Perched high in the Spanish Pyrenees, this old stone building needed major changes to become habitable again. The strucutre was not incredibly robust: for instance, the south-facing windows couldn't be expanded to admit more sunlight for risk of weakening the wall. However, that didn't stop architect Josep Bunyesc from seeing an opportunity to turn this home into a model of energy efficiency.

Originally built in a 1900 as a hostel of sorts, the building had since fallen into disrepair. Bunyesc was asked by a family of four—the couple have a 1 and 2 year-old—to transform the aged building into a modern home. The architect wanted something energy-efficient, and had several renovations under his belt, but he turned to an unusual material to keep the sun warm: polycarbonate, a rugged plastic common to industrial architecture, sheds, and other lightweight construction.

The south-facing facade is covered in 8 layers of polycarbonate plastic, in total about 3 inches thick. During winter, the polycarbonate absorbs sunlight during the day, heating up to about 130°F. That heat is then transferred to the stone walls underneath and slowly radiated into the inteior, keeping it around 70°F even as night temperatures drop to 25°F. A wood-burning stove compensates for cloudy days. (For the thermally-savvy, the polycarbonate has a U value of 1.1 W/m²K).

During summer, the sun is too high above the home to heat the polycarbonate: it simply reflects the light—and its energy—back into the atmosphere. In fact, during the summer the stone wall can absorb the interior heat and release it into a small air gap—about 2-4 inches wide—between the polycarbonate and the wall.