During summer, the sun is too high above the home to heat the polycarbonate: the plastic simply reflects the light—and its energy—back into the atmosphere. In fact, during the summer the stone wall absors the interior's excess heat and releases it into an air gap—about 2-4 inches wide—between it and the polycarbonate.

The home's furniture is economical: the table seen here is just a wood board stabilized on two A-frames.  Photo 5 of 6 in Guess Which Humble Material Made This 100-Year-Old Building Super Energy-Efficient