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 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.
If it was worthwhile, we wouldn't need to wait for a paint, we'd all have mirrored chromium surfaces on our roofs.
"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."
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. ” 
“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.” 
This may never leave the lab. It's hard to say at this point.
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.
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.
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.
That's pretty good for a roof in direct sunlight but it's not going to eliminate AC.
I suspect roof paint will be much less important in coming decades as solar panels proliferate, however.
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.
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).
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.
And whether we're willing or not to make aesthetic roof changes has little impact on the climate.
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.
Installing a couple solar panels would probably be much more effective.
> 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?
And how many years will the paint last?
and counterpoint: White Roofs, Green Myth? ( https://www.huffpost.com/entry/white-roofs-green-myth_b_2901... )