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Economical and environmental benefits

Click photos to see larger images Photo: © Richard Davies At Portcullis House (above and below) in London, the windows are not operable, so air is drawn in at the chimney bases and rises through facade air shafts. Photo: © Richard Davies Photo: © Ian Lawson Nottingham University's tower-top cowlings rotate in the wind. Pressure differentials across the building and cowlings create draw.

As demonstrated in the Inland Revenue Center and Queen’s Building, natural ventilation enjoys considerable advantages: air-conditioning equipment can be downsized initially. This reduces electrical consumption, peak demand, and carbon dioxide emissions at the electrical generating plant. The results, confirmed over the last few years, are more sustainable buildings with operating budgets lower than the norm. Inland Revenue consumes about a quarter of the energy a conventional building would utilize on the same site, with a conventional air-conditioning system accounting for about half that energy. Monitoring at Queen’s reveals similarly impressive results.

Part of the economic and environmental success of these buildings stems from the fact that natural ventilation strategies tend to work well with other sustainable practices. For instance, the high spaces and narrow floor plates necessary for ventilation also work well with daylighting. Naturally ventilated buildings such as these also depend on thermal mass—concrete and masonry—to provide a stable mean radiant temperature. Not only does mass temper incoming air, but ventilating it after hours, or “night flushing,” dissipates the heat built up during the day. Mass provides a thermal damper, so the building requires less overall energy to heat and cool.

Perhaps the real strength of natural ventilation is that architects have found it can be a new source of inspiration. The spring point for Alan Short’s recent renovation of Manchester’s Contact Theater was scrapping the air conditioning, which was always too noisy during performances, and replacing it with ventilating chimneys. Ventilation there consists primarily of five extract stacks built on the roof. Square terra-cotta inlet flues at ground level, revealed rather than concealed, are made from standard chimney liners, a building component rarely visible at all, but celebrated in this design as a direct expression of an inventive servicing approach.

Computer and physical modeling

Empirical insights are the starting points for design, but technology is pushing further. Arup and Max Fordham have both developed proprietary computer programs to help determine tower and chimney parameters. Multiple factors impact airflow: The amount of heat absorbed by the tower or chimney dictates airflow rates; the size of intake grilles into each space limits the amount of air that can pass through them; room geometry and openings to the stack itself affect air currents. A computer model of the proposed design, with weather data integrated into the program, can simulate the myriad factors determining airflow.

Physical models are also used to cross-check the computer simulations, which are not perfect and are not powerful enough to model the airflow through the complicated shapes of some rooms. Wind-tunnel testing of scale models has proved to be an effective design tool to analyze air movement through a building. Another method employs saline solutions. These sink in water in exactly the same way that hot air rises in colder air. In this method, a clear plastic model of a building is immersed in a water bath. When the saline solution is added, its flow reveals how increasing stack size or the number of air inlets can boost airflow. If a room’s shape or partitions hinder airflow, this will be indicated by the physical model.

The development of expertise and design tools contributes to an expanding range of naturally ventilated projects. In Short’s recent completed Coventry University Library, the ventilating chimney vocabulary is applied to a new building type; at Hopkins’ Saga Headquarters it is applied to a corporate facility. Other practitioners are also exploring the ventilation concepts: In a dorm project in Durham by Arup architects and engineers, the buildings cluster around an iconic ventilation tower. A row of stainless-steel chimneys in Feilden Clegg’s Building Research Establishment in Hertfordshire punctuates and reinforces the building’s bay structure. Battle McCarthy Consulting Engineers has worked with architects to explore ventilating towers and chimneys for shopping malls and other projects.

These projects encompass a range of climates where passive, low-energy ventilation is most applicable, but it is important to note that sites such as these should have access to fresh air. Even this limitation is being challenged, however, at Hopkins’ Portcullis House, which contains offices for members of Parliament and is located right across from Big Ben. London’s air and security concerns dictated inoperable windows, suggesting a conventionally air-conditioned building. Instead, 14 bronze chimneys and connecting ductwork send spent, stale air out the chimney caps. Fresh air is brought in at their bases, where it is cooled—with cold ground water drawn from 450 feet below the building—before it is delivered to office spaces.

Natural ventilation goes global

While circumstances favor development of naturally ventilated buildings in the U.K., the principles are applicable to other cultures and climates. Eastgate by Pearce Architects with Arup in Harare, Zimbabwe, illustrates stack-ventilation concepts in an office block. The capital and maintenance costs of imported air conditioning, along with other factors, led designers to develop a passive ventilation alternative—the first of its kind in Africa. Harare’s climate is moderate, characterized by sunny, warm days and cool nights, yet it is quite distinct from the climate in the U.K. A myriad of strategies—shading, good daylighting, and ventilation chimneys—contribute to a low-energy building made of local materials.

Depending on building program and type, natural ventilation is applicable throughout the U.S.—at least for parts of the year. Yet in much of the country, natural ventilation cannot totally supplant air conditioning for spaces requiring full conditioning, given humidity levels in the peak cooling season. The concept is most appropriate for mountain climates, with low humidity and large diurnal temperature swings. For a proposed classroom and laboratory facility at Montana State University in Bozeman, BNMI Architects plans to use stack ventilation, expressed in the towers, for ventilation and passive cooling. The area’s cool summer nights, which are often 30 degrees Fahrenheit lower than the daytime highs, mean that night flushing can cool the building sufficiently. According to calculations, no air conditioning will be required.

In the hands of talented architects and engineers, vertical gestures are becoming distinctive elements wedding architectural design and building service systems. Bridging these concerns in this way is not new: Wright’s Larkin Building and Kahn’s Richard Medical Labs both have a striking vertical expression of mechanical services. What is novel, however, is how chimneys and towers become key components as alternatives to hermetically sealed buildings. While the flat-roofed, horizontal aesthetic—whose ascendancy as a predominant design style coincided with the popularization of central air conditioning—is coming under challenge, the new vocabulary can only expand as strategies underlying these building are explored further.