Refocus Heat Action Plans To Mitigate Indoor Heat Risks

As the issue of climate change persists and worsens, extreme heat events are becoming more common. Research has explored the effects of climate change on outdoor temperatures and urban heat indexes (UHIs), but little research explores the factors that affect indoor temperatures.

In "Safe at Home?," (Journal of American Planning Association, Vol. 89, No. 3), Larissa Larsen, Carina J. Gronlund, Kaan Cem Ketenci, Sharon L. Harlan, David M. Hondula, Brian Stone Jr., Kevin Lanza, Evan Mallen, Mary K. Wright, and Marie S. O'Neill explore the possible effects of various building-level factors as well as how tree canopy affects indoor temperatures.

Rethinking Heat Action Plans for Buildings

The researchers pose a critical question: "What factors can assist urban planners in identifying homes and individuals at higher risk of indoor extreme heat?" The findings of this study lead the authors to recommend a shift in the focus of heat action plans towards enhancing building-level characteristics and improvements, advocating for a departure from sole reliance on tree-planting initiatives.

The authors sampled 140 data sets from households across three U.S. cities: Detroit, Atlanta, and Phoenix. They purposefully sought participants who would represent a mix of single and multifamily housing, masonry and non-masonry facades, and low– and middle-income neighborhoods with racial diversity.

Using available tax assessor data to assign each land parcel with a building year of construction, total interior area, and building type (single or multi-family). The percentage of tree canopy cover within a 400-meter radius was assigned as the tree cover around the unit. The percentage of tree canopy cover in the surrounding concentric ring between 400 and 1,600 meters was assigned as the tree cover of the nearby neighborhood.

Finally, indoor and outdoor air temperatures were measured with calibrated Onset Corporation HOBO Temperature/Relative Humidity 2.5% Data Loggers. Each sensor collected both air temperature and relative humidity so that heat absorption caused by evapotranspiration (the evaporation and transpiration of water through tree leaves) could also be indexed.

Table 2: Building and tree canopy characteristics by Detroit, Atlanta, and Phoenix.

Climate Impacts Vary in Cities

The authors found that the effects of their predictors on indoor temperatures were highly dependent on each city's climate. As hypothesized, tree canopy was a more influential factor in indoor temperature in cities with temperate climates such as Detroit.

In Detroit, tree canopy and masonry building facades had roughly equivalent impacts but in opposite directions. The 10°C reduction in indoor temperature due to higher amounts of nearby tree canopy was offset by the 10°C increase when the house had a masonry (brick) facade.

Even in Atlanta, which has a warmer climate than Detroit and where the authors expected building typologies to be the most influential, the tree canopy percentage within a 400-meter radius around homes reduced indoor temperature the most. However, higher percentages of tree canopy coverage in the neighborhood increased indoor temperature. The negative impact of the neighborhood tree canopy could be attributed to lower wind circulation.

Lastly, in Phoenix, building typologies had the greatest impact on indoor temperatures. Single-family and smaller homes had higher indoor temperatures compared to multi-family and larger homes. While tree canopies were not effective at decreasing indoor temperatures, building characteristics were.

As expected, air conditioning has the greatest effect on decreasing indoor temperatures. However, in Detroit, only 35 percent of the respondents regularly used air conditioning. With Detroit's older housing stock, many respondents reported that they either lacked air conditioning or could not afford the electricity costs.

While summer temperatures are higher in Atlanta and Phoenix, planners working in cities in temperate locations, such as the Midwest, need to recognize the vulnerability of their residents to extreme heat.

While air-conditioning is the most effective cooling strategy, planners need to highlight the importance of using electricity from non-fossil fuel sources and address the vulnerability of lower-income residents. Planners and municipalities should advocate for enforcing maximum indoor temperature thresholds and recognize that structural interventions are generally more effective in reducing indoor temperatures compared to tree planting.

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