A Termite Mound Taught a Building in Zimbabwe to Cool Itself

Out on the grasslands of southern Africa, some termite mounds rise taller than a person: reddish towers of baked earth with ridges and small openings running down their sides. Inside, a huge colony of tiny insects tends its fungus gardens in the dark, and not one of them has ever switched on a fan. For a long time people have wondered how such a crowded home manages to stay fresh under the African sun.

One of the people who wondered was the Zimbabwean architect Mick Pearce. In the early 1990s he was asked to design a large building in the centre of Harare, with shops on the lower floors and offices above, and the owners did not want to pay for a full air-conditioning system. Pearce turned to the termites. The story he had heard was that their mounds keep a steady temperature inside while the air outside swings from cold nights to baking afternoons, by drawing air in near the bottom and letting warm air escape through the top.

The Eastgate Centre opened in 1996, built with the engineers Ove Arup and Partners. Its heavy concrete and brick walls soak up heat during the day, much as a mound's earth does. At night, fans draw the cool outside air into the building and through ducts in its structure, chilling the thick walls and floors so that they are ready to absorb the next day's heat. Warm air rises and leaves through rows of chimneys on the roof, and in the morning the offices start the day already cool.

It works. According to AskNature, the Biomimicry Institute's library of nature-inspired design, leaving out a conventional air-conditioning plant saved about ten percent of the building's 35-million-dollar cost at the start, and tenants pay lower rents than in comparable buildings nearby. Estimates of its energy use vary, but it is often quoted as needing only around a tenth of the energy of a conventionally cooled building of its size.

There is a lovely twist. When scientists went back and measured real mounds, the termites turned out to be doing something slightly different from the story that inspired Pearce. In 2015 a Harvard team led by L. Mahadevan, studying mounds in India, found that a mound does not hold a steady temperature at all. Instead it uses the daily swing. During the day, sunshine warms the thin outer ridges, so the air in them rises while the air in the cooler central chimney sinks. At night the pattern flips. The mound, as Mahadevan put it, breathes once a day, and that slow breath carries away the carbon dioxide that builds up inside, which can reach four or five percent at the centre.

So the building and the mound are cousins rather than twins. Both use heavy walls and the difference between day and night to move air without a single compressor, even if the termites were never really running a thermostat. The architect learned from a slightly wrong story, and it still gave Harare a building that keeps its cool. The termites, meanwhile, simply carry on breathing, once a day, as they have for a very long time.