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The Return of Natural Ventilation
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As architects rediscover the benefits of fresh air as an alternative to hermetically sealed, air-conditioned buildings, they discover new architectural forms.
By Todd Willmert

Natural ventilation is not a new idea—for thousands of years wind scoops and towers have been an integral part of vernacular Middle Eastern architecture. These structures moved air either up or downward, depending on the prevailing winds, and helped make homes and buildings habitable in the hot, harsh climate.

During the Victorian era, the English became obsessed with clean air. London and other cities were plagued with smoke- and dust-saturated air, and buildings such as Pentonville Prison and Parliament were designed with chimneys and towers that were used not only to expel smoke and to serve as observation points, but also to be part of the ventilation systems.

After World War II, the advent of central air conditioning and its progeny, the sealed building, made natural ventilation an anachronism. Today it is making a comeback, however, owing to rising energy costs and the worldwide movement toward buildings that employ “green” strategies. Architects and engineers, mostly in England, are using advanced computer and modeling techniques to refine the physics of heating, cooling, and ventilating. Chimneys and towers are key architectural elements for harnessing pressure differentials by employing the stack effect and other air-movement principles.

The following case studies provide lessons for American architects, because the design strategies, which were motivated by client mandates to reduce energy costs, go beyond the implementation of their efficient environmental control systems. Such projects work because naturally ventilated buildings have a certain appeal that sealed buildings do not. In the U.K., there is a long tradition of designing well-ventilated buildings to promote health and hygiene; typically in such structures large quantities of diffuse air are delivered at low velocity at floor level. This contrasts dramatically with the common U.S. practice of delivering forced air at high velocity near the ceiling, a more energy-intensive strategy. Furthermore, the temperate U.K. climate—not too hot, cold, or humid—makes natural ventilation a relevant concept. Ventilation also helps to remove moisture; by code, buildings are ventilated at a background rate (24 hours a day) to alleviate dampness.

Buildings that breathe

Click photos to see larger images
Photo: ©Graham Gaunt/Arup
At the Inland Revenue Offices in Nottingham, U.K., fresh air, assisted by fans, enters through full-height, operable windows and is exhausted through the top of the stair towers (below).

Photo: ©Graham Gaunt/Arup

Photo: Courtesy of Arup
Eastgate, a large office block in Harare, Zimbabwe, relies on long, narrow floor plates for maximum daylighting and ventilation.

Photo: © Margaret Waller
A large atrium, covered by a glass canopy provides fresh air to the ventilation system, as shown in the energy section (below).

Photo: Courtesy of Arup

These circumstances have fostered a new approach to mechanical servicing in the design of large offices and other building types. In particular, two projects in England—the Inland Revenue Center in Nottingham, by London-based Michael Hopkins and Partners with engineers Arup, and the Queen’s Building at De Montfort University in Leicester, by Short Ford Associates with Max Fordham engineers— are excellent examples of a new trend in which architectural form purposefully exposes mechanical function.

Vertical chimneys and towers are the noteworthy elements of these buildings, but they are only the culmination of a complete planning effort that includes three-dimensional section development. In fact, a low-energy, passively ventilated building must fully address total airflow patterns, from intake to exhaust, with the chimney or stack effect the primary, but not sole, principle employed. The other key consideration for enhanced ventilation is displacement ventilation (harnessing air’s natural buoyancy to facilitate its movement). The principle is simple. Fresh air is introduced at the bottom of a space. As it is warmed, primarily by people and equipment, it rises and collects against the ceiling, where it can flow to the exhaust chimneys or towers. Key factors in calculating stack ventilation include both total and net stack height—the distance from the top-floor ceiling to the top of the stack.

At the Inland Revenue Center, a 400,000-square-foot government office complex, wings are 45 feet wide and 240 feet long, to maximize exterior exposure. The long, narrow floor plates of the building facilitate cross ventilation when windows are open. When the windows are closed, intake louvers draw in fresh air and allow stale air to travel through the building to the roof ridge and towers at the end of each wing. On the top floor, spent air is expelled by a skylight ridge, instead of the stair towers, which would have to have been at least 20 feet higher than the ceiling to draw air adequately. Each of Inland’s three floors has parallel airflow. Fans within the raised floor on each level pull fresh air through louvers directly into the cavity. The air travels over heat exchangers, where it is heated if necessary, then moves through a nearby floor grille, where it is introduced in the offices at floor level. Stale, warmer air collects at the ceiling and is drawn along the ceiling until exhausted through the ridge, or stair towers, whose roof raises and lowers to regulate rate.

At the Queen’s Building, window, louver, and chimney forms demarcate the various ventilation strategies—which are principles taught in the classrooms of the building itself. Multiple atrium chimneys exhaust air, supplementing other chimneys in the high-bay lab spaces and auditoriums. The great variety of spaces and their usage at Queen’s calls for a more varied ventilation approach. Here, 100,000 square feet of labs, classrooms, auditoriums, and offices housing the university’s engineering program are either high, narrow spaces exposed on two or more sides, or they open to an atrium. Two small labs for precision work require mechanical ventilation, but aside from these spaces, more passive means are fully explored. Offices utilize simple cross ventilation where possible, with deeper spaces relying on stack ventilation. Underfloor ventilation provides fresh air to auditoriums; as the warm air rises it is pulled out the stacks. Rooms overlooking the atrium have walls punctured with operable panels that can be changed to control ventilation.

These projects illustrate the nuances of chimney caps and tower tops, which are critical to ventilation design. The towers at the Inland Revenue Center absorb solar energy to create and assist draw. By contrast, another project by Michael Hopkins and Partners, this one at Nottingham University, uses tower-top cowlings that rotate in the wind. With openings facing downwind, the wind pressure differentials over the building and across the cowlings create draw. At Queen’s, the chimneys, with four faces, are designed to draw regardless of wind direction and are solar-assisted. Much recent work utilizes this principle, instead of the temperature differences that drive the stack effect to foster air movement. Potential advantages of this strategy include chimney diameters that are smaller than those usually required to create the stack effect.

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