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Kitchen, Colorful Cabinet, and Pendant Lighting 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