A new tech twist on ancient knowledge22nd Nov 2020
Houses in warm parts of the world are traditionally painted white – think of Greek islands, or Cape Dutch houses. Now that might be through lack of alternatives – or lack of imagination – but it is actually a good idea. White paint reflects sunlight, which means the walls absorb less heat.
(Of course, the fact that traditional walls were nearly a metre thick also helped.) And – sticking with the traditional motif – thatched roofs offer fantastic insulation from insolation (say that fast ten times). Insolation, by the way is incoming solar radiation. Because thatched roofs were dark, and traditional clay roofs were red, roofs have remained, mostly, darker than houses. Let’s face it, a white pitched roof does look – well – unusual. So how can we make roofs more reflective? Or, in tech-speak, how can we increase the albedo of roofs?
A number of established paint companies market heat-reflective paint specifically for roofs. Most are white – or even silver – but some are offered in a variety of shades, including grey, green and a traditional-looking red. The heat-reflecting capacity is due to tiny reflective particles, or beads, included in the paint. It is hard to quantify how well these work, but some manufacturers claim that their paints can reduce internal temperatures by as much as 25% – or 10°C. These paints do come at a premium, and some have been shown to not live up to their marketing promises. What is inarguable, though, is that – all else being equal – a light paint will be cooler than a dark one.
Don’t lose it, use it
Of course, deflecting, reflecting, blocking or otherwise avoiding insolation is just one way to cool your house – but that’s being a tad negative. Rather than pushing the sunshine away, use it. Yup. Adding solar panels to your roof is a smarter way to cool it than paint. Depending on the design and type of panel, it will cool in a number of ways. Firstly, solar panels do absorb insolation, but, instead of then conducting it through the roof as heat, they turn it into electricity. If the panels are slightly raised from the roof they will create shade, and – okay, this last one is a cheat – you can use the insolation to create electricity to run an air conditioner.
Heat reflecting paint on steroids
But buildings – especially tall ones – absorb heat through the walls as well as the roof, so heat-reflecting paint is still something worth thinking about. And that’s exactly what engineers at Purdue University have done. They have produced a paint that has been shown to be significantly more effective at reflecting insolation, and therefore heat, than existing heat-reflecting paints. What’s interesting here is how the engineers achieved this. After testing a variety of products, they settled on using calcium carbonate (CaCO3) fillers in a commercially available white paint. Due to its unique atomic structure and varying particle sizes, the CaCO3 scatters a wide range of wavelengths of visible and infra-red light. Another advantage of CaCO3 is that it is widely found on earth in rocks and seashells.
Back to the future
I can’t help smiling at this. Think back to those whitewashed Cape Dutch houses, Greek villas and Overstrand fisherfolk cottages. Whitewash is made from CaCO3 – usually made from crushing and burning seashells. (A little aside, Kalk Bay is called Kalk [lime or chalk] Bay because that was where most of the whitewash for the early Cape Dutch houses was made.) Anyhow – it seems our ancestors knew a thing or two about keeping houses cool, and now engineers are catching up to them.
According to one of the researchers, Xiulin Ruan, surfaces painted with this paint can be up to 10°C cooler than those painted with commercial white paint. Sadly, they did not use traditional whitewash as a control. Because much of the insolation that is reflected back to space can escape the earth’s atmosphere through the greenhouse gases, extensive use of genuinely light-reflective paint can have a significant effect on ambient temperature and, some researchers believe, could compensate for the decrease in albedo due to melting ice caps.
The surface of the earth would get cooler, thus halting (or at least slowing) global climate change, said the researchers in a press release from Purdue. Whether this would be the case if these surfaces were merely painted white – or, even better, whitewashed – is moot, but there is a good argument for opting for light and white. Many studies show that increasing urban surface albedo by painting rooftops white, having light pavements, and even – as suggested by some eco-engineers – the roofs of cars can significantly reduce local temperatures, thus decreasing the need for air conditioners, and thereby slowly spinning into a virtuous cycle. What the tipping point would be is anyone’s guess, but it is certainly worth working towards, and it’s likely that, even within one estate, extensive use of white or light surfaces could contribute to an overall cooling. If it’s a big enough estate. Especially if anything that is not light-coloured is natural vegetation.
Something to think about at your next AGM when someone questions the aesthetics guidelines calling for black roofs.