Eight layers of polycarbonate plastic are used for the three-inch think cladding of the south-facing façade of this renovated house in the Spanish Pyrenees. Inn winter, the polycarbonate absorbs sunlight during the day, heating up to about 130°F. This heat is then transferred to the stone walls, which slowly radiated into the interiors, keeping it around 70°F even as night temperatures drop to 25°F. Photo 6 of 9 in 9 Eco-Friendly Homes With Smart, Sustainable Features

9 Eco-Friendly Homes With Smart, Sustainable Features

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Eight layers of polycarbonate plastic are used for the three-inch-thick cladding of the south-facing facade of this renovated house in the Spanish Pyrenees. During the winter, the polycarbonate material absorbs sunlight during the day, heating up to about 130 degrees. This heat is then transferred to the stone walls, which slowly radiates warmth into the interiors, keeping it around 70 degrees—even as night temperatures drop to 25 degrees.