When Adrian Bejan, a mechanical engineering professor from Duke University, visited the town of Benevento in southern Italy, something caught his eye—the roofs of the older buildings all looked very similar.
They had shallow peaks and wide bases. At first, he thought it might just be an old architectural style.
But as a thermodynamics expert who studies how heat moves, Bejan began to wonder if there was a deeper reason behind these roof shapes.
So, he sat down with pencil and paper and started doing calculations. He looked at how heat flows through two roof shapes: one with a triangular cross-section like a typical A-frame roof, and one shaped like a circular cone.
He worked with Pezhman Mardanpour, a fellow engineering professor from Florida International University.
Together, they discovered something fascinating: the shape of a roof can make a big difference in how much heat a building keeps or loses.
Their research, published in the journal International Communications in Heat and Mass Transfer, showed that air trapped inside a roof acts as an insulator, like the air in an attic.
But how that air behaves depends on the roof’s shape. If the peak of the roof is under three feet tall, the air moves smoothly, helping to keep the building warm. But if the peak is taller, the air becomes more chaotic and doesn’t insulate as well.
To save energy, they found two ideal shapes. For shorter roofs (with peaks under three feet), the best design is to make the roof three or four times wider than it is tall. For taller roofs (over three feet), the best design is an equilateral triangle, where the height and width are equal. These shapes help trap warm air better and reduce heat loss.
Interestingly, Bejan noticed that many old homes around the world, not just in Italy, already follow these proportions—even though the builders likely didn’t use physics or equations.
Back then, keeping warm in winter wasn’t about saving energy—it was about staying alive. If one family had a warmer house, neighbors would copy the design. Over time, the most effective roof shapes spread naturally.
Bejan believes modern architects could learn from this. Today, we focus on insulation materials and fancy technology, but we often forget about the basic shape of buildings. According to physics, the shape itself plays a big role in energy efficiency—and it’s a lesson worth remembering.
