An anonymous reader quotes a report from the Guardian: Australian scientists have developed roof coatings that can passively cool surfaces up to 6C below ambient temperature, as well as extract water from the atmosphere, which they say could reduce indoor temperatures during extreme heat events. One coating made from a porous film, which can be painted on to existing roofs, works by reflecting 96% of incoming solar radiation, rather than absorbing the sun's energy. It also has a high thermal emittance, meaning it effectively dissipates heat to outer space when the sky is clear. Its properties are known as passive radiative cooling. [...]

In a study, published in the journal Advanced Functional Materials, the researchers tested a prototype for six months on the roof of the Sydney Nanoscience Hub, pairing the cool paint with a UV-resistant topcoat that encouraged dew droplets to roll down into a receptacle. As much as 390 milliliters per sq meter per day could be collected for about a third of the year, the scientists found. Based on that water capture rate, an average Australian roof -- about 200 sq meters -- could provide up to 70 liters on days favorable for collecting dew, they estimate. [...]

In well-insulated buildings, a 6C decrease in roof temperature "might result in a smaller fraction of that cooling being reflected in the top level of the house," [said the study's lead author, Prof Chiara Neto of the University of Sydney], but greater temperature reductions would be expected in most Australian houses, "where insulation is quite poor." She said the coating could also help reduce the urban heat island effect, in which hard surfaces absorb more heat than natural surfaces, resulting in urban centers being 1C to 13C warmer than rural areas. The researchers found that the prototype coating was comprised of poly(vinylidene fluoride-co-hexafluoropropene), which is used in the building industry but was "not a scalable technology going forward" due to its environmental issues. However, they are now commercializing a water-based paint with similar performance that is affordable and environmentally safer, costing about the same as standard premium paints.

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