He covered the south-facing facade with 8 layers of polycarbonate plastic, in total about 3 inches thick. During winter, the sun is nearly perpendicular to the facade, beaming its energy directly into all its layers. The polycarbonate can reach 130°F even as outside temperatures peak at 50°F.  Photo 3 of 6 in Guess Which Humble Material Made This 100-Year-Old Building Super Energy-Efficient