Paint Yoghurt on Your Windows

An eccentric idea from Max.

I feel lucky to have been a member of Max Fordham LLP, the engineering design practice that Max set up in his name. When I started, Max was still somewhat active in the business. I enjoyed our interactions, which revolved around him answering my posts on the company’s online engineering discussion forum by telephone.

Max was enthusiastic and had a fantastic laugh. He himself was eccentric. Whilst often bold, the ideas that I saw him present were typically practicable.

I heard a story about him in the 1990s that helped spark this blog. How true is it? I'm not sure.

Max was part of the design team for a renovation of the Geffrye Museum (now the Museum of the Home, and since renovated again). All designed in an era before any regulatory measures existed to limit overheating in building designs. The renovated building incorporated a fantastic diagrid roof, full of glazing.

There’s a rule of thumb that if a building looks like a greenhouse, it will perform like one, too, and get spectacularly hot in summer. The story goes that this building was no different, and the client was spectacularly annoyed about it.

At an emergency meeting, Max proposed his solution: “Stand on the glazed roof and pour yoghurt all over it—it’ll reflect the sun’s heat off the glazing and bathe the interior with a milky glow of light. When summer is over, just hose off the yoghurt and repeat it next year”. The client thought this was wild. And I did, too - painting your windows with yoghurt is wild.

But then I kept thinking about it. What if it worked? Maybe it’s a good idea after all.

I looked on the internet, but I could only find it painted on the inside. I found US students who had used this to mimic privacy film in their student flats. I found people who had used it as a screen and then sculpted images into the yoghurt using a razor blade. Both looked fantastic.

I tried it myself. It was remarkably easy to apply with a gloss roller to the outside of a window. The yoghurt dried quickly and consistently. And it struck me that the result looked exactly like architectural glazing with a ceramic frit—a product I’ve specified on building projects to limit solar gains.

So, the more I’ve thought about applying the yogurt to the outside of a window, if it’s effective at keeping the heat out of a home, maybe it’s not such a wild idea. Yoghurt is readily available, easy to apply, cheap, easy for a window cleaner to wash off once the heatwave is over and totally biodegradable. Replacing glazing for a few weeks a year would be an incredibly expensive enterprise. But painting a little yoghurt on the outside of some glass? It’s super cheap. Is a landlord really going to notice or care? Nah - very doubtful.

I recall the g-value of fitted glazing I’ve specified is about 0.2. This means the glazing would reflect 80% of incident solar heat, really high-performance solar control glazing.

Has anyone tested the g-value of yoghurt on standard single or double glazing? I can’t find a paper testing it. Please get in touch if you know differently, have a means of finding out, or fancy funding such an investigation!

As you can tell, I’ve changed my mind. I think Max’s idea could be an important one.

Get more information on this idea, and other ways to mitigate the impacts of a heatwave, on the solutions page.