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!

Yes greenhouse shade paint is definitly a thing in the UK too. I'd not thought of painting the house windows, but it would be a quick, cheap and temporary means of reducing the light getting in and hence the heating effect of the sun. Probably only needed on any South facing windows - much easier than trying to rig up shade cloth I should think.

Nancy Reading wrote: Probably only needed on any South facing windows - much easier than trying to rig up shade cloth I should think.

Much of what I've read, it is west facing windows that are a big issue in many ecosystems. This appears to be true in my ecosystem, partly because the west sun is coming in at a lower angle, so less is blocked by overhangs, and I think partly because the air has heated up more compared to the overnight low.

This video also suggests west windows are a big issue, and suggests that although temporary painting jobs will help, even a small air gap makes a big difference, which is where some sort of screening/shade cloth may have an advantage.



All that said, I'm no engineer, and my engineering son hasn't gotten back to me with his evaluation of the information in this video. Also, this may be a better long term solution, right now during a serious heat wave, *anything* that's cheap, quick and easy, should be used, not just by yourself, but if possible, be kind to neighbors and help them, as not everyone is safe or comfortable on ladders!

I'm wondering about a cheap, bulk source of chalk to try this.

I have some glass skylight windows that add too much heat in summer (but are wonderful in all other seasons). Previously I covered them with aluminized bubble wrap insulation, which is a pain to install because of the wind that rips along our roof.

I know weightlifters have bags of chalk, for "gription." Where can I get this stuff cheap?

Douglas Alpenstock wrote:I'm wondering about a cheap, bulk source of chalk to try this.

I have some glass skylight windows that add too much heat in summer (but are wonderful in all other seasons). Previously I covered them with aluminized bubble wrap insulation, which is a pain to install because of the wind that rips along our roof.

I know weightlifters have bags of chalk, for "gription." Where can I get this stuff cheap?

Looks like hardware and feed stores sell it as 'line marking powder'or powdered agricultural lime.(Stone) - chalk is calcium cabronate. A lifetime supply, or 50 lb bag, for about 20 bucks.

You could also use yoghurt and paint it on the outside of your window as a quick and easy film, that will biodegrade and probably last as long as you need it to.

This makes me think of my grandma who doesn't wear hats because she doesn't like to muss her hair, but given a 35 degree day in Kent (England) she will gladly steal mine and enjoy the comfort over the glamour. Besides... yoghurt windows are so eco chic ;0)

Burra Maluca wrote: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.

***I would be afraid that birds would fly into this reflective film. Already too many birds crashing into windows and dying.

Gaurī Rasp wrote:***I would be afraid that birds would fly into this reflective film. Already too many birds crashing into windows and dying.

We did have issues with birds flying straight into the west-facing window when we first moved here, but fitting a fly-screen has completely solved the problem. We fitted it before the reflective film so I'm not sure how that would have effected things without the screen.

But yes, point taken!

I use another way of shading windows. I've tacked white cotton sheets/fabrics in front (outside) of our hottest windows and it helps a lot. I get light but also a reduction in direct sunlight. It is also moveable if I want to peek outside to check on our animals. It only works for those who can tack items outside their windows (maybe not for 2nd floor rented apartments for example) but is an easy fix and probably as cheap as paint. Works better in times where I don't get a lot of summer rain. I've gotten my sheets at estate sales etc. for $2 or so a piece and take them down in winter. Something to keep around and always handy!

My home made triple glass windows with a blind between it and storm windows. and my air conditioner siting on my upper window + lower window + upper storm window + lower storm window and insulation on the units sides.

I wonder how bentonite clay would work.  Dissolved in water to what ever thickness seems right.

And you could maybe apply it in an interesting pattern , or once dried scrspe out a pattern you don’t mind looking at, with strategic peek holes.  It seems like a basic project that could lead to fun, creativity and even joy!

Mark Reed wrote: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.

In South Africa, semi desert  with summer temps normal and regularly exceeding 40 odd celcius, we use coolaroo shade netting (we call them shade sails) with great success. Works for lower light plants, keeps kids from getting burnt in the pool (without leaves from shade trees making cleaning a challenge) and keeps summer sun off and high above all our north facing windows. About to install a few more as we head into spring and add rabbits to our garden, which doesn't have that many trees (or at least decent sized ones).

I like this idea of painting the roof. It gets freaky hot in my house. Does anybody know if it will badly affect cement fibre 'slates'? The community was painting the roads with chalk paint to stop the road melting too much and heavy tractors and wood lorry traffic.

I have used hologram gift wrapping foil on the inside of my windows. I just spray rhe window with water and it sticks like magic.  Easy to remove at the end of the season

Steph Walker wrote:... I've tacked white cotton sheets/fabrics in front (outside) of our hottest windows and it helps a lot. I get light but also a reduction in direct sunlight. It is also moveable if I want to peek outside to check on our animals. It only works for those who can tack items outside their windows (maybe not for 2nd floor rented apartments for example) but is an easy fix and probably as cheap as paint...

If high winds aren't an issue, and depending on how the windows are built, a "spring rod" such as is sometimes used for shower curtains or sheers, or a "clamping adjustable rod" such as Hubby bought for the back of his truck so stuff wouldn't slide all around, could be used when you don't want to install hooks or use tacks.

Douglas, paint the inside of the window. The rain won't wash it off.

John Duffy wrote:Douglas, paint the inside of the window. The rain won't wash it off.

Thats an interesting idea.  I’m Just wondering about whether or not the glass itself heats up when painted on the inside instead of the outside….

And if that results in any significant amount of radiant heat indoors.

John Duffy wrote:Douglas, paint the inside of the window. The rain won't wash it off.

Yes! I was thinking that also, but the press articles weren't clear. It's so obvious.

The cleanup may be more messy though, requiring containment measures. Airborne chalk does not interact well with modern tech.

Thekla McDaniels wrote:...  I’m Just wondering about whether or not the glass itself heats up when painted on the inside instead of the outside…

I agree with Thekla here. The chalk will be much more effective on the outside. If you need something that will cope with rain, I would try some sort of material, or the mesh in the video I posted, or even strings or poles with bean plants growing up them. Anything that intercepts the light *before* it goes through the glass, because the goal is to keep the heat outside.  

Lace “glued” to the outside of the window might be lovely… maybe starch could act as an adhesive…. Hmmm, when was the last time I heard of starch - the laundry product as opposed to in reference to food.  They used to sell it in the grocery stores.  Maybe they still do🤷🏻‍♀️.  I think you could make laundry starch out of cornstarch and water, mix the lumps out, or strain the persistent ones.  Then slowly heat, stirring constantly.  Seems a lot like making gravy… or wait until you have cooked it before straining the lumps out.  

Dip the old lace curtains from the thrift shop in the liquid starch, and then apply to the windows.  Hmmm, before putting the starchy curtains up, devise a way to hold them in place until it dries.  I don’t know for sure they wouldn’t stick on their own… just thinking it through as a thought experiment.

I like lace on the windows😊

As we think this whole idea of outside window coverings through, so many things come to mind!  For me it seems to become what to put up and how to keep it in place.  And is there a way to embellish or enhance the effect so that it also brings beauty or whimsy or humor or some other element to the environment…. although it does seem that permies people already live rich lives!

To remove chalk paint or any other type, simply requires a razor blade scraper with a sharp blade "carefully" applied in a gentle motion. Brute strength could result in broken glass...I have found that running the blade at a 45 degree angle will produce a desirable result...FLIP the blade over after each pass so as to maximize the paint removal..If your blade seems to be getting dull, you can strop the blade on some charged leather or, Pull the blade away from the cutting edge on a piece of wet/dry sandpaper...I learned this after leaving masking tape on a South facing window for 2 weeks...I think i invented a few new cuss words after that debacle

Thekla McDaniels wrote:And is there a way to embellish or enhance the effect so that it also brings beauty or whimsy or humor or some other element to the environment…. although it does seem that permies people already live rich lives!

I think lace curtains would be quick and effective. Alternatively add a little pigment to the paint and try some painted windows or stained glass window effects.


tutorial here


etsy

I can think of two ultra white paints that could be used.

The first, best is Barium Sulfate.  It sounds exotic, but it is perfectly harmless and is used as a contrast dye for certain medical imaging.  It has to be consumed in the form of an ice cream shake, typically flavored chocolate or strawberry.

Barium sulfate is extremely white, reflects across a very broad spectrum but re-radiates in a narrow spectrum—conveniently the spectrum of infrared which the atmosphere is highly transparent during a cloudless sky.  

The other option is calcium carbonate (feel free to check me here).  It is slightly less reflective than barium sulfate but far more common.  Ideally, the particles should be ground to a point where there is a mix of sizes—not one uniform size.   Like Barium sulfate, it radiates through that same narrow window.

The net effect of either of these paints is that a panel (or roof) painted with either can actually cool below ambient temperature on a hot, bright sunny day—not a bad trick.  It’s a phenomenon called passive solar cooling.

Might be worth a try?



Eric

That's good information Eric! Barium sulfate (suphate) is fairly easily obtainable as it is used in soil tests to aid flocculation (makes the particles stick together so you they precipitate out more quickly)

It's not just chalk! There's a little clay in there, which would make a paste that would stick.

From MSN dot com:

Meudon whiting is composed of calcium carbonate — the main component of chalk and limescale — and it also contains a small amount of clay. It takes its name from the quarries on the outskirts of Paris, where it was first thought to be extracted in the 18th century. You may also hear it referred to as Spain whiting, Champagne whiting, Troyes whiting and ‘white polishing stone’.

Artisans and households first used the powder to polish delicate surfaces, such as glass, marble and silverware. It’s also been traditionally used as a base pigment or additive in paints and putty.

What about double paned upstairs condo windows?  Double paned windows cannot have anything applied on the inside as it heats up the air between the panels and can void your warranty and also cause damage & discoloration or cloudiness between panes.

Do they open Shannon? If so then paint and/or netting could still be an option on the outside.