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Comment by kumarvvr | Hacker News Reader

This concept is used all over India to cool down homes that are on the top floors.

https://www.amazon.in/EXCEL-CoolCoat%C2%AE-Reflective-Coatin...

Basically, have a highly reflective white coat on your roof, to reduce temperatures by about 3 Degrees Celsius.

Almost all homes in Urban India are made from concrete and bricks, which can hold a lot of heat.

I myself have been in houses that use this to cover only some rooms of the house (mainly the bedroom), and the temperature difference is definitely noticeable. It also makes the room livable in the extreme hot summers in India.

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This is the opposite. It says, "Refelects [sic] 90% of solar infrared rays," because of its "High IR reflective Pigments [sic]," so its emissivity in the infrared is 0.1, but the IR-selective paints we're talking about here are optimized for high infrared emissivity, which means they absorb a lot of infrared.

Maybe there's some wiggle room here because solar infrared is mostly near IR and MWIR, and the place where we want high emissivity (absorptivity) is longwave IR, but to the extent that the advertisement makes any claims about infrared emissivity, it claims very low infrared emissivity, not high.

A paint with low emissivity across the spectrum will slow down the temperature rise when the sun is up, but also slow down the temperature drop when the sun is down. This can still make rooms livable, but it isn't the same as what you get with regular whitewash, where the temperature of the roof is actually lower than the temperature of the air around it.

It kind of blew my mind when I first learned about this whole phenomenon (mostly from the YouTube series I posted). Not all white paints are equal and it’s kind of interesting to think that something that looks mostly identical to our eyes has very different (passive) properties in the infrared.

I think one of the things in the paints that Ben adds is a set of microspheres that reject incident incoming infrared beyond a certain angle but allow it to pass through when radiated. Something like that.

He usually garbles the scientific theory in his videos, but I trust that he's honestly reporting his experiments, and that his theoretical errors are honest mistakes rather than intentional attempts to mislead.

You should be aware that there are rigorous constraints on how much absorptance can differ from emittance, known as Kirchhoff's Law of Thermal Radiation: https://en.wikipedia.org/wiki/Emissivity#Absorptance because without them you could get mechanical power generation from a uniform bath of thermal radiation, which would give you a perpetual-motion machine.

To be fair it does say "reflects solar infrared rays," which I would interpret as "the IR from the Sun" (aka NIR).

The product datasheet[0] claims a thermal emittance (aka LWIR) of 0.82. Having such a high value is typical for non-metallic surfaces.[1]

[0] https://5.imimg.com/data5/CA/RO/MY-653008/excel-cool-coat.pd...

[1] https://ntrs.nasa.gov/api/citations/19840015630/downloads/19...

Thank you for finding this! I was wrong, and I'm pleased that the product is in fact what the great-grandparent claimed. And the NASA publication is very helpful for putting this in context.

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