Materials: Roof that Sweats

Mammals have the unique ability to moderate their body temperature through perspiration—
evaporation on the surface of our skin that helps to keep us from overheating. But if sweating works so well for humans, why couldn’t it work for buildings, too? A team of Swiss researchers from ETH Zurich has developed a new type of synthetic roofing material that holds water and sweats when it heats up, effectively cooling down the building without the use of electricity.

For the roofing material, the researchers used a special water-permeable membrane that fills with water when it rains. If the material becomes warmer than about 90 degrees Fahrenheit in direct sunlight, it begins to “sweat,” releasing water at the surface, where it evaporates and helps to cool the building. The water-permeable mats actually shrink when they get hot, forcing water out. According to the team’s calculations, a sweating roof could save up to 60 per cent of the energy used by air conditioning on hot days.

The researchers used small models to test the material, covering the roofs of small railway model houses with 5-mm-thick mats of the special material. They then shined a bright light on the roof to simulate intense sunlight while measuring the temperature inside the model houses. As expected, the model with the special ‘sweating’ roof stayed cooler. “The house insulated with the PNIPAM mat warmed up much more slowly,” said ETH-Zurich doctoral student Aline Rotzetter. Although the findings appear to be quite promising, there is still a lot of research to be done before this type of material could have any practical application.