Comparative Thermal Performance of Old Sana'a Tower-House Archetypes: A Moisture-Coupled Reduced-Order Simulation Study
This study employs a moisture-coupled reduced-order simulation framework to demonstrate that the three-story compact tower archetype in Old Sana'a offers superior thermal resilience and adaptive comfort compared to taller variants, identifying window-to-wall ratio as the primary overheating driver and advocating for reversible, heritage-sensitive interventions.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the high-altitude desert of Yemen, where the sun beats down with intensity but the nights can turn surprisingly cool, the ancient city of Sana'a stands as a testament to human ingenuity. For centuries, its inhabitants have lived in towering structures built from sun-dried mud brick, a material that breathes and changes with the air around it. These buildings are not just shelters; they are complex machines designed to keep people comfortable without electricity. The science of how these structures work relies on two main ideas: thermal mass and moisture. Thermal mass is the ability of a heavy material, like thick mud walls, to soak up heat during the day and release it slowly when the air cools, acting like a thermal battery. Moisture adds another layer to this process; because these walls are made of earth, they absorb water vapor from the air, which changes how much heat they can store. Understanding how these physical properties interact is crucial for preserving these historic homes, as modern changes to their windows or ventilation could accidentally ruin the delicate balance that has kept them cool for generations.
Researchers set out to understand exactly how different shapes of these traditional tower houses perform under today's climate conditions. They focused on three distinct types of buildings found in Old Sana'a: a short, compact three-story tower; a standard five-story tower made of mixed materials; and a tall, slender seven-story tower. Instead of building physical models or waiting for years of weather data, the team created a sophisticated computer simulation. This simulation was unique because it did not treat the mud walls as static blocks of stone. Instead, it modeled them as living materials that change their heat-storing ability based on how humid the air is inside the room. The researchers ran thousands of scenarios, tweaking variables like the size of the windows, the thickness of the walls, and how much air could flow through the building, to see which design held up best against the heat.
The results revealed a clear winner in the race for thermal comfort. The compact three-story tower proved to be the most resilient, staying cool the longest and offering the most comfortable living conditions for its occupants. In a typical year, this smaller tower experienced only about 70 hours where the indoor temperature rose above a comfortable threshold of 30 degrees Celsius. In contrast, the standard five-story tower struggled with 111 hours of overheating, and the tall, slender seven-story tower fared the worst, enduring 128 hours of excessive heat. The study found that the taller buildings, despite their height, actually trapped more heat because their large surface area and higher number of windows allowed too much solar energy to enter, overwhelming the cooling effect of the night air. The compact shape of the three-story tower minimized its exposure to the sun, allowing its thick walls to do their job effectively.
A key discovery was that the moisture in the mud walls played a significant role in keeping the buildings cool. When the simulation accounted for the fact that the walls absorb and release moisture, the overheating hours dropped noticeably across all three building types. This effect was most pronounced in the tallest tower, where the moisture interaction reduced the time spent in uncomfortable heat by nearly 12 hours. This suggests that the traditional material itself is a vital part of the cooling system, and that ignoring its ability to interact with humidity leads to an incomplete picture of how these buildings work. The researchers also identified that the size of the windows was the single biggest factor influencing how hot a building would get. Larger windows meant more heat entering the home, regardless of the building's height or wall thickness.
The study offers a clear path forward for preserving these historic structures without damaging their natural cooling abilities. The author argues that before making any permanent changes to these buildings, such as adding insulation or altering the walls, conservationists should focus on reversible measures. This includes restoring traditional wooden shutters to control sunlight, fixing windows to ensure they open and close properly for ventilation, and encouraging the use of night air to flush out heat. The research emphasizes that the most effective way to keep these towers cool is to work with their original design rather than against it. By understanding that the compact three-story form is naturally superior in this climate, and that the moisture in the walls is a helpful ally, we can protect these architectural treasures while ensuring they remain comfortable places to live in a changing world.
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