, but I chose white). The sun can be directly hitting it for hours, and it does what it says it does – stays perfectly cool to the touch, while sorrounding areas that have regular paint, are uncomfortably hot.
    Unfortunately, couldn’t find any information (granted didn’t look hard enough) on how the stuff works.

  5. It’s a long video, but the coolest part is not just that it works, but that it’s made from normal kitchen and hardware store ingredients, and the preparation is something anybody can do at home. It’s not just a hack, but real approachable research.

    It also looks like there’s going to be some followup tests, and I suspect tech ingredients is either collaborating or going to do some friendly competition

  6. I watched it (and all the fuss in making the stuff) and my brain went “why not just put mirror tiles on sun-exposed structures”. eg clear polycarbonate sheets with the silvering on the back.

    1. I think the answer is because, paradoxically, a mirror tile would only be reflecting visible light which gets re-absorbed by the atmosphere (and then the mirror itself) anyway. In the IR spectrum that the atmosphere is actually transparent to, the polycarbonate mirror is opaque. So even though the mirror is reflecting visible light the energy from that light isn’t escaping, it’s still stuck on earth at least. Like playing ping-pong against a wall – you can keep hitting those photons but they just keep coming back!
      This paint acts more like a mirror in that it reflects (emits?) IR in the part of the spectrum the atmosphere allows through, so it escapes into space.

      I once looked through a camera with no IR filter at the original Oculus Rift and was amazed that the apparently opaque black plastic shell was totally transparent in near IR. It’s crazy how materials can be so different to other wavelengths of light.

      1. I used to design NIR imaging systems and had a pretty extensive collection of glass and plastic filter materials. It was interesting to look out the window at various wavelengths.

      2. Tell the designer of every building in the west that has mirrored windows that they are not helping… Now painting the rest of the building around the windows with this may cost less than mirroring the whole building, and I do suspect there is some variation in how different mirror materials handle IR. I know for a fact the film they use on buildings and cars etc helps a lot, but I also have a light source that drives a fiber optic cable, and tight out of the first thing the light goes through is a mirror that lets the IR pass through and the visible light go into the lenses that focus into the fiber.

        1. Those windows, semi-mirrored or not, can be and generally are coated to reflect the near-infrared and the uv components of sunlight even more than the visible. That’s the big benefit, and that’s what the films you’re talking about do, but it’s nothing to do with longwave IR. Low-e glass is what you call it when windows do not emit much longwave infrared (the infrared emitted by things around room temperature). Otherwise, regular glass emits a fair amount although not the maximum that any material could emit. And it usually lets in more heat than it emits when it’s sunny.

    2. Well, even apart from Noah’s explanation, mirrors aren’t as reflective as white paint. They just reflect all in the same direction. So a mirror actually will get warm regardless of infrared, and in the mean time people will be blinded at certain angles.

  7. Combining the cool apitn with hot paint eg a perfect absorbed and a termopile for energy generation. But most likley solar would generate moar. then again using a solar panel as the black hot part and then coat it white underneath and then a thermopile there could add some extra energie.
    Thinking about it there was an article with cooled solar panels which where more efficient so there would needs to be alot fo tests done to find the optimal energy efficiency.

  8. Plank’s Law describes the frequency of radiation from a body as a function of temperature. The magic paint would radiate in the infrared because that is where objects at normal room temperatures normally exist.
    It may appear cooler because it is white and absorbing less of the sunlight, but it doesn’t cool to less than the environment – that would be a violation of the Second Law of Thermodynamics.ddd

    1. It can cool without violating thermodynamics because it’s emitting light in a wavelength that the atmosphere is completely transparent to, and therefore that light escapes into space (taking the energy with it).

      This form of radiative cooling has been know for millennia (used to create ice overnight), but, has never been able to function during the day because it absorbs more energy from sunlight than it can emit into space as IR.

      The primary issue is mostly a material sciences issue, creating something that reflects a high enough percent of sunlight, while emitting a good amount of IR and also being durable enough to survive outdoors without damage.

      1. The point I was making is that Plank’s law determines the wavelength of the radiated energy, so there is no magic – any surface will radiate at the same wavelength given the same temperature. It has no special wavelength of radiation. In fact, however, it doesn’t need to be transparent to the atmosphere – if it radiates more energy than it receives, it will get cooler until the radiated energy reduces to match the absorbed energy. Heating the atmosphere or sending it to space doesn’t matter to the surface once the energy is radiated.

        Consider this: If it radiated more than other surfaces, you could face it to something else, and more heat would travel from it than from the other surface; it would get cooler while the other surface warmed. The second law of thermodynamics is pretty clear that this is not possible.

        1. So, first excuse my ignorance Ive forgoten Planks Law and my quick wiki dive has not enlightened me. However, seems to me that while Black Bodies follow Planks Law, the whole point of emission spectroscopy is that you can tell which materials are which by the EMR they emit, as different materials have different excitation emissions. Thats how we tell which elements make up stars. That would suggest that, even though raising the energy of a material does shorten their peak emmisions, they do have a characteristic EMR emission based on their material.

          Regardless, the way the material is described, your panels facing eachother would not violate the secone law, because the non- skycooling panel would radiate back what it got at the same frequency it absorbed (the same way absorbtion spectrum analysis works) and so it would return the energy to the sky panel with the same efficiency that it was delivered.

          My question is, why isnt there enough of this radiation in sunlight to cancle out the output from the panel. It SHOULD absorb light in the same spectrum as it produces.

          1. Thanks for an interesting replly. It forced me to review some stuff. As for the spectroscopy, this page gives some clues with Kirchoff’s Laws of Spectroscopy – a hot dense object produces a continuous spectrum, a hot low-density gas produces a line spectrum.

            The panels at the same temperature will radiate the same amount of energy. If they didn’t, facing two in a closed system would cause one to get warmer than the other, until there was a balance. If that were to occur, that free temperature differential could be used to run a heat engine for free energy.

            In a variation on this, one could paint one side of a thermally conductive panel with this heat radiating paint, and the other side with ordinary heat absorbing paint. This, in place of window glass would provide free air conditioning, or heating, depending on which side faced in.

            In summary, you can’t get paint that radiates more heat than other surfaces without violating the laws of thermodynamics. I believe your question at the end hits on the “magic” of this paint. It reduces the solar heating by being very reflective and absorbing less energy; instead of radiating more, it reflects sunlight better.

            Again, thanks for the interesting reply.

    2. “The environment” is a tricky phrase here. It’s easy to imagine that it means the local air temperature, but when we’re talking about radiation, up to 50% of our environment is the blackness of space.

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