In the heart of Benevento, a quaint town nestled just beyond the sprawl of Naples, Italy, an intriguing architectural mystery caught the eye of Adrian Bejan, a distinguished professor of mechanical engineering at Duke University. While visiting the area, Bejan observed a peculiar uniformity in the design of the region’s roofs—each one exhibiting a similar shape and proportion that seemed at odds with modern aesthetic diversity. This was no mere stylistic coincidence, Bejan suspected, but rather a subtle testament to ancient engineering wisdom embedded in the fabric of these dwellings, shaped by centuries of empirical knowledge and survival instincts.
Adrian Bejan, with his extensive expertise in thermodynamics and heat transfer, embarked on a journey to decode the physics behind these familiar rooflines. Applying equations that describe the complex movement of heat and air, Bejan and his collaborator Pezhman Mardanpour from Florida International University analyzed two primary roof forms: the classic triangular cross-section typical of an A-frame and the smoother curve of a circular cone. Their investigations revealed compelling evidence that the proportions of these roofs were not arbitrary but finely tuned to maximize thermal efficiency by manipulating the airflow and insulating properties of the attic spaces beneath.
The underlying principle is both elegant and scientifically profound. The pocket of air trapped beneath a roof acts as an insulator, slowing down heat loss from the building below. Bejan explains that the shape and size of this air pocket directly influence the manner in which heat is retained or emitted. When the roof’s peak is shorter than approximately three feet, heat transfer occurs in a smooth, laminar flow akin to water streaming gently down a basin. However, once the peak exceeds this critical threshold, turbulent air patterns emerge—much like chaotic smoke billowing wildly in the wind—that drastically increase heat loss.
This transition between laminar and turbulent airflow has precise geometric repercussions. For roofs with peaks under three feet tall, Bejan’s calculations suggest the optimal design ratio is to make the roof about three to four times wider than its height, thus creating a shallow but broad surface that promotes steady and efficient heat retention. Conversely, taller roofs with peaks higher than three feet should adopt the geometry of an equilateral triangle, where height and width are equal, facilitating controlled airflow patterns that help maintain thermal stability.
Remarkably, such ratios are not found only in Bejan’s scholarly calculations but resonate deeply with the vernacular architecture seen across Italy and many other parts of the world. The older generation of builders—without advanced thermodynamic knowledge—intuitively reached these optimal configurations, perfecting roof shapes over generations through trial, observation, and practical necessity. This intuitive engineering marvel highlights the deep link between physics and traditional construction, emphasizing how ancient practices embodied principles now formally understood through science.
Bejan’s research, recently published in the prestigious journal International Communications in Heat and Mass Transfer, underscores the power of cross-disciplinary thinking, blending mechanical principles with cultural and historical observations. Their study not only affirms long-standing architectural conventions but also challenges modern designers to reconsider the role of shape in energy efficiency. While current building standards focus heavily on insulation materials and HVAC technologies, the nuanced impact of physical form in controlling thermal dynamics has largely been overlooked.
In vibrant detail, the study quantifies how the geometry of a roof influences internal air patterns and thus heat retention, revealing how small differences in shape can significantly alter a home’s energy profile without added cost. This insight has profound implications for sustainable architecture, especially in an era where energy conservation is critical to mitigating climate change. According to Bejan, the architectural profession could benefit enormously by integrating these fundamental principles into design protocols, potentially revolutionizing how buildings are shaped and oriented.
Studying the fluid dynamics of air within attic spaces, Bejan’s team discovered that the internal air pockets behave like a fluid system governed by classical mechanics principles. When roof peaks are low and broad, airflow is steady and predictable, reducing convective heat loss. Conversely, roofs with narrower, taller peaks induce air vortices and chaotic motion, accelerating the loss of heat to the environment. These patterns echo fluid behaviors observed in other natural and engineered systems, reinforcing the universality of thermodynamic laws.
Although the historical builders lacked formal training in heat transfer or fluid mechanics, their construction choices were likely guided by observable effects: homes with certain roof shapes retained warmth better in winter and remained cooler in summer. These practical lessons, passed across generations, inadvertently optimized building envelopes through an empirical understanding of environmental physics. This phenomenon exemplifies how culture and science can converge in unexpected ways to solve fundamental human challenges.
Modern architecture’s neglect of shape as a variable in energy efficiency is an oversight Bejan highlights with urgency. While extensive efforts focus on advanced insulation, innovative materials, and cutting-edge HVAC systems, the geometry of structures remains a largely untapped resource for energy conservation. By revisiting and rigorously quantifying the relationship between form and function, architects and engineers may uncover simple yet powerful strategies to reduce energy consumption simply by adapting shape.
This research also opens the door to broader applications beyond residential buildings. Vehicles, animals, and other systems where heat transfer is critical might similarly benefit from a physics-informed approach to shape optimization. The convergence of thermodynamics, fluid dynamics, and design, as championed by Bejan, presents a promising framework for enhancing efficiency across multiple domains, marrying aesthetics with scientific rigor.
Funded in part by the U.S. Air Force Office of Scientific Research, the study stands as a testament to interdisciplinary innovation and the enduring relevance of physics in everyday life. By decoding the hidden language of roofs, Bejan and Mardanpour invite designers, historians, and engineers alike to rethink assumptions about form, value tradition, and harness the complexity of nature to build a more sustainable future.
Their paper, "Why people shape roofs the same way," published online in March 2025 in International Communications in Heat and Mass Transfer, challenges us to look beyond materials and technology and recognize shape as a fundamental factor in thermal performance. It’s a powerful reminder that sometimes, the wisdom of the ages, encoded in simple shapes and proportions, holds the key to solving contemporary challenges.
Subject of Research: Not applicable
Article Title: Why people shape roofs the same way
News Publication Date: 28-Mar-2025
Web References: 10.1016/j.icheatmasstransfer.2025.108909
References:
“Why people shape roofs the same way.” A. Bejan, P. Mardanpour. International Communications in Heat and Mass Transfer, Volume 164, Part B, May 2025, 108909. DOI: 10.1016/j.icheatmasstransfer.2025.108909
Image Credits: Adrian Bejan, Duke University
Keywords
Building heating, Heat, Thermal energy, Heat transport, Energy transfer, Ancient architecture, Mechanical engineering
