That Ultra-White Paint That Helps Cool Surfaces? Make Your Own!

It started with [KB9ENS] looking into paints or coatings for passive or radiative cooling, and in the process he decided to DIY his own. Not only is it perfectly accessible to a home experimenter, his initial results look like they have some promise, as well.

[KB9ENS] read about a type of ultra-white paint formulation that not only reflects heat, but is able to radiate it into space, cooling the painted surface to below ambient temperature. This is intriguing because while commercial paints can insulate and reflect heat, they cannot make a surface cooler than its surroundings.

Anecdotally speaking, this painted battery section of a solar recharger gets too hot to touch in full sunlight. But when painted over, it was merely warm.

What really got [KB9ENS] thinking was that at its core, the passively-cooling paint in the research is essentially a whole lot of different particle sizes of barium sulfate (BaSO₄) mixed into an acrylic binder. These two ingredients are remarkably accessible. A half-pound of BaSO₄ from a pottery supply shop was only a few dollars, and a plain acrylic base is easily obtained from almost any paint or art supplier.

[KB9ENS] decided to mix up a crude batch of BaSO₄ paint, apply it to some things, and see how well it compared to other paints and coatings. He wetted the BaSO₄ with some isopropyl alcohol to help it mix into the base, and made a few different concentrations. A 60% concentration by volume seemed to give the best overall results.

There’s no indication of whether any lower-than-ambient cooling is happening, but according to a non-contact thermometer even this homemade mixture does a better job of keeping sunlight from heating things up compared to similarly-applied commercial paints (although it fared only slightly better than titanium dioxide-based white paint in the initial test.)

[KB9ENS] also painted the battery section of a solar recharger with his homemade paint and noted that while under normal circumstances — that is to say, in full sunlight — that section becomes too hot to touch, with the paint coating it was merely warm.

Actual passive cooling can do more than just keep something less warm than it would be otherwise. We’ve seen it recently used to passively and continuously generate power thanks to its ability to create a constant temperature differential, day and night.

49 thoughts on “That Ultra-White Paint That Helps Cool Surfaces? Make Your Own!

  1. I think it would have been better/more accurate to use a contact probe to measure temperature in this case. Those IR-type thermometres are getting their readings based on emissivity of the surface. Most often they are setup for an emissivity of 0.95 which corresponds to a black painted surface (1.0 corresponds to theoretical super-vanta black).

    1. Yea. These thermometers get a little weird when you point them at some types of reflective surfaces like chrome or polished metal. At my old work (a green sand iron foundry) were were trained to point at certain targets when recording results, so they would be consistent.

    2. Two heat sinks, each with a thermocouple fastened to the underside in the same spot with thermal paste as well, warmed in a oven until the temps matched to an acceptable level, allow to cook to ambient in an enthalpy controlled space, time the return to ambient, ensuring cooling rate also matches to desired accuracy. Apply paint to one. Repeat heating/cooling and note the deltaT and rate of drops.

    3. I was logging on to make the same comment.
      There are three components to any material: reflectivity, transmittivty, and emissivety. (Transmissivity would be for things like glass, as it described the amount of light that can pass through a material).

      reflectivity + emissivety=1

      Most IR thermometers actually recommend using a piece of scotch tape on whatever you are measuring, as it typically has an emissivety of .90.

      But either way, I wouldn’t use an IR thermometer to figure out the temp. When you have a reflective surface, it will be reflecting the IR light of nearby objects. So, it generally will read as room temp.

    1. Yes. Modern usages of “DIY” has come to mean “to make or construct” as oppose to purchase. Just like with “ATM machine”, the acronym has been given a meaning of it’s own.

          1. The problem is that most people are dumb, and thus that leads to definitional drift, which undermines the meaning of words; as words mutate, they become less precise for communication. People can’t communicate well when the language is corrupted. It also leads to more disagreement. And so now we see (for example) silly semantic argument between agnostics and atheists (and atheists even had to add all kinds of adjectives a dozen years ago) and even silly semantic arguments between atheists.

            Anyway, dictionaries used to wait decades before accepting the latest corruption of a word to become recorded. The new usage had to have staying power, some utility.

            But then the internet. Worse, UrbanDictionary.

            Every idiot got to “yeet” his nonsense out there and you never knew what was going to stick. Dictionary sales have been crashing since at least around 2005. Dictionary publishers, hair on fire, decided they needed to be trendier and hipper, like UrbanDictionary.

            Merriam-Webster, arguably the skid row of the dictionary line, was among the first to just start cramming kids’ internet trash into their pages. This just demonstrated that their commitment to no standards was getting worse.

            Sadly, due to attrition, eventually even the once-great Oxford English Dictionary fell to this idiotic mindset. So words quickly go from meme usage to the OED. Goddamn kids get off a my lawn!!

  2. This “paint” drives me nuts. I’ve been seeing it floating around for years, but never with any critical analysis or long-term tests. This is for a good reason: if you did long-term testing you wouldn’t want to use it! Barium sulfate is a filler pigment; it doesn’t have enough hiding power to useful on its own in a proper paint, it only works here because this particular paint is underbound–it’s basically primer. If you’ve ever left primer on an exterior surface you know what happens next: it won’t stay white for very long. Calcium carbonate works for this purpose too, and can be formulated in such a way that it serves as its own binder: this is known as whitewash, and has been used for millennia for this exact purpose. There is nothing new under the sun, especially when it comes to pigments and paint.

    1. If you watch the videos, it requires an air gap with glass covering to work so that the cooling effect doesn’t radiate into the environment. This will also protect the paint, though the glass will need regular cleaning.

    2. If you’ve looked closely at these various paints, they have made whiter paint per thickness by using multiple sizes of particle for better packing and by producing lots of surface edges to get the same effect as snow, which is white despite being made of ice. You’re right, calcium carbonate seems much easier, but I don’t find it impossible to believe that the barium sulfate version can’t be protected well enough for intentional radiators, rather than random walls and such. It still wouldn’t be quite like whitewash if you’re using the packing and surface reflection strategies.

  3. I think one of the intimations in this write up is somewhat problematic. That a pain can cause the surface to be cooler than it’s surroundings. The example was a solar panel electronics box. In classical thermodynamics I think this would be impossible. An isolated box would be at best at the temperature of it’s surroundings, if it is absorbing some energy from solar radiation (instead of reflecting it, it would be hotter. Only if it is in contact with a cool surface below such as moist soil etc. could it be cooler than the surroundings. Then the sun exposed surface would reflect nearly all the radiation, and the box would conduct ambient heat away to the cooler surface it is in contact with. It is true that there are recent claims of devices working on a quantum mechanical level that appear to break the laws of thermodynamics (one example: These are fascinating and I don’t know whether to believe the research. However I very much doubt that this barium sulphate paint could be workng on that level.

    1. It seems to actually work, via infrared radiation of heat. This paint radiates on a frequency that isn’t absorbed by the atmosphere, so it’s literally leaking heat away into space.

    2. I think the best way of thinking of it is that this paint broadens what counts as its surroundings. Most of the time, a terrestrial object would mainly have thermal interactions with the sun, the atmosphere, and to a degree the earth/other nearby object. This paint just allows for a small amount of “thermal contact” with space, As Jonathan correctly stated, it is done by radiating/absorbing IR light at a frequency at which the atmosphere is more transparent, allowing non-negligible radiative heat transfer from the panel to the cold of space. No violation of the 2nd law here!

    3. (My this blurb is unrelated to the original article. I wish to touch on some rampant misinformation regarding the capabilities of these (or similar) coatings, specifically the allegation of them transmitting heat directly back into space, bypassing the atmosphere)

      100% what Michael Avison said. You cannot cool anything below thermal entropy without using energy to (re)move energy or similarly requiring “work” to be performed on some fundamental level to achieve that goal. Here’s my addition:

      “Every action has an equal, and opposite, reaction.”

      I would posit that the articles reporting these coatings are being very-much EMBELLISHED and that ignorant minds are mistaking/misinterpreting/misreporting the normal thermal radiation the planet leaks into space due to thermal entropy, as some newfangled technology that magically transmits thermal energy into or from space.

      Due to thermal entropy, ALL energy will be absorbed, as heat, into space eventually, and I believe that impressionable minds misunderstand a coating that reflects thermal energy back into the atmosphere, and then the atmosphere emitting thermal energy into space, as a coating that is somehow able to BYPASS the atmosphere and emit directly into space: such a notion defies so many laws/fundamentals of thermodynamics and other schools of physics, it’s LAUGHABLE to imply such a thing (sub-ambient temperature, space-transmitting, atmosphere-bypassing paint) is remotely feasible.

      If you want to understand WHY such a coating is fundamentally not feasible, read and understand a small Wikipedia article on “BLACK-BODY RADIATION” (protip: It may say “black” but the laws and theorems apply to WHITE as well, usually in the inverse). It explains all the science behind WHY you cannot feasibly transmit the earth’s (or ANY surface’s) absorbed solar radiation back into space without it almost entirely being absorbed by earth’s 7+mile-thick atmosphere.

      For a further proof-of-concept, research why we haven’t simply created a solar-powered laser that emits energy at whatever magical wavelength these COATINGS magically turn infrared thermal energy into that somehow magically bypasses the atmosphere yet is also somehow magically absorbed by space, which is made-up of the same stuff our planet/atmosphere is. I call it “laser cooling.”

      I mean, think about it. If such a coating were actually feasible, why not use it to transmit thermal energy to/from cooler/warmer locations? Why not do my solar-panel-powered radiator to cool on a large-scale? Why not use these coatings in conjunction with heat pumps to deliver ENORMOUS heating/cooling potentials? Why not use it for electricity production via MSR (or similar)? Why not use such coating to make “clean water condensers” which pull easily-cleanable water from humid atmospheres FOR FREE? I hope you see my point.

      1. You’re missing a couple of key points. Heat energy flows from hot to cold. In certain frequency ranges (look up atmosphere window) the atmosphere is totally transparent so there’s nothing to push against except the cosmic background radiation, which is like 3 degrees kelvin. This means that something with high reflectivity and high emissivity in those bands will drain heat out to space faster than the surroundings can warm it, which results in sub-ambient temperatures.

        Oak Ridge has explored using it to increase nuclear reactor efficiency.

        It’s being used to produce electricity, at least at small scales:

        Several devices have been built that condense water for free:

        If you got about 3% of the world’s surface covered with them, global warming would end.

        There’s products out there for enhancing heat pumps:

  4. “… read about a type of ultra-white paint formulation that not only reflects heat, but is able to radiate it into space…”
    Outer space ? Hardly. There is no proof that this paint will send the heat back into outer space. More likely the infrared heat will be reabsorbed and reradiated by particles in our atmosphere. If this paint were widely used on rooftops, etc. we would still end up with a warming atmosphere. To take this a step further, say we go totally green and reduce the CO and CO2 in the air… hydrogen, fission, fusion in producing energy have an ignored byproduct… waste heat. The excess heat will someday exceed the Earth’s atmosphere ability to radiate it into space. Satisfying our future energy needs will result in a warming atmosphere even with reductions in heat trapping CO2.

    1. You are gravely and utterly mistaken. Human heat (or any form of energy) production does not make smallest dent in Earth’s overall energy budget. It is really sad that people like you keeps spreading that BS. (It falls to the same category of tragicomical technical misconceptions like that Earth’s gravity shuts down at certain altitude and that Blue Origin is flying payloads to “space” and competing with Arianspace/SpaceX/ULA/etc..)

      Earth receives on average 173 quadrillion watts (173 000 TW) from which 122 is not reflected back to space (that is 122 quadrillion Joules every second!) from Sun. Let that sink in. Humans are warming the planet through enlarging CO2 concentration which can absorb *certain* IR wavelengths very very well. It can’t absorb all wavelengths very well though even in the IR part of spectrum… (That is how the paint works.)
      Any human energy output is puny compared to that.

      Massive amount is also being emitted (not reflected!) back to space 121 quadrillions watts on average.

      Educate yourself

      1. In other words enough solar energy strikes Earth to power multiple Katrina type hurricanes every second, while the amount of energy that one hurricane unleashed over the course of a week was equal to about the previous 20 years of all Human energy use/production.

  5. Suppose you could control ideally perfect insulation and shielding such that your surface would receive no heat input from its (immediate) surroundings, but could still radiate toward outer space.. Then it would get cooler as it moved toward thermal equilibrium with the 4 K of cosmic background. Then you let some heat flow in. Your temperature will increase, and emission along with it, until outbound emission equals the inbound influx. As the heat naturally moves from a warmer place (your lab) to a cooler place (cosmic background), you have a heat engine!

    In practice there are intractable concerns about where the heck you’re going to find that ideal insulation, and what to do about the Earth’s atmosphere radiating back down your sky-tunnel. But that’s “just” engineering, not fundamentals.

  6. ” He whetted the BaSO₄ with some isopropyl alcohol …”

    Believe me, I can misspell, mix metaphors, and mangle grammar like the best of ’em, but shouldn’t that be: “He wetted the BaSO₄….?”

    “Wetting” something moistens it. “Whetting” sharpens it.

  7. People always talk about thermodynamics when they doubt things. Case in point would be solar electric panels, it once was laughed at because it would disagree with that bible verse. I only see people testing hypothesis they believe in, so let them believe. Remember indoctrinated fools will follow the fools path of belief without merit. Science is changing constantly.

    1. I’ve been informed that some of science is ‘settled’.
      The people involved have lawyers, so watch yourself with that ‘Science is changing constantly.’ language. Very close to slander. You could find yourself looking at a human rights commission.

  8. This thing has been made for centuries (if not millenia), most mediterranean countries have the traditional way of painting the houses with a mix of lime (calcium oxide, not the fruit) and water. Have you ever seen in films or TV these small mediterranean villages near the sea with pure white buildings? That’s It, lime and water.

  9. Bone to pick with article. There are 2 commercial paints available. One for cool patios that won’t burn your feet. And one from a French company for painting roofs. I could look it up again and put the info here, but you can also google it yourself or follow the links from the YouTube comments section.

    1. Mythoughts62 provided above two links describing how people in the desert cooled water below ambient, to make ice. Here’s the more descriptive link:,though%20the%20air%20temperature%20never%20dropped%20below%20freezing

      Not just “possible”; it’s been done for centuries. Radiative cooling, no work is done.

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