Researchers at the University of Sydney and commercial start-up Dewpoint Innovations have created a nano-engineered polymer coating that not only reflects up to 97% of the sun's rays, but also passively collects water. In tests, it was able to keep indoors up to 6 °C (~11 °F) cooler than the air outside.

That temperature differential results in water vapor condensing on the surface – like the fogging on a cold mirror – producing a steady trickle of droplets.

In trials on the roof of the Sydney Nanoscience Hub, the coating captured dew more than 30% of the year, generating as much as 390 mL of water per square meter (roughly 13 fluid ounces per 10.8 square feet) daily. This might not sound like a lot, but a 12-sq-m (about 129-sq-ft) section of treated roof could produce around 4.7 L (around 1.25 US gallons) of water per day under optimal conditions.

Most houses have a lot more roof than that. "Over an average residential roof," reads the Dewpoint website, "you can expect enough water per day to cover your basic water needs." That's in addition, mind you, to the rainwater you'd be collecting as well, since you do need to have a typical rainwater collection system installed to capture the dew. In Sydney for example, assuming an average annual rainfall around 1 m (3.3 ft), The Tank Factory tells us we could expect to collect somewhere around 6 times more rainwater than condensation - but that equation would certainly look very different in drier areas.

"This technology not only advances the science of cool roof coatings but also opens the door to sustainable, low-cost and decentralized sources of fresh water – a critical need in the face of climate change and growing water scarcity," said research lead Chiara Neto, a professor at the University of Sydney's Nano Institute and School of Chemistry.

The team has recently finished up a six-month outdoor trial, with panels featuring the polymer paint-like coating set up on the roof of the Sydney Nanoscience Hub building. During this period, minute-by-minute data was collected on the coating's cooling and water collection abilities, and found that dew could be collected over 32% of the year, suggesting water could be harvested from the air during periods without rain. What's more, the coating withstood the challenging test of the harsh Australian sun, and showed no signs of degradation over the six months.