Physics reveals optimal roof reports for energy efficiency

While being a guest teacher in Bénévent, outside Naples, in Italy, Adrian Bejan noticed something on local architecture: all the roofs were alike. With what seemed to be too much peaks on smaller and older structures grouped together, it was perhaps just the style of the time.

Or maybe the former Roman manufacturers had been on something. An expert in thermodynamics and the movement and heat flow, Bejan, the distinguished professor of Ja Jones of mechanical engineering in Duke, was the perfect person to detect an answer.

Sitting with a pencil and paper, Bejan has crossed the equations and calculations that govern the heat flow and the transfer in two similar forms: a long roof with a triangular transverse section and a circular cone.

The results, obtained in collaboration with Pezhman Mardanpour, assistant professor of mechanical engineering and materials at the Florida International University, are published in International communications in heat and mass transfer. They have shown that there are indeed roof forms that maximize heat retention – the older generation of Italian architects knew what they were doing.

“The air pockets are good insulators, and the granaries are essentially only air pockets of different shape,” said Bejan. “While energy conservation is a popular word today, for years, it was a matter of survival.”

The details of how squat or a roof line determine how the air inside will act. Given a single peak on an A A A or a circular cone, if this peak is less than three feet, the air will pass gently and uniformly through, like water cracking on the side of a sink. But if the peak measures more than three feet high, the air will tumble chaotically like suckling smoke in the wind.

Based on the physics of these air flows and heat transfer, if a roof peak is shorter than about three feet, it should be about three or four times wider than it is large to minimize heat loss. And if a roof peak is three feet higher, it should be an equilateral triangle with a height / width ratio of one.

Unsurprisingly, it is almost the same ratios that can be found in countless old and modest housing created around the world. And they are quite close to the roof lines that Bejan saw that day in the south of Italy.

“This type of insight is not difficult to rationalize, but it is easy to ignore even if there are examples everywhere,” said Bejan. “It is important for our students – and their teachers – to open their imagination and ask why things are the way they are.”

While Bejan doubts that the architects of past days applied thermodynamics to their roof conceptions, he does not think that their forms were accidental. It is not difficult to imagine, he said, discovering that the house of a neighbor is warmer than that of another and copies his design several times for many years.

It is a lesson, he says, that modern architects could also consider.

“Today’s houses and buildings are designed to be as energy efficient as possible,” said Bejan. “But to my knowledge, no one considers the physical form of the building, or any” thing “as a vehicle or an animal, as a variable that could help with this efficiency, and perhaps we should be.