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By Nadav Malin

Designs for natural ventilation typically require open areas in which large volumes of air flow naturally through a building. This openness may make it more difficult to pressurize corridors and prevent smoke migration in case of fire. "Fire code issues come into play particularly in high-rise buildings," McGregor adds. "Often more fan power is needed to control the pressure with the windows open."

Buildings that rely solely on natural ventilation may also expose occupants to unwanted noise and dirt from outdoors. "If you're designing a building that relies on open windows, then chances are you'll have a noise issue," McGregor says. Buildings that depend on open windows must either be located in quiet settings or have clients willing to tolerate some degree of noise intrusion-asking someone on the telephone to hold while a fire truck goes by, for example.

Acoustics can also be an issue indoors. The additional openings provided for airflow increase noise transmission. This problem, common to many open-plan spaces, can be remedied with noise-absorbing finishes on interior surfaces. Rarely are these acoustical issues insurmountable, but they do require careful attention from the early stages of design. Also, many occupants site a lack of noise from mechanical systems as an advantage of natural ventilation.

Public buildings often contain activities less interrupted by noise and temperature variations than typical office buildings, and they are therefore likely candidates for natural ventilation. A good example is the Martin Luther King, Jr. Civic Center in Berkeley, Calif. A city landmark designed by James Plachek and built in 1940, the building was recently renovated by Elbasani & Logan Architects. Their work retains the building's reliance on natural ventilation, while enhancing comfort with improved airflow and reduced solar gains.

Finally, interior air quality is normally improved with natural ventilation. Concerns about sick-building syndrome and off-gassing of furnishings and other interior elements are greatly diminished. The exception would be a structure in a heavily traveled area, where dust and fumes would enter the space. Another concern is pollen infiltration in rural settings. Without the filters provided with conditioned air, allergy sufferers are likely to be more uncomfortable.

Operable windows

Randy Croxton, AIA, of Croxton Collaborative Architects in New York City, tries to provide operable windows in all the buildings he designs. "You cannot underestimate the value of operable windows for the sense of individual control in terms of productivity and comfort," says Croxton. He also insists on providing high volumes of outside air-100 percent when climate conditions permit-via the mechanical systems, so the occupants rarely feel the need to open the windows. The outside air passes through high-performance filters and is delivered, via ducts and diffusers, at a speed high enough for occupants to feel the air moving.

"If you have these layers of quality built into the air, people aren't inclined to open windows anyway," Croxton reports. He has used this combination of high-quality mechanical ventilation and operable windows successfully on several projects, including Chattanooga's new Development Resource Center, designed in partnership with Chattanooga-based Artec, and currently under construction.

Mixed-mode buildings sometimes include locks that prevent occupants from opening windows when the air conditioning is on, or stops that prevent the windows from opening fully. The windows in Croxton's 1995 American Association for the Advancement of Science building in Washington, D.C., for example, are restricted in how far they can open. In other instances, the occupants are trusted not to undermine the mechanical system. This is the case at the new computer sciences facility at York University in Toronto, designed by Van Nostrand and Di Castri Architects of Toronto in joint venture with Busby + Associates Architects of Vancouver. Occupants will be encouraged to use the windows during swing seasons, when natural cooling will be relied upon. "There will be a 'how the building works' card for every office," says Mike McColl of Busby + Associates. The card will tell occupants when to keep their windows open.

The building also has an atrium with openings at the base and at the top. These will be automatically opened or shut to optimize or limit airflow through the space, depending upon ambient temperatures. In midsummer and winter, when conditions are not conducive to natural ventilation, the mechanical system will draw naturally cooled or warmed air from underground circulation tunnels, which connect the buildings on campus, and pump it into the building.