Engineering Rome

Thermal Respite in Ancient Rome: A Study of the Baths of Caracalla

By Lila Munn

Figure 1. The ruins of the baths of Caracalla. Photograph by Lila Munn (2022).

1 Introduction

Each morning, we wake up in our air-conditioned homes. We drive in our air-conditioned cars to work in our air-conditioned offices. An invisible mechanical system raises and lowers the ambient temperature of our spaces, so we may never suffer thermal discomfort for longer than the five or ten minutes it takes to travel between house and car, or classroom and coffee shop. We have engineered a thermally neutral world— perfectly 70 degrees— but in doing so, we have also severed our connection to the natural world. Author and architect Lisa Heschong makes the comparison that, to create the perfect thermal environment, we submit to living in a “monochromatic” world without variation, joy, and interest. We may be guaranteed comfortable, but we have lost all opportunity for pleasure. Our thermal sense, while not as noticeable as sight or perhaps smell, is “intricately bound up with the experience of our bodies,” creating the backdrop for which all other experiences are informed [3]. My own memories of Rome are greatly impacted by thermal sensations. What I recall most vividly is not the details of the frescos decorating the Sistine Chapel, but the cool Mediterranean Sea soothing my sunburnt ankles. I remember the sublime drop in temperature between a wide-open street and shaded alley. I remember the cool nighttime air on my arms, and the warmth on my legs from the sunbaked stone church steps on which I sat. Each of these thermal delights contains a lesson in the simple brilliance of Ancient Roman design, and captured my interest as an engineer. Heating and cooling at will—what we consider to be a modern comfort—in truth is hardly modern at all. The Ancient Romans enjoyed this same comfort, and perhaps the best example of this are the baths of Caracalla.

The ruins of the baths of Caracalla stand, tucked away and quiet, in the bustling heart of Rome. The sounds of the city fall away as you enter the large complex, leaving only the echoes of crunching gravel underfoot and the distant chirping of birds. Brick walls stand proud nearly at their original height, and arches of monumental proportions span overhead, alluding to the grandiosity of its past life. It is easy for one to imagine the riches that were once here; ornate marble cladding, vibrant frescos, lines of statues and gurgling fountains spouting fresh spring water. These structures were considered to be the palaces of the people, and were born from the idea that the prosperity of the empire should be enjoyed by its citizens indiscriminately. Some have even hailed these once gold and glittering structures as the most exemplary instances of true democracy that we have. Of course, all that remains today are remnants, but even in its ruinous state the baths still serve their original purpose: to inspire.

Figure 2. The ruins of the Baths of Caracalla. Photograph by Lila Munn (2022).

Construction of the thermae commenced in 206 AD by emperor Septimius Severus and was opened in 216 AD by his son and successor, Caracalla, shortly before his death in 217 [4]. Emperor Caracalla was not a well-liked man and was known mostly for the murder of his brother, as well as an Alexander the Great-like disposition with none of the military success to support it. It is theorized that his work on the baths was an attempt to redeem himself in the eyes of the public. Caracalla understood that in order to maintain power, his citizens needed to remain comfortable, well-fed and well-entertained, therefore the baths were free to enter. He spared no expense in the ornamentation of his bath house. The walls were clad in the most ornate of materials and decoration. Various types of marble were imported from across the Mediterranean, and iconic works of art were housed in this grand structure. Beyond a show of wealth, the baths were most importantly a place of refuge for the people of Rome. On the hottest days of summer and coldest days of winter, one could find relief from the thermal environment in the baths of Caracalla.

Figure 3. Artist’s rendition of the great hall of the Baths of Caracalla. (Rome: Baths of Caracalla: Reconstruction Drawing. 212-216 A.D. JSTOR, Accessed 25 Sep. 2022.)
Figure 4. Decorative marble tile mosaic floors. Photograph by Lila Munn (2022).

After three hundred years of operation, the baths of Caracalla found their end in 537 AD following the siege of Rome by the Goths. Aqueducts were destroyed, and the baths lost their supply of water, thus falling into disrepair. The building was also the victim of looting, and many of its original adornments became repurposed into other structures around Rome. Several capitals were reused in the church of Santa Maria in the Trastevere neighborhood of Rome [4]. The Piazza Farnese contains two of the granite basins that belonged in the frigidarium. Due to the bath’s location south of the historic city center, once the valuable materials were stripped from the structure, the skeleton of the thermae remained relatively undisturbed. This is how we find it today, occupying its own space of solitude, removed from the chaos of a modern city, and a refuge for birds.

Figure 5. Aerial view of the ruins of the Baths of Caracalla. Photograph by Nancy Le (2022).

2 Architecture

That which is most distinctive of Roman architecture is the consideration of the senses. Vitruvius outlines the fundamental principles of architecture as (1) order, in terms of symmetrical agreement between the components and the whole, (2) eurythmy, or beauty, (3) symmetry, (4) propriety, or the adherence to principles constructed by the natural environment, and (5) economy, communicating in particular the significance of the visual sense [2]. This can be seen clearly in the plan view of the baths shown below in figure 6. These principles translated into physical walls and rooms communicates to us today how people moved through the structure.

Figure 6. Plan view of the Baths of Caracalla. (Thermae (Baths) of Caracalla Plan. ca. 212-235 C.E. JSTOR, Accessed 25 Sep. 2022.)

The baths are symmetric along a singular axis, and unfold, like the wings of a butterfly, to reveal two main flows through the building. The experience begins on the north face of the building, where entrance halls are located on the left and right side. Upon entering the complex, bathers would be directed to the apodyterium, or changing rooms, where they would remove their street clothes in preparation for bathing. They would then progress into the palestra, or gym, for exercise. There are entrances and exits in the palestra so that people might enjoy both the large gardens as well is the inside area. Next, one would enter the sauna to relax after exercising. This completes the first movement, north to south, through the outermost rooms of the central bathing complex. Next, leaving the sauna, the hot baths of the caldarium would be enjoyed, beginning the second movement, now south to north, through the central core of the complex. It was typical to move from hot to cold, as the design of the building facilitated movement along the central axis beginning in the hot baths, then to the tepidarium for the warm baths, and finally to the frigidarium, or cold baths [1]. After leaving the pools, one would return to the apodyterium to collect their street clothing, and leave the main building. From here, one could exit the complex entirely, or those wishing to linger could visit the rooms on the perimeter of the structure. Here, lectures from scholars could be enjoyed in the theaters, or one could sit with a book from one of the many libraries located inside the complex. There were even shops at the main entrance of the building, making the baths of Caracalla a one-stop destination for its users— a city within itself.

3 Engineering the Environment

Beyond serving as a microcosm of all the best in life, the baths of Caracalla were a technological marvel—astounding to us even in today’s modern age. Our intuition tells us that such tall and expansive spaces must be cool and drafty, but in truth they could maintain incredibly warm temperatures regardless of outside weather conditions. This can be predominately attributed to two clever engineering solutions employed by Romans: the hypocaust, and passive solar.

In winter, temperatures in Rome could drop into the low-forties Fahrenheit during the day, and even colder at night. The hypocaust, a subfloor furnace-based heating system, was the primary way these cold temperatures could be combatted. Beneath the floors in the sauna and caldarium, a 2’ deep air cavity circulated hot air from a furnace underneath the floor tiles, which then heated the space from below. To more efficiently distribute the hot air, the floor of cavity was sloped up from the furnace such that the air would more easily flow back into the furnace as it cooled. This hanging floor was supported by short, stout pillars called pilae, which were made of standard 8” square bricks. In the figure below, you can see the layout of the air cavity of the hypocaust in the Stabian baths at Pompeii. Note the layout of the pilae; a standard 2’ center to center spacing was typically used.

Figure 7. Stabian baths at Pompeii. (Stabian Baths. Archeological park of Pompeii, Accessed 01 November 2022.)

Flues, called caliducts, were installed in the walls to draw air from the hypocaust up and out through the height of the structure. This served two important purposes. First, blowing hot air through the caliducts would allow heat to accumulate in the thick stone walls in addition to the floor, contributing to the overall climate of the space and pleasure of the bathers. Second, and perhaps most important, the caliducts facilitated the movement of toxic gasses from underneath the floor up and out of the building. Any leak in carbon monoxide or smoke would make the baths unusable. Shown below are an example of caliducts from the archaeological park of Ostia Antica.

Figure 8. Wall flues at Ostia Antica. Photograph by Lila Munn (2022).

Structural elements of the caldarium were designed to be responsive to the warmth emanating from the hypocaust. Vaults in hot rooms were typically double height, so the steam and humidity would circulate through the building instead of rotting the wood roof members. Furnaces were typically installed on the perimeter of the building. This allowed for attendants to easily access the furnace, as they required constant attention. The fires ran on small twigs and branches, as larger wood logs burned too slow to generate the required amount of heat. This in turn required an incredible amount of manpower. Additionally, workers would need to frequently clean out the ash and soot from the furnace, as it could smother the fire and was a safety hazard [5]. Shown below is a hypocaust furnace in Ostia Antica. A similar furnace would have been used at the baths of Caracalla.

Figure 9. Hypocaust furnace at Ostia Antica. Photograph by Lila Munn (2022).

Compliment to the hypocaust was passive solar heating. Passive solar was a technique widely implemented by the Greeks with their winter rooms, but an art expanded upon and perfected by the Ancient Romans. Passive solar can be described as “the use of natural processes, such as radiation, conduction, and convection, to distribute thermal heat provided by the sun,” as was a critical aspect to the success of the baths [6]. The hot rooms of the were placed along the southwest facing wall and featured large, glazed windows to allow maximum possible sunlight to intrude into the building. As bathing activities typically occurred midday to evening, this southwest orientation was critical. The sunlight would enter the structure through the windows and strike the thick stone floors serving as the thermal mass. The stone would then reradiate the sunlight as heat, which would drift slowly upwards warming the air. To increase efficiency, Vitruvius described the use of a large bronze disk suspended from the ceiling to help regulate temperatures in the room and reflect the radiation back to the pools [2]. The bronze disk could be lowered or raised depending on the desired temperatures.

But how effective is passive solar truly? Well, let’s look at some numbers. Assuming an outside temperature of 30° Fahrenheit, an ideal inside surface temperature of 100° F, with 250 btu/ft2/hr coming into the space from the sun, and 400 btu/ft2/hr from the subfloor hypocaust, it was entirely possible to maintain 100 degrees Fahrenheit inside the caldarium with glazed windows [8]. Even after the sun has gone down for the day it was possible to maintain temperatures due to amount of thermal energy able to be stored in the floors and walls. To understand how the heat accumulates, let’s look at inflows and outflows; Into the caldarium, the hypocaust alone can supply 400,000 btu/hr. Solar radiation into the room will supply 70,000 btu/hr, summing to a total of 470,000 btu/hr influx of heat. From conduction, 40,000 btu/hr will flow out of the space, and an additional 40,000 btu/hr will be lost from the windows [8]. This results in a total outflow of 80,000 btu/hr. Comparing inflows with outflows, we see a total net heat flow of 390,000 btu/hr into the space. Solar radiation alone contributes a net influx of 30,000 btu/hr.

Figure 10. Heat inflows into the baths. Ring, J.W. (1996)
Figure 11. Diagram of heat outflows from the baths. Ring, J.W. (1996)

The art of passive solar also extends to cooling as well as heating. As seen with the narrow Roman alley example earlier, the sun can be blocked deliberately to decrease the temperatures of a space. One way this was done was by simply planting umbrella pine trees in the gardens surrounding the baths. On a hot sunny afternoon, after a long day’s work, one could find solace underneath an umbrella pine, who’s canopy provides a particularly welcome spot of shade—sometimes the simplest of solutions are most effective. This idea, too, extends to the structure itself. The large, cavernous spaces in the bathing complex allowed for the heat to rise above the occupants. Thick concrete and brick walls also served to insulate against the outside heat. In the throes of summer, the baths were a place to hide from the sun.

4 Conclusion

Warmth is what is alive at the very center of things, and it is this warmth that turns a utilitarian structure into a sanctuary—a place to rest, or to gather with friends and family. Therefore, it is no surprise that the thermae became a foundational aspect to ancient Roman life and culture. Upmost care went into creating comfortable places for occupants, which was made possible through the subfloor heating system and clever passive solar design. There are many lessons we can learn from the baths of Caracalla, and indeed over the centuries many have found inspiration in their walls. Architects of New York City’s original Pennsylvania station borrowed heavily from the floor plan of the Baths of Caracalla, and the station boasts many similar architectural features such as coffered groin vaults and clerestory windows [7]. The floor plan also shares many similarities to the layout of the baths, as the station consists of a central block surrounded by a ring of shops, food stalls, and general places to congregate.

Figure 12. New York City’s Historic Pennsylvania Station. (Pennsylvania Station. ca. 1910. Museum of the City of New York (MCNY), JSTOR, Accessed 26 Sep. 2022.)

But beyond aesthetics, the baths of Caracalla also teach us about sustainability in the built environment, particularly in the context of passive solar design. Famous American architect Frank Lloyd Wright integrated passive solar design principles into his Usonian houses, which focused on living in harmony with the natural environment. The Usonian houses are very distinctive in style, featuring flat roofs with large overhangs to maximize solar radiation in winter and minimize the heat in summer. Underfloor heating was used in the houses to compliment solar heating, similar to the hypocaust in teh baths of Caracalla. The most famous example of Wright’s Usonian model is the Jacobs II “Solar hemicycle” house. Pictured below, the house is semi-circular in plan and opens southward to a circular sunken garden.

Figure 13. Herbert Jacobs II house, exterior front view. Wright, Frank Lloyd, 1867-1959. Herbert Jacobs House II. Exterior Front. 1948. JSTOR, Accessed 2 Nov. 2022.

The south wall is fully glazed to admit solar radiation throughout the day. On the North, East and West faces of the house, the land is sloped up to the height of the clerestory windows to protect the house from strong, cold winter winds coming from the North. Additionally, the sunken gardens in combination with the sloped land created a pressure differential which deflects snow and wind away from the south windows to preserve warmth [9]. The interior walls are made of local limestone which serves as a thermal mass. The first story features an open floor plan to help the hot and cool air circulate efficiently, and the second story loft area is suspended from the ceiling and open to the floor below, so hot air will naturally rise and heat the space.

Figure 14. Herbert Jacobs II house. Interior view from first story living room. (Wright, Frank Lloyd, 1867-1959. Herbert Jacobs House II. Living Room. 1948. JSTOR, Accessed 2 Nov. 2022.)

With the Jacobs II house, now in its 74th year of continuous and successful operation, Frank Lloyd Wright shows us how we might live better connected to our environment. Today, we are reliant on HVAC to warm and cool our homes at the expense of immense carbon emissions. Perhaps it is time we shift focus to more natural systems of heating and cooling. And, when designing for the future, like Wright and the architects of the Pennsylvania station in New York, we should look to the past for inspiration.


[1] DeLaine, J. (1992). “Design and Construction in Roman Imperial Architecture: The Baths of Caracalla in Rome”, University of Adelaide Department of Classics. Adelaide, Australia.

[2] Vitruvius. (1914) “Ten Books on Architecture”. Translated by M. H. Morgan. Harvard University Press. Cambridge, Massachusetts, USA.

[3] Heschong, L. (1979). “Themal Delight in Architecture” Massachusetts Institute of Technology Department of Architecture, MIT Press. Massachusettes, USA.

[4] Museum guide: sign text, The Baths of Caracalla Archeological Park, Cooperative Society Cultures. Rome, Italy.

[5] UNVR. “Roman Hypocaust,” United Nations of Roma Victrix. [Online] Availiable at: (Accessed September 2022).

[6] Barber,S. “History of Passive Solar Energy”, East Carolina University. [Online] Available at: (Accessed September 2022).

[7] Jones, R. (2014). “AD Classics: Pennsylvania Station/McKim, Mead & White”, ArchDaily. [online]. Available at: (Accessed September 2022)

[8] Ring, J.W. (1996). “Windows, Baths, and Solar Energy in the Roman Empire,” American Journal of Archaeology, 100(4), pp. 717-724. (Accessed November 2022).

[9] N/A (2011). “Frank Lloyd Wright and the ‘Solar Hemicycle’ (Jacobs II)” Fieldman Architecture. [online]. Available at: (Accessed November 2022).

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