emergency blankets works great and are cheap too

however, you get even better cooling with a coating that radiates the heat, it cools below the air temperature which feels so wrong

NighthawkInLight on YouTube has explored this and created recipes, the results still feel impossible to my intuition

You can also get UV/IR-reflective coatings on your windows, either in the glass when they're installed or as aftermarket foils on existing ones. We have them on all of our sun-side-facing windows and they do a great job. They also provide privacy if you use the heavier-duty commercial-grade ones, which look like a mirror from the outside.

Using CaCO3 which is calcium carbonate, like the article.

This science-lecturer-esque guy is using Barium Sulfate: https://www.youtube.com/watch?v=dNs_kNilSjk

Have you been to Paris? Go on google earth and poke around. Theres no place to put the condenser. Most people rent so even if there was they can’t drill holes for the lines and even if they could, most buildings are historic so they can’t. Central AC/heatpumps on some rooftops could work.

If you live in a historical rental flat you often can't install AC.

Also we do tend to consider the environment here. All the heat kept out doesn't have to be pumped.

It's not either-or