Perched high in the Spanish Pyrenees, this old stone building needed major changes to become habitable again. Originally built in 1900 as a hostel of sorts, its strucutre was not incredibly robust: for instance, its windows couldn't be expanded for risk of weakening the wall. But architect Josep Bunyesc still saw a chance to turn it into a model of energy efficiency.  Photo 1 of 6 in Guess Which Humble Material Made This 100-Year-Old Building Super Energy-Efficient