Natural Ventilation and A/C in the Tropics

Natural Ventilation and A/C in the Tropics

In the last 30 years, air conditioning has gone from a rare luxury to a widespread necessity in the South American tropics. As climate change brings more extreme weather to these regions, we are faced with a paradoxical dilemma: fossil fuel consumption, the very same cause of rising CO2 levels in the atmosphere, is also the energy source that powers the systems that keep indoor spaces comfortable and, in extreme cases, safe. Is there an alternate way? Are passive strategies passé, do they no longer align to our comfort standards? Is the standardization of building systems around the world responsible for creating spaces that need to be air-conditioned by their very design? Could looking at “vernacular” construction systems give us some clues on how we could more passively design and build in these areas?

This research study proposes to investigate these questions by studying different types of buildings in the following cities:

– Cartagena, Colombia

– Anapoima, Colombia

– Lima, Peru

– Arequipa, Peru

– Tarapoto, Peru

These cities are located in very different altitudes and climatic conditions, where different vernacular ventilation strategies are customary. I compared the thermal performance and comfort between buildings that use passive ventilation against those that use mechanical ventilation and air conditioning.

For the study, I used a thermometer, a hygrometer, and a camera.

Let’s dive in!

Cartagena—Hot Seaside Tropics, Old and New

Cartagena is a vibrant coastal Colombian city that enjoys warm sea breezes throughout the year in its colorful streets.

Average Temperature (December): 87°F

Average Relative Humidity (December): 81%

Thermal Comfort: Imagine stepping off the plane into a warm embrace (some would say a bit too forceful of an embrace). That is how it feels to be welcomed by Cartagena’s tropical sun. The cobbled streets of the Old Town are deserted from midday until the sun starts to set. The colonial buildings’ thick walls—crafted to withstand pirates and time—keep the heat at bay through their large thermal masses. Newer neighborhoods, like Bocagrande, are made up of high-rises that shimmer in the heat. These new buildings are air-conditioned, heavily reliant on blinds, and often oriented for beach views.

Building 1: Private single story home, concrete and plastered brick construction

Like many private weekend homes built in the last 50 years in Cartagena, this house was built with the Latin American materials of choice: concrete, plastered brick, natural stone, and tile. This home in particular was oriented with large apertures towards the south to catch the sea breeze, a technique many locals follow. The front door is located on the north side, which along with its high ceilings allow the breezes to cool the main spaces of the home. The bedrooms, however, are air conditioned.

The non-air conditioned social area was almost at the same temperature as the air conditioned rooms, (90.68°F vs 86.4°F, respectively) but the relative humidity did vary (68% vs 45%, even when the air had been turned off for some time). Even at these high temperatures, the rooms felt quite comfortable while the social area felt humid but manageable. Relative humidity made a very large difference in the comfort of the spaces, especially when the large windows were closed off.

Building 2: Private home workspace, wood construction

This small workspace was located in the same lot as Building 1. The former owner of the home was a navy officer and he felt most at home in small, wooden spaces so he built a small appendix to the house. The hut is built with wooden walls and a metal roof. The doors and windows were closed when we entered and it was HOT.

Even though the temperature was the same as in the Building 1 social area (90.68°F), the relative humidity was 77% and it felt stifling. The building was not doing much to ward off heat (no thermal mass walls and low roofs) and since the windows were closed, there was no cooling breeze. Humidity was higher indoors than outdoors.

Building 3: Former convent (current hotel), thick adobe and stone construction

The Sofitel Legend Santa Clara was originally built as a convent in 1621. Over the years, it transformed into a charity hospital, a prison, and even a medical school. In 1995, it underwent a remarkable metamorphosis, becoming the Santa Clara hotel. The building retains its Spanish colonial style, preserving features like crypts, wells, confessionals, and hidden windows.

I entered what seemed to be a former chapel with very high wooden ceilings and beautiful tile floors. You can see the thickness of the stone and adobe walls, which help to ward heat off in the midday fry. This room had no air conditioning, but it was less humid than outdoors. I have heard from some sources that many of the rooms that are located in the old parts of the building, which are air-conditioned (this is a luxury hotel after all) suffer from humidity issues. The thick walls probably perform very different with air conditioning inside and some condensation likely occurs when there is such a large temperature differential between the interior and exterior of the thick walls.

Building 4: Private apartment, concrete, glass, and steel construction

This was our Airbnb in Cartagena, a relatively new building made of concrete, steel, and, of course, glass. We were on the ground floor of the building and the heat was pretty unbearable without having the fan or the air conditioning on. All the units have louvers on the side opposite of the large windows to promote breezes, but the building is oriented in the wrong direction to catch sea breezes. The apartment where my parents were staying next door had the right orientation and was on a higher floor. It enjoyed significantly more natural breeze.

Anapoima—Tropical Midlands Affected By Climate Change

Anapoima is a small town two hours down the mountain from Bogota, where weekenders from the city go to seek refuge from the capital’s cold and rainy weather.

Average Temperature (December): 88°F

Average Humidity (December): 76%

Thermal Comfort: Anapoima experiences a tropical climate. It enjoys warm weather throughout the year. The average temperature hovers around 70.3°F. The temperature variation is minimal making it challenging to distinguish between hot or cold seasons.

Building 5: Private single story home, concrete and plastered brick construction

The thermal comfort differentials in this house are felt rather than recorded. The main social area has very high ceilings and is wide open on the south side. The rooms have average-sized windows and the doors allow for some mild cross-ventilation. Although the social area remains fresh and comfortable for lounging, the rooms are simply too hot. One can feel the heat radiating from the plastered brick walls at night.

Lima—Escaping the Fog Line

Peru’s capital has a desert climate with a few fertile valleys. It is a vibrant foodie’s paradise and in the last few years it can get uncomfortably hot in the (southern hemisphere) summer.

Average Temperature (December): 73°F

Average Humidity (December): 80%

Thermal comfort: Lima has pleasant weather in December. The average daytime temperature hovers around 73°F, providing a comfortable warmth. Nights cool down to approximately 64°F, making it pleasant for evening strolls. The last few years has seen these temperatures go higher for longer, rendering night cooling less effective and forcing citizens to buy more split or A/C systems for their spaces.

Building 6: Private three story home, concrete and plastered brick construction.

For the most part, this house remains comfortable for the entire day if one simply opens the windows. Humidity can get high in Lima, particularly close to the coast, but this house is further out into the mainland. The house’s air flow management works very well on the first floor, with large openings towards the east and west. Upstairs, rooms get warmer and warmer, as the warm air rises and spaces are more closed off. Unless a room is bathed in direct sunlight, no A/C is required to keep comfortable—at least in December.

Arequipa—Highland Bliss

Home of the Andean condors, the beautiful mountains where Arequipa is perched gives it a dry but fertile atmosphere. Sillar, a white stone quarried from nearby mountains, gives the city its distinctive white Spanish colonial look.

Average Temperature (December): 64.4°F

Average Humidity (December): 64%

Thermal comfort: Arequipa experiences pleasant weather in December. Throughout the day, temperatures typically settle at around 64.4°F. However, by evening, they drop to ~ 48.2°F, creating a comfortable transition from day to night.

Building 7: Private three story home, concrete and plastered brick construction

The weather in the Andean highlands is surprisingly similar to San Francisco. Bright days are followed by cool nights. No A/C is needed here with good design. This house was built around 30 years ago. It has relatively small windows and thick masonry walls. Overall, the house remained comfortable, even a little cold sometimes. Going out into the direct sunlight felt like a treat after being inside for too long. The house stays fresh throughout the day.

Tarapoto—Amazonian Search for Ventilation

In the area denominated “ceja de selva” (rainforest fringe), Tarapoto is accessible by plane and is characteristic for being considered part of the Amazonian mountainous region. Multiple rivers also give access to this privileged spot.

Average Temperature (December): 82°F

Average Humidity (December): 72%

Thermal comfort: Tarapoto experiences a tropical climate that remains very hot throughout the year. December is the hottest month—the average temperature during the day reaches around 82°F, with a minimum of 72.2°F and a maximum of 92.3°F. Nights remain warm, making it comfortable for outdoor activities. Despite being the dry season, Tarapoto still experiences some rainfall in December.

Building 8: Small lodging cottage, wood construction

Boy, was I apprehensive about this one. After seeing how hot the wooden hut in Cartagena had been, I was not looking forward to sleepless humid nights in this wooden cabin. I knew that, in theory, its strategic location by the river would bring some comfortable breezes, but I did not think it would be enough. I was wrong.

The high glass-less open areas around this hut do a lot of work. Whatever breeze the river brings (which is surprisingly strong) makes its way beautifully through the large doors of the cabin and out through the back, especially when helped by the small ceiling fan. Even though the height differential is not super pronounced, this little piece of marvel remained fresh and comfortable throughout our stay.

Discussion

This study helped to solidify my understanding of thermal comfort as a factor of not just temperature but also humidity and air speed. We learn about these concepts in a theoretical way, but it was another experience altogether to go out, report it, and feel it. To me, some of the greatest discoveries were when temperatures barely changed yet spaces felt completely different due to their relative humidity.

Something that was surprising to me was how a wood building could feel so different in one context versus another. I think we understand the thermal workings of concrete buildings intuitively, but seeing how such a simple structure in Tarapoto could work so well was very inspiring.

One of the biggest takeaways for building comfortably in the tropics is the importance of orienting buildings towards prevailing winds, providing a large enough cross-ventilation outlet for these winds, the importance of having high ceilings to allow some room for the stack effect to happen, and pairing those high ceilings with good ceiling-mounted fans. Buildings that checked all of these off were comfortable even without the need for air-conditioning, regardless of their materials or building systems.