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By Nadav Malin

How it works

Natural ventilation may simply refer to operable windows within a building, or it may imply carefully engineered airflows driven through the building by wind and temperature differences or by fans. Natural ventilation may be intended to replace air conditioning entirely or, as is more often the case in large buildings, to coexist with mechanical systems in what is called "mixed mode."

Buildings that rely on natural ventilation may be more expensive initially, due to the higher costs of operable windows and solar controls. But this is offset by avoiding expenditures on mechanical ventilation equipment and the energy it consumes. In mixed-mode buildings, which require operable windows and other natural-ventilation systems, as well as mechanical ventilation equipment, construction costs tend to be higher.

Natural ventilation is more successful in moderate climates and with certain building types. "A combination of climate and function determines when passive ventilation is doable," says Alisdair McGregor, a specialist in natural ventilation with Ove Arup & Partners in San Francisco. For example, coastal California, Oregon, and Washington are climatically appropriate-if the building is not too big and internal and if solar gains are not too high.

Another controlling factor is humidity. In steamy climates, chillers work as much or more to remove moisture from the air as to reduce the actual air temperature. Introducing humid air-even if it is relatively cool-into a partially air-conditioned building will make the chiller work harder to remove that moisture. Also, dampness is stored in the materials and furnishings of the building. High humidity in paper causes problems for toner in copiers and laser printers, for example. All this means that it takes a long time for a space to cool down and dry up after an influx of moist air.

Increasing airflow makes people more comfortable at higher temperatures, whether the breezes come from windows or fans. This is called comfort ventilation. Higher indoor air speed is particularly effective in a humid environment because sweat evaporation from the skin makes occupants feel cooler.

Building design influences how much air enters a space and at what velocity. Natural-ventilation design requires a return to design principles that were commonplace before mechanical air conditioning took over in the post-World War II era. Kenneth Yeang, an architect with T.R. Hamzah & Yeang in Malaysia, together with Phil Jones, an architecture professor at the University of Cardiff in England, and Richard Aynsley, a wind consultant and professor at James Cook University in Queensland, Australia, researched the subject and developed techniques that boost natural cooling. One of these is building configuration-most naturally ventilated buildings are designed with narrow, unobstructed floor plans and windows or openings positioned so that occupants are within easy reach of an outdoor-air source. Desks are a maximum of 23 to 26 feet from a window. Erik Ring, a researcher at the University of California in Berkeley, says successful, naturally ventilated spaces typically only have a plan depth of 40 to 50 feet, quite a bit narrower than most modern commercial buildings.

Other techniques to boost cooling include building orientation in relation to the path of the sun and the wind; facade design, including the use of balconies, windows, and air intakes; solar protection, including sunshades and other solar-deflection devices; use of passive lighting, such as skylights, which saves energy and lowers internal heat buildup; vegetation and landscaping to provide shade; and the color of the building.