The Eastgate building in Harare, Zimbabwe, is world-famous for its biomimetic passive cooling system, inspired by termite mounds. The fan-assisted network of thermal labyrinths and chimneys cools the space economically and “uses about 10% of the energy” consumed by a mechanically conditioned building next door, architect Mick Pearce told EBN.
Necessity was the mother of Pearce’s invention. The expense of importing the equipment needed for a mechanical HVAC system drove the strategy. Ten years later and seven thousand miles away in Melbourne, Australia, Pearce employed natural ventilation again for Council House 2, with profoundly different results. That’s because the natural ventilation system pulls in air “for breathing, not for cooling.” Instead, radiant cooling makes the ceiling “like the roof of a cave.”
In Melbourne, the economic driver wasn’t the cost of equipment; it was worker productivity. “The building actually cost about 20% more than the cheapest office block at that time,” he said. The team expected a ten-year payback, but revised that down to seven years after a couple years of data had come in. Energy savings were actually weaker than anticipated—about a 60% reduction compared with the building it replaced, not the 85% modeled—but Pearce attributes savings to a decrease in sick days due to the amount of fresh air. “Air-conditioned offices recirculate the air at least six times; otherwise you waste so much energy. In my system, there is no recirculation at all; it’s all fresh air.”
As the contrast between these two buildings demonstrates, the reasons for natural ventilation differ by project, and the system may cost more or less upfront than a conventional mechanical system. It may save more or less energy than the project team anticipated. Most importantly, a successful natural ventilation system must be attuned to the local microclimate and the occupants’ microculture in a way that most other design strategies simply don’t require.
All this makes some project teams balk at the idea from day one, and most never even consider it—yet even in the U.S., where climates and cultural expectations typically make mechanical HVAC a given, there are project teams pursuing natural ventilation for its energy and air-quality benefits.