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The whitest paint could eliminate the need for air conditioning (yahoo.com)
97 points by graderjs 36 days ago | hide | past | favorite | 84 comments

Barium sulfate based coatings have been used for decades, maybe a century, as the 'whitest' reflecting standard for optical instruments. I hate to be that guy, but there's nothing that new here. Though Purdue and this prof sure are hyping the heck out of this, it's pretty funny for anyone in the optics world.. we buy it all the time for integrating spheres! This wasn't published in a high profile academic journal for a reason...

The actual challenge with any solar reflective paint is that it loses its solar reflectivity after multiple years of outdoor exposure. This can be seen at the third party Cool Roof Rating Council's website: https://coolroofs.org/directory

Outdoor white paints that start at 90+ % solar reflectance drop to the low 80s with time. Same thing will happen with this one, so the added benefit of the high reflectance will decrease quickly. Figuring out how to maintain this high reflectance in a paint exposed outdoor remains a very hard problem.

I don’t know anything about this, but wouldn’t low 80s still be far better than standard roof material? I’m guessing your average roofing, like shingles, absorbs +90% of solar energy?

Yes but at that point you may simply use classic white paint I guess. Also one thing to consider is that going front 90% to 80% may not look like a big difference but that's still doubling the heat that comes in

Yes you can make you own by adding barium sulfate to white paint, not as good as the expensive integrating sphere paint but pretty good for large white diffuse panels on a budget.

I watched as the rooftop out my window was painted white, it was extremely reflective. After a year, visibly the roof is dark gray. I wish I would have taken a daily picture and spectral measurement, maybe next time.

"Busting the myth on solar thermal reflective paints, and having a cool roof":


If it was worthwhile, we wouldn't need to wait for a paint, we'd all have mirrored chromium surfaces on our roofs.

That link doesn't really support what I think you are claiming. Their models show solar reflective paint reducing heat over a black roof. Most of the myths they bust are about it keeping a house warm, which it doesn't.

Reflective roof paint only helps if you have no insulation (which might be true of a shed, detached garage, etc.). Basic R-4 insulation performs dramatically better.

"A cool roof does not improve energy efficiency if your ceiling is [...] insulated, the roof area temperature is not lowered enough using a paint to make a quantifiable difference."

I feel like dirt is going to completely defeat the whiteness improvement on this within a matter of days.

> Cue roof washing drone industry

saw it here first, folks

edit: depends on enviro toxicity of barium? Only use I know of it is gastro contrast studies. Probably other industrial uses I’m ignorant of..

edit2: You have now subscribed to barium facts: “ Surprisingly barium shows up in our oil production. When drilling for oil, a substance called drilling mud is used. There are various types of drilling mud that serve a multitude of purposes. Crude barite is used to create a specific type of drilling mud called thixotropic mud. Its purpose is to help remove rocks from the borehole and keep them from settling when the mud is not flowing. ” [1]

“Barium is not an extensively used element. Most is used in drilling fluids for oil and gas wells. It is also used in paint and in glassmaking.” [2]

[1] https://info.noahtech.com/blog/how-barium-is-used-in-everyda...

[2] https://www.rsc.org/periodic-table/element/56/barium

Before we get that far, we need to know how fragile and/or toxic the paint is. And a whole host of other properties.

This may never leave the lab. It's hard to say at this point.

for sure. It’s used in oil drilling, it must be safe right? :)

Barium is from the same group of elements with Calcium and Magnesium so I imagine it's not very toxic. But it has many radioisotopes and a long lived one that's naturally occuring, Ba-130.

Your imagination is incorrect. Barium is indeed toxic, as is the case for pretty much every metal at the bottom of the periodic table even if they aren't radioactive.

However, it is one of the more mild toxins, and its toxicity is heavily mitigated by the fact that it's one of the least soluble cations (i.e., barium-anything is pretty insoluble), so it would be relatively difficult to actually ingest.

Lead is in the same group of elements as carbon and silicon so I imagine it’s not very toxic. — Flint, Michigan City Manager, probably

We use barium sulfate for enemas and swallow studies. I don’t know if that means all barium is safe though. It’s certainly not like gadolinium.

Bright white roofs are currently used to reduce urban heat island effects. They're great for the building occupants but are a literal eyesore for anyone above looking down. I can imagine this paint taking that to a whole new level.

I stained the deck at my old house solid white because I was sick of having to re-stain so frequently. It looked good and held up well but it could really be almost blinding.

I feel like green roofs are a better option here

My first thought was that dirt would immediately make this useless.

But since we are only talking about heat from radiation and not convection then putting a layer of glass above it would solve most of that issue. Bonus points if the paint and layer of glass are separated by a vacuum.

This world greatly increase the cost but would insulate very well and still reflect all the heat. The paint layer itself would never get dirty.

Wouldn't you just use mirrored glass?

The glass would still get dirty though.

Maybe you can put another layer of glass on top of it? Or, disposable plastic? (joke)

Yes, it would, and it would absorb more heat. It would then radiate that heat and the paint would reflect it.

If the glass gets dirty to the point that it is fully covered then it would have no benefit. But if it is dirty where say 20% of light is absorbed, then it the reflective layer underneath would still be hugely effective.

It was a joke - there are self cleaning glass products on the market but I don't know how well the self-cleaning properties hold up on horizontal installations - you will still get some sediment buildup. The self-cleaning coatings may also change the heat reflective properties of the installation.

> During field tests, the BaSO4 film stays more than 4.5 °C below ambient temperature or achieves an average cooling power of 117 W/m2.

That's pretty good for a roof in direct sunlight but it's not going to eliminate AC.

A black (e.g. asphalt shingle) roof would rise significantly above ambient temperature, though, right? So the 4.5 degrees C could be a significant improvement.

It would at least significantly reduce demand.

So like it's done for at least 1000 years?


I have a flat, foam roof, painted white. It really does work to keep the house cool, although the flat roof geometry would be problematic in climates with more precipitation.

I suspect roof paint will be much less important in coming decades as solar panels proliferate, however.

Very similar thing here, I live in the Bay Area and have a flat, bright white roof. On an 85 degree summer day, it’s a wave of cool comfort to walk through the front door, and I pay nothing for it.

i live in an apartment building in LA with a vented attic and a flow-through design, as the building was built before air conditioners became popular. it’s cooler than ambient by a few degrees (haven’t rigorously measured it though) even without a white roof.

This is a silly headline, but what can I expect from yahoo. Help certainly, but it does nothing for humidity. Down south we have to leave ac on somewhat even we go out of town, because the humidity damages stuff. I also wonder what this much reflective stuff might cause as other effects. Would reflecting all your sunlight onto other, non super white houses count as a negative externality? Seems like you'd double your neighbor's AC bill to halve yours.

> This is a silly headline, but what can I expect from yahoo. Help certainly, but it does nothing for humidity. Down south we have to leave ac on somewhat even we go out of town, because the humidity damages stuff.

Wouldn't it be wiser to get a dehumidifier for this purpose? A cursory glance seems to indicate that it would consume less energy for the task of managing the humidity-levels, when cooling is not required.

>I also wonder what this much reflective stuff might cause as other effects. Would reflecting all your sunlight onto other, non super white houses count as a negative externality? Seems like you'd double your neighbor's AC bill to halve yours.

This would likely be trivial, given the relatively small amount of light that would get reflected onto the building in relation to all of the reflected light - most reflected light would not go towards neighbouring buildings. Plus, they can paint their houses with the same type of paint as well, negating the issue entirely.

Dehumidifiers are ACs, just with the condenser indoors and exhausting heat back into the cooled air stream (so heating the room overall).

A particular dehumidifier can (of course) be more or less efficient than a particular AC system. But it's probably more efficient, given similar technology, if you want a drier, warmer room.

Hardly ever want that warmer room bit during the Southeast US humid season. So you'd wind up running the AC too, to exhaust the excess heat (including the extra heat from the dehumidifier).

I'm specifically only thinking about the case where you run an AC while not at home. By all means, run your AC if you are at home and need to manage the temperature.

I run two dehumidifiers in my basement and they consume similar energy to an a/c (400-500W), but separate from that they also increase the temperature because the exhaust has no where to go, defeating part of the purpose of the white roof. Main purpose of my humidifier is to keep my basement from growing mold, not keeping cool.

>I run two dehumidifiers in my basement and they consume similar energy to an a/c (400-500W)

Am I correct in understanding that two humidifiers use the same amount of electricity as one AC, making them twice as efficient for the task at hand?

>but separate from that they also increase the temperature because the exhaust has no where to go, defeating part of the purpose of the white roof.

If you're looking to cool and dehumidify, then by all means, use an AC. My question was on the use of an AC while no one was home, ie where the cooling effect would go entirely wasted, and only the dehumidification was valuable, in protecting from humidity-induced damage.

It won't eliminate A/Cs. People could use white or silver paint right now. They don't. It "looks bad".

Not to mention that it won't do anything about humidity. Even when it's not that hot, high humidity feels awful. Also not good for the house and the items inside.

Yeah, reflective paint isn't obstacle to be overcome here, it's the almighty HOA.

This is a place where the coercive hand of the state might be helpful, if white paint is really so good that it's going to eliminate what is a major contributor to global warming.

It doesn't only look bad, it reflects the sun and probably into your neighbours/road etc at different times of the day. Silver roofs are banned here for that reason.

I assumed anyone sane would use flat/matte.

It was corrugated iron, and was very common, it's not polished and looks matte, but still when the sun hits at the right angle it reflects. It has to be painted now.


Could still be a big deal in South America, Africa, and Asia. They have fewer issues with buildings that look bad, but are also less able to afford A/C. Those people are also going to be most affected by climate change, and would want cheap ways to reduce the impact of intense heat waves.

It does seem a little grotesque if we're going to stick them with the problem and not even be willing to make aesthetic changes. Particularly when we're disproportionately responsible for the problem in the first place.

Stick them with the problem? I'm not sure what you mean.

And whether we're willing or not to make aesthetic roof changes has little impact on the climate.

Saying "hey guys, tough luck, you're on your own, we refuse to change anything to reduce our wildly high per capita emissions" is sticking them with the problem. The claim of this article is that this white paint can reduce roof temperatures by 4.5 degrees centigrade, which is plausibly enough to substantially reduce demand for air conditioning, which is a major contributor to global warming. If the _only_ problem with this plan is that some people don't like the aesthetics -- I'm not saying that is the case, but granting it in this discussion -- it sounds like a frivolous objection.

Painted my workshop roof white, temperature drop was noticeable.

Needs a repaint though, after 6 years it's washed off. Then again I used the cheapest paint I could find, just to test it.

Air conditioning uses a bit more power than I thought, but it's still not that much.

Installing a couple solar panels would probably be much more effective.

I don't think they're exclusive strategies, and paint has some other things to recommend it like much simpler, lower-cost installation.

They don't have to be exclusive but you'd obviously want to do the more cost-effective one first.

> paint has some other things to recommend it like much simpler, lower-cost installation.

I wouldn't say painting a roof is simpler than mounting 4-6 solar panels. Are you sure it's cheaper?

Just taking the first Google results at their word, a roof painting is around $3000 and a solar panel installation around $15,000. Of course there are many variables to consider but I think it is likely to hold true in most cases.

That's for bigger solar installations. A 2kW system would cost about $4k after tax credit and it would probably save you a lot more energy.

And how many years will the paint last?

Weird yahoo link from today?. Here is the the Purdue link from April 15th 2021:


Related articles : L.A.’s New Reflective Streets Bounce Heat Back into Space ( https://reasonstobecheerful.world/reflective-white-streets-u... )

and counterpoint: White Roofs, Green Myth? ( https://www.huffpost.com/entry/white-roofs-green-myth_b_2901... )

Pretty neat, it can passively cool surfaces because it absorbs less solar energy than it emits. And it's made from common chemicals used in cosmetics, among other things.

Not anywhere south of the Mason-Dixon line, it won't.

https://rovashield.com/rova-shield-insulation/ Aerogel paint can also effectively do the same thing. Cost is usually the issue with construction.

So we just have to go paint the whole earth surface with this and climate change is sorted.

I am very happy to see this, although it has many other characteristics that must be considered as a building material (such as durability, I don’t want to paint the wall once a week)

Would be great to know the cons too. Eg. environmental impact, wildlife (birds, insects), air travel, radio-waves ...

Does this reflect more light (or different wavelengths) than a mirror?

very cursorily, apparently this paint reflects ~99% of light and a decent mirror will reflect about the same. white surfaces scatters light randomly while mirrors reflect it incidentally (initially thought white might somehow ‘even out’ reflectivity across wavelengths vs. a mirror, but quickly realized that makes no sense). so if you could sufficiently flatten a white surface so that light bounces off it uniformly, it becomes a mirror.


It won’t eliminate AC for me; I run my air conditioner at night.

Just don't look in the direction of any buildings.

Zima White

Who calls it an 'aircon'?

Australians definitely do.

In many countries I have visited, its the norm. eg- Singapore.

Submitted title was "Scientists created the whitest paint. It could eliminate aircons". We've since changed it.

I’ve heard this before but very rarely. I think it’s a regional thing.

I had a housemate from Korea who called it that.

The anglosphere outside of north america

Should be called room cooler.

Very common in Australia.

What do you call it?

Oh, in the US it's 'air conditioning' or 'air conditioners'. Surprised to hear so many people affirm it!

in japanese it's エアコン (eakon)


Negative comments in this thread and HN in general are exhausting. I very much wish this crowd were more optimistic and inspirational.

I find it absurd as well.

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