September 1996 Issue #12

Urban Heat Catastrophes

Last year's Chicago heat wave created a great deal of human discomfort and, by some estimates, caused over 700 deaths in three days. Epidemiological studies of the heat wave deaths have uncovered some cultural, social, and institutional explanations, but the role of the building itself and its inside conditions had not been carefully examined. Curiously, most of the deaths occurred on the top floors of the buildings. Researchers in the Building Energy Measurement and Performance Analysis, led by Joe Huang, investigated the physical conditions, specifically how indoor conditions during extreme heat are exacerbated by poor thermal characteristics and improper operation of the buildings. Eventually, Huang wants to identify mitigation strategies. His goal is to propose and test practical, low-cost measures to keep such buildings from becoming dangerously hot during future heat waves.

Huang intially focused on the benefits of ventilation. These simulations demonstrate that mechanical ventilation can prevent temperature build-up in subsequent days (see figure). Without sufficient ventilation, air temperatures can rise to fatal levels.

A computer simulation of temperatures in a typical Chicago apartment during the heat wave. Note how the temperature in the top floor is higher than outdoors and far above the comfort range.

The role of surface radiant temperatures on thermal discomfort is much more important during a heat wave. During the day, the sunshine on the roof can raise the radiant temperature of the top apartment's ceiling well above body temperature. During the late afternoon, west-facing brick walls absorb heat and then re-radiate it into the apartment all night. Together, these large radiating surfaces prolong unhealthy thermal conditions even after the air temperature has dropped.

The result of this study will be practical guidelines for reducing the danger from future heat waves that can be implemented either as a stand-alone retrofit activity or incorporated into a weatherization program. For example, lightly colored roofs greatly reduce the ceiling temperatures.

Although the research focus is to reduce deaths from urban heat catastrophes, the same strategies will obviously greatly reduce thermal discomfort for many more people. Finally, many of the mitigation strategies will also reduce heating and cooling energy use that may pay for their installation through the energy savings.

-Alan Meier

What to do during a heat wave from WeatherLabs, Inc.

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