That heat is then transferred to the old stone walls underneath. From there, the stone slowly radiates the warmth into the interior, 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). Photo 4 of 6 in Guess Which Humble Material Made This 100-Year-Old Building Super Energy-Efficient

Guess Which Humble Material Made This 100-Year-Old Building Super Energy-Efficient

4 of 6

That heat is then transferred to the old stone walls underneath. From there, the stone slowly radiates the warmth into the interior, 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).