Preventing Urban Heat Catastrophes

Figure 1. Indoor temperatures in a prototypical 1940s two-story apartment building in Chicago during the July 1995 heat wave. In the existing building, top floor temperatures reached 108oF and remained high even after the outdoor temperatures had started to drop. The addition of attic insulation, white paint on the roof, and a ventilation system brought top floor temperatures below outdoor temperatures during the days. Ventilation alone lowered the peak temperatures by 8-10 degrees. A weatherization package of light-colored roofs, roof insulation, and better shading further reduced expected indoor temperatures by 3 degrees on the top floor and 6 degrees on the bottom floor.

The heat wave in Chicago last summer created a great deal of human discomfort and, by many estimates, caused over 500 deaths in three days. The overwhelming majority of these deaths occurred in buildings with indoor conditions that were reported as stifling. To prevent such urban heat catastrophes in the future, we need to understand how indoor conditions during such extreme weather conditions are exacerbated by poor thermal characteristics and improper operation of the buildings. By identifying vulnerable housing structures, we can develop strategies to keep such buildings from becoming dangerously hot during a heat wave.

Epidemiological studies of last year's heat wave deaths have revealed many cultural, social, and institutional factors. But the role of the conditions inside the buildings is attested by the fact that almost all those who died lived on the top floor and either did not have air conditioning or could not use it due to blackouts or insufficient building wiring.

Multifamily buildings in urban areas like Chicago, Philadelphia, and St. Louis are often the ones most likely to become excessively hot. The buildings' brick walls store heat over several days and radiate it into the apartments. Particularly vulnerable buildings can be identified by their construction materials, insulation, roof and wall colors, window orientation, apartment configuration, and lack of operable windows or mechanical ventilation.

The role of the radiant temperatures on thermal discomfort is much greater during extreme conditions. One particularly dangerous feature of many apartment buildings is an uninsulated, west-facing brick wall. This wall absorbs solar gain during the late afternoon and radiates the heat inward after the sun sets. This large radiating surface prolongs unhealthy thermal conditions even after the air temperature has peaked.

Researcher Joe Huang, at Lawrence Berkeley National Laboratory (LBNL), investigated and modeled the physical conditions in prototypes of these buildings (see Figure 1). Huang's simulations demonstrate that mechanical ventilation is the most effective way to prevent heat buildup from day to day, and that light-colored roofing significantly lowers ceiling surface temperatures. Insulation helps to keep temperatures down on both the top and bottom floors.

Practical guidelines to reduce deaths from future urban heat waves can be implemented by cities either as a stand-alone retrofit activity or by incorporating them into a weatherization program. Obviously, these same strategies will greatly reduce thermal discomfort for many more people. Finally, many of the mitigation strategies will also reduce heating and cooling energy use enough to pay for their installation.

-Alan Meier

Home Energy can be reached at: contact@homeenergy.org
Home Energy magazine -- Please read our Copyright Notice