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

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By Zachary Edelson / Published by Dwell
Tasked with renovating a dilapidated structure, an architect saw the opportunity for an unconventional eco-friendly design to shine through.

Perched high in the Spanish Pyrenees, this old stone building needed major changes to become habitable again. The strucutre was not incredibly robust: for instance, the south-facing windows couldn't be expanded to admit more sunlight for risk of weakening the wall. However, that didn't stop architect Josep Bunyesc from seeing an opportunity to turn this home into a model of energy efficiency. 

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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.

Originally built in a 1900 as a hostel of sorts, the building had since fallen into disrepair. Bunyesc was asked by a family of four—the couple have a 1 and 2 year-old—to transform the aged building into a modern home. The architect wanted something energy-efficient, and had several renovations under his belt, but he turned to an unusual material to keep the sun warm: polycarbonate, a rugged plastic common to industrial architecture, sheds, and other lightweight construction. 

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Bunyesc was asked by a family of four—a couple with two young children, aged 1 and 2—to transform this aged structure into a modern home.

The architect wanted something energy-efficient, and while he had several renovations under his belt, he turned to an unusual material to harness the sun's energy: polycarbonate, a rugged plastic common to industrial architecture, sheds, and other lightweight construction.

The south-facing facade is covered in 8 layers of polycarbonate plastic, in total about 3 inches thick. During winter, the polycarbonate absorbs sunlight during the day, heating up to about 130°F. That heat is then transferred to the stone walls underneath and slowly radiated into the inteior, 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).

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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.

During summer, the sun is too high above the home to heat the polycarbonate: it simply reflects the light—and its energy—back into the atmosphere. In fact, during the summer the stone wall can absorb the interior heat and release it into a small air gap—about 2-4 inches wide—between the polycarbonate and the wall. 

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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).

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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.

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The rest of the building is insulated with recycled cotton—4.75 inches in the walls, almost 8 inches in the roof—to keep the interior temperature stabilized.