Shaping the City: Good design promotes efficient air conditioning

(Robert K. Lewis)
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By Roger K. Lewis
Saturday, July 17, 2010

"It's not the heat, it's the humidity!" a complaint often heard in Houston, where I grew up, is wrong. In fact, it's the heat and the humidity.

Houston would still be a small Gulf Coast city if its dreadful climate were not made tolerable by air conditioning. Likewise, as this summer's steamy weather reminds us, Washington might be a somewhat-less-populated city if air conditioning didn't exist.

The mid-20th-century advent of air conditioning encouraged the growth of major cities and their suburbs in many parts of America characterized by semitropical and tropical climates. The invention of the automobile also helped, but without air conditioning, even a car in every garage might not have offset the reluctance of many Americans to endure months of climatic misery.

From a sustainability perspective, air conditioning is a mixed blessing. Air-conditioning equipment -- condensers for homes, chillers and cooling towers for large buildings, pumps and fans -- consumes huge amounts of electricity produced mostly by burning fossil fuels. And thanks to air conditioning, millions of not-very-green residential and commercial buildings have been designed and built with little or no concern for natural cooling and ventilation techniques.

Air-conditioning systems lower indoor air temperatures and humidity. But heat and humidity inside buildings are not caused only by weather. Heat indoors is also caused by electric lights, machines and human bodies. Interior moisture is emitted by kitchens, bathrooms and other spaces in which water is present. In buildings where people assemble, such as auditoriums or large office settings full of lights, equipment and workers, a building's internally generated heat can necessitate air conditioning even during winter.

Given the economic and environmental costs of electricity, we need buildings to be less dependent on electromechanical air-conditioning systems. In climates such as Washington's, we will always need to install such systems, but we could design and construct buildings in which they would run less frequently and more efficiently. We know how to do it.

A fundamental strategy for reducing heat transfer from outdoors to indoors is to configure wisely a building's overall form. The form must be artful but sufficiently compact to avoid excessive exterior surface area. Solar orientation also greatly affects seasonal cooling loads. For example, the summertime solar load on a building's south and west facades is much greater than on its north and east facades. Consequently, architects should compose each facade to respond to its solar orientation.

Beefing up insulation in walls greatly reduces thermal conductivity, as does use of high-performance windows and glazing. Well-insulated, vegetated flat roofs and reflective roofing materials on sloping roofs are likewise essential for keeping out heat. Fixed louvers, canopies, awnings or shutters can shade windows and keep summer sun out.

For homes and other small buildings, deciduous tree screens are especially effective, providing shade much of the year while admitting the sun's warming rays only during winter.

Improved technologies help. Compact fluorescent lights use less power and generate less heat, as do incandescent lights controlled by dimmer switches. Energy-utilization monitoring devices, programmable thermostats and room-based sensors and controls conserve energy by enabling HVAC (heating, ventilation and air-conditioning) systems to respond more precisely to occupancy and air-conditioning needs.

The thermal inertia of the Earth itself can help save energy. Geothermal heat pumps use naturally cool water piped through a heat exchange loop from deep underground to cool and dehumidify indoor air. Since temperatures underground are considerably lower than summer air temperatures, habitable spaces partially below grade will always be naturally cooler.

Nevertheless, we know that on days when the temperature hits 100 and humidity is visible, air-conditioning systems will need to run. What we don't know is whether future climate change will mean many more such days.

Roger K. Lewis is a practicing architect and a professor emeritus of architecture at the University of Maryland.

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