It's an Air Con
How sad that air-conditioning, perhaps the definitive building technology of the 20th century—responsible for the appearance of more architecture than all of the “isms,” genius practitioners, and political dictators put together—has increasingly become a dirty word among some architects. And how ironic that the more energy-hogging air-conditioning systems we build, the hotter the planet becomes.
In his 1969 book The Architecture of the Well-tempered Environment, Reyner Banham considered the air-conditioning unit a “portent in the history of architecture.” But might we be on the cusp of a decline in its dominance? Although basic, time-honored design strategies such as exterior shading, interior thermal mass, operable windows, and careful site orientation have reemerged in the past decade under the guise of green architecture, passive or natural ventilation remain exceptions.
According to McGraw-Hill Construction’s 2006 Construction Outlook, America built more than 150 million square feet of office space in 2005 alone, with the largest portion in Phoenix. We can safely assume that all of this space—and that in Phoenix in particular—came equipped with air-conditioning. This overwhelming evidence of our addiction to what Willis Carrier, the father of modern air-conditioning, called “man-made weather,” has not prevented an adventurous set of architects and engineers from aggressively pushing for more passive design strategies.
They call it the “Windy City”
Devon Patterson, AIA, a principal with Solomon Cordwell Buenz (SCB) in Chicago, calls the implementation of passive ventilation strategies—basically, anything to reduce dependence on air-conditioning—a “forward-thinking” consideration. In the firm’s design for the 69,000-square-foot Information Commons and Digital Library at Chicago’s Loyola University, scheduled to open in November 2007, the need for energy efficiency that wouldn’t sacrifice the building’s transparency led the architects to knock on Matthias Schuler’s door. Schuler, a mechanical engineer who runs the Stuttgart-based climate engineering firm Transsolar, assisted SCB in developing a double-skinned glass curtain wall as part of an integrated system of radiant slabs, underfloor ventilation, and operable windows that would result in smaller overall mechanical systems. Patterson says Chicago’s extreme weather conditions—hot summers, cold winters—prevented an entirely natural scheme, but that so-called “mixed-mode” systems that combine conventional heating, ventilation, and air-conditioning (HVAC) with various levels of natural ventilation represent the best attempt to combat air-conditioning’s prevalence.
“We were initially going to install some sunshades, but we found this was a better way to mitigate heat gain,” Patterson says. Air enters the building off of Lake Michigan from the east through automatically operated clerestory windows on the glass curtain wall. It then moves across the interior to louvers at the top of the west wall, where it enters the 3-foot-wide cavity of the double-skinned curtain wall, a design-build point-supported glass system. This warmed air then exhausts via a natural stack effect through a large vent on the top of the building. There are basically two kinds of natural ventilation—the stack effect and wind. Loyola represents a combination of the two.
Loyola is a simple enough solution—a tunable glass box wrapped around a concrete structure—but complexity resides in its details. For one, the east wall includes interior low-E shades that form a mini-double-skinned facade when early morning sunlight might otherwise overheat the space. (Europeans often place these shades on the exterior, Patterson says.) Two systems sandwich the air of the interior, which consists mostly of open space for computer workstations.
A raised-floor displacement ventilation system provides conditioned air designed to handle the first 8 feet of vertical space, as opposed to conditioning the entire 12 feet to the ceiling. By locating the air supply in the floor, the architects could install in the exposed poured-concrete, barrel-vaulted ceiling the plastic tubing needed for radiant cooling and heating. Vaulting not only contributed more surface area for the radiant system, it also optimized reflection for the efficient T5 fluorescent indirect lighting system.
Of course, chilled slabs lead to worries of condensation and indoor rain showers, but Transsolar’s modeling showed that with a minimum of 67 degrees Fahrenheit for the slabs, there would be only 10 days each year where the slabs would be colder than the dewpoint. The building’s conventional HVAC system easily accommodates dehumidification for these instances. The west facade, though, remains the lynchpin in the design. Early computational fluid dynamic (CFD) modeling of this side of the building showed air would flow over the top of the extended curtain wall and create a negative pressure zone on the backside that would pull air out of the wall’s cavity. The curtain wall sits atop a trench, which pulls air in to facilitate the stack effect. In winter months, with the trench closed, warm air builds up in the wall cavity to help heat the building.
In 2004, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) revised its bible of air-conditioning—Standard 55, Thermal Environmental Conditions for Human Occupancy—to account for studies that proved that when people have more control over their environment, they typically accept a wider range of temperature conditions. In 2000, one influential study led by Gail Brager at the University of California at Berkeley’s Center for the Built Environment showed that traditional research in human comfort expectations neglected to adequately consider three modes of adaptation: physiological, behavioral, and psychological. While Brager did not find that physiological adaptation, or how a body adjusts to a climate over a long duration, could account for much, she did find that behavior—removing clothing, turning up fan velocities—could significantly alter environmental perceptions with occupants. Brager also found that psychologically, occupants of naturally ventilated buildings grow accustomed to less consistency in temperature, eventually coming to expect interior conditions that more closely hew to outdoor weather.
These shifts in perception undoubtedly encouraged SCB’s architects to take advantage of Chicago’s more temperate weather periods for natural ventilation. “A weather vane on the roof can open the facades and induce ventilation when outdoor temperatures fall within a range of 55 to 65 degrees,” Patterson says, noting that the building’s fullest potential lies in spring and fall. Still, he estimates that’s enough time averaged across the year for the building to beat ASHRAE’s 90.1 Energy Standard by 50 percent.
Green for open, red for closed
For architects and engineers, this sort of mixed-mode design probably represents the future direction of passive ventilation schemes. For risk-adverse developers, mixed-mode designs offer flexibility—tenants who prefer air-conditioning can choose to ignore the windows—as well as a buttress against the unrelenting onslaught of humidity. At NBBJ’s new offices in Seattle, Allan Montpellier, a mechanical engineer with the local office of Flack+Kurtz, developed a mixed-mode approach for an underfloor HVAC system that depends on occupants to operate windows when the air-conditioning is off. Green and red indicator lights, tied to the building’s HVAC control system, alert staff to when they can open windows. The approach can also backfire. “If you put a desk up against a window, that person will then ‘own’ that window,” Montpellier says.
“You really need space between desks and windows—you have to think psychologically about the space.”
But before Montpellier could get to this point, he undertook a comprehensive review of what he terms a “typical meteorological year” in Seattle. He developed a chart for NBBJ that showed the number of hours in a year when the temperature would stay in a specific range; ever the jargonists, many engineers call this “binning up” temperatures. For example, the chart indicated 432 hours annually for a bin of 60 to 62 degrees, but Montpellier mainly focuses on the values above 80 degrees. “You really start to get uncomfortable around 80 to 85 degrees, even with a ceiling fan,” Montpellier says. “Above that, people will complain.” He found there were only 64 working hours per year where the temperature exceeded 80 degrees, which helped convince NBBJ to go with the mixed-mode design, though he notes each client tolerates different environmental conditions.
Having established a case for natural ventilation, Montpellier then had to determine if he could secure enough openings in exterior walls to get the minimum amounts of air the scheme would require. The International Mechanical Code requires that for every square foot of floor area, you need 4 percent of the wall area for operable windows. Montpellier says he designs closer to 10 percent, as the code suggests a minimum that doesn’t adequately address occupant comfort issues. At this point, Flack+Kurtz developed a CFD model of the building to gauge airflow patterns around and through the structure to ensure a natural ventilation scheme could accommodate the interior recesses of the building. Relatively standard design tactics—shallow, open floor plates no wider than 50 feet and a central atrium—facilitated air movement.
Engineers often refer to CFD modeling as definitive—a design tool producing results as good as the built thing. Paul Linden, chair of the department of mechanical and aerospace engineering at the University of California at San Diego, says CFD modeling can generate skewed results since most existing software, such as the widely used EnergyPlus from the Department of Energy, contains programming code that doesn’t take into account internal heat gains. “EnergyPlus in its traditional form is for sealed, air-conditioned buildings,” he says.
Linden, whose research has focused on airflow modeling, says updating EnergyPlus to consider natural ventilation schemes, transient environmental conditions, more complicated geometries, and estimates of comfort levels for spaces will be key to the software’s usefulness in design. Still, he says he worries that many consultants don’t know how to read CFD analysis results, and even worse, hardly anyone monitors buildings post-occupancy to determine the accuracy of CFD predictions. “CFD models produce nice pictures,” Linden says, “but who’s to say what’s right or wrong.”
William McDonough + Partners developed a similar scheme as that at NBBJ for a mixed-use office project in Barcelona, with manually operable windows that allow cool air to filter into a central atrium and then exhaust through the roof. The architects produced a user’s manual for the building, now under construction, explaining the role each occupant would play in how the building functions. John Easter, a director at McDonough, says the busy Barcelona streets surrounding the project certainly produce problems with indoor air quality and noise—two of the chief complaints of natural ventilation—but the city’s commitment to reducing pollution should help in the long run. “There are times with heavy traffic where they will need to close up the building, but it’s a system the tenants monitor and will have to adapt to,” he says.
The term “risk” pops up frequently in discussions of natural ventilation and unconventional building systems. Matt Herman, an energy-modeling consultant with Buro Happold’s New York office, worked on the McDonough project. He says the emergence of better performance-based analysis tools has instilled engineers with more confidence.
After typical building energy modeling, or thermal analysis, is complete, Herman usually embarks on creating an airflow network. This relatively simple analysis, which he performs using the IES Virtual Environment software, models bulk air transfer around and through a building across an entire year. An airflow network primarily yields graphs that compare indoor to outdoor air temperatures throughout a year. It takes into account materials, solar obstructions, internal heat loads, and occupancy. Although it takes time to set up, running the program can take a few minutes to a few hours. A CFD model, on the other hand, can take days to calculate and, while it produces a high level of information about airflow, its data accounts for only one specific instance in time, usually a worst-case situation.
Although software tools have helped justify designs to risk-averse clients, some architects have returned to the pre-air-conditioning era for inspiration. Architects and engineers must discard a lot of cultural baggage to implement natural ventilation schemes. Modern air-conditioning, arguably first installed in Brooklyn in 1902 by Carrier, took decades to become what Philip Johnson would call a crutch—an easy solution allowing architects to block out the real world, hermetically sealing our daily lives in a cocoon of ignorant bliss. Not until movie theaters widely adopted the technology in the 1920s did the public begin to demand it. ASHRAE first institutionalized occupant comfort levels with a chart for engineers in 1924. That became Standard 55 in 1974, which for years has supported the case for air-conditioning for nearly every building in America.
Grimshaw Architects looked at 19th-century European rail stations to find design tactics for the Southern Cross Station, completed in Melbourne, Australia, in 2006. Victorian stations typically had high barrel vaults that would help force a plume of smoke up away from travelers, with linear clerestory vents along the top of the arch for exhaust. Grimshaw implemented a more high-tech version of this with a domed, double-skinned roof that would absorb heat and contaminants into an open, gridlike inner layer, which would then exhaust through a 16-inch cavity out of “moguls” at each peak. CFD modeling showed that wind sweeping across the top of the roof would engender a pressure differential, or stack effect, at the mogul openings, naturally pulling the heat out of the cavity. Engineers considered a variety of airborne contaminants—sulfur dioxide, nitrous dioxide, and carbon monoxide—to ensure the system would work under every condition. Keith Brewis, a director in Grimshaw’s Melbourne office, says a 1-foot opening along the ground plane of each wall’s facade supplies the station’s air for the system to work. “During the competition phase, we hadn’t done the CFD analysis, so we made a provision in the plan and cost to put a fan in the apex of the domes,” Brewis says. “But we’ve been open and there hasn’t been a concern.”
Although Brewis says Grimshaw seeks to implement natural ventilation schemes on every project, he feels governments should lead the call to action since leasing agents can thwart a developer from even considering unconventional design approaches. While enlightened clients help, in nearly every case discussed in this article designers stressed the need to educate clients. SCB’s Patterson says Loyola approached the topic unenthusiastically until the architects took the university’s key players to visit similarly ventilated buildings Schuler had designed in Germany. Scott Frank, a mechanical engineer with Jaros Baum & Bolles in New York, says it can be a tough sell since owners view the components that make mixed-mode systems function as adding directly to cost and maintenance. “In the end, facades of buildings are going to have to be a more active component of design,” Frank says, suggesting perhaps the integration of radiant systems into facade components as one potential solution. Although eliminating air-conditioning sounds radical in 2007, the possibilities for an architecture unburdened by its demands clearly remain open to discussion.