Chalk paint to cool windows and roofs

I never thought of using temporary chalk paint on windows. I wonder if there is an easy way to make it a bit more durable (rain resistant).

https://www.bbc.com/future/article/20260625-why-the-french-are-painting-chalk-on-their-windows

"Why French people are painting chalk on their windows in the heat"

"As record-breaking heat sweeps over France, some shops are running out of a simple, cheap and unexpected product – crushed chalk.

Known as Blanc de Meudon, or Meudon whiting, it is normally used to make paints or as a cleaning product. But faced with punishing temperatures, there are reports that ingenious people have been using the chalky material as a home remedy against the heat, covering windows in schools and private homes.

Mixed with water, then painted on glass, the result is a milky, whitish coating that lets in some light but reflects the heat. And a growing body of research suggests that there may be some solid science behind the DIY cooling hack."

Saw this recently, along the same lines, lime washing roofs.  Something that would brighten, but not seal, asphalt shingles would be great.  Normal elastomeric roof coatings can seal in moisture and rot the roof decking.

We use outdoor curtains made of shade cloth. Something called "coolaroo" shade cloth works very well. We bought it in a roll and made our own, lots cheaper than buying ready-made ones. I think it is made of recycled plastic bottles and is much more durable that I expected, we've used it for years. It really keeps the heat out, but some light gets through and you can see though it, a little bit.  A lot of things would probably work, old shower curtains or bed sheets, anything light in color and made of something like polyester although I guess cotton would work too.  If one layer not enough just double it up. Keeping the heat outside is much better than trapping it between the glass and an indoor curtain.

Douglas Alpenstock wrote:I never thought of using temporary chalk paint on windows. I wonder if there is an easy way to make it a bit more durable (rain resistant).

https://www.bbc.com/future/article/20260625-why-the-french-are-painting-chalk-on-their-windows

"Why French people are painting chalk on their windows in the heat"

"As record-breaking heat sweeps over France, some shops are running out of a simple, cheap and unexpected product – crushed chalk.

Known as Blanc de Meudon, or Meudon whiting, it is normally used to make paints or as a cleaning product. But faced with punishing temperatures, there are reports that ingenious people have been using the chalky material as a home remedy against the heat, covering windows in schools and private homes.

Mixed with water, then painted on glass, the result is a milky, whitish coating that lets in some light but reflects the heat. And a growing body of research suggests that there may be some solid science behind the DIY cooling hack."

I wonder if mixing it with washable kid's school glue would make it last a little longer (while still being easy to eventually remove).

I love the idea.

(And yes, exterior shade cloth makes a huge difference, too).

We fitted all our windows with sun-protection film. Not as natural as chalk paint but you can leave it up all year round and it's surprisingly effective. During the day, you can see out through it perfectly but people can't look in as it's mirrored. During the night, if you have the light on inside, it's the other way around so bear that in mind if you have neighbours!



Image from temu

It claims to filter out 90% UV, 89% IR but only 20% of the visible light.

I'm not sure it's quite that effective but it certainly helps and is very affordable. I think ours has been up a couple of years now and looks as good as new, except for the bit near the cooker where it's been splashed with hot fat, but that's hardly surprising.

We also extended the roof when we changed it so that the south-facing windows are shaded from the sun during the hottest months of the year. The only problem window now is the west-facing bedroom window but pulling the blind down during the late afternoon seems to help with that.

In my ecosystem, people use an Agricultural Lime wash on their greenhouses. I think that the theory is that our soil can use the lime, or at least isn't hurt by the lime, so even if it gets washed off before they want it to be, it just means reapplying it. It washed off intentionally when the greenhouse needs more light and heat.

I don't know the chemical difference, or the pH difference between the chalk and the Ag Lime, but I'm sure some nice permie (Joseph Lofthouse maybe?) will enlighten me!

I would be aware if trying this, that it likely isn't going to wash off evenly, so when the crisis is over, they may have some work to be done. That said, if it helps keep more people alive in a heat wave, that's worth any amount of effort after the fact - stay safe everyone!