Beating the Heat

by Marcy Marro | October 3, 2022 12:00 am

Extreme heat is also a racial and social justice issue. The U.S. Centers for Disease Control and Prevention[1] (CDC) reports that from 2004 to 2018, Black and Native American communities in the United States experienced the highest rates of heat-related deaths. A 2020 study[2] (Hoffman, J.S.; Shandas, V.; Pendleton, N. 2020) found strong correlation between heat islands and century-old racially biased housing policies in many U.S. cities. These historically disadvantaged zones have significantly higher temperatures (as much as 13 F) than wealthier neighborhoods. It should also be noted that globally, wealthy northern countries are primarily responsible for the greenhouse gas emissions currently causing climate change, while developing countries are already suffering the worst effects, including heat waves, drought and flooding.

The first reaction to the increasing incidence of heat waves is often to add or increase use of air conditioning (AC), especially in previously temperate regions where it is less common. This may be a necessary measure, especially in housing for the elderly who are more susceptible to temperature extremes. However, increasing the use of AC drives unfortunate feedback loops. Using more energy for AC leads to more climate changing emissions, further increasing heat extremes. Additionally, AC is designed to move heat from inside buildings to outside, and higher concentrations of mechanically cooled buildings will raise outdoor temperatures. A 2007 study by the Okayama University of Science found that cooling equipment can raise temperatures in downtown Tokyo by as much as 2 F, and Arizona State University (Salamanca, F. et al, 2014) modeled the effects of AC on air temperatures in Phoenix and found a similar nighttime temperature increase. These impacts will be magnified as the quantity of AC units and the duration of use increases.

Increasing the use of AC will exacerbate heat waves and can lead to brownouts and blackouts due to spiking power demand. While we cannot avoid this entirely, we need to design new buildings and retrofit existing buildings to passively reduce indoor temperatures and mitigate urban heat islands to limit the need for mechanical cooling in our rapidly warming planet. The strategies to accomplish this include:

Cool roofs–Darker colored roofing absorbs more solar radiation that is transferred into the building and reradiates heat to the local environment, contributing to urban heat islands with higher outdoor ambient temperatures. Cool roofs with a solar reflectance index (SRI) of at least 82 for low-slope roofs, and over 39 for steeper roofs (>2:12) address both issues. Vegetated or green roofs have a similar benefit and help to manage stormwater. Cool roofs can be 50-60 F cooler than a typical roof, reducing the air temperature at rooftop outside air intakes, thus reducing the cooling load.

Exterior shading–Windows are an important feature in buildings, providing natural daylight and views for occupants, contributing to indoor environmental quality. They are also the weak point in a building’s thermal envelope, resulting in heat gain that increases indoor cooling load and radiant thermal asymmetry, creating discomfort for occupants close to exterior walls. Exterior shading devices for windows exposed to direct solar exposure significantly reduce solar heat gain and air-conditioning demand, while also improving comfort and preserving daylight and views for occupants.

Improved building envelope–While insulation and air sealing are most often considered as a means to keep heat in during cold-climate winters, these are equally important for doing the opposite in the summer months and warm climates. Insulation slows the transfer of heat through walls and roofs to occupied interior spaces, and more importantly air sealing prevents hot, humid air from infiltrating the enclosure and causing moisture and comfort issues. Building enclosures in new construction can be designed to exceed code minimums, and existing buildings can receive insulation and air sealing enhancements to reduce the demand for mechanical cooling and improve occupant comfort. Air infiltration undermines the effectiveness of insulation, so air sealing should be prioritized in all cases.

Night flushing–In arid, temperate, and higher-elevation locations, even on the hottest days there is a significant diurnal temperature swing (25-50 F). Residential and commercial buildings can take advantage of this with night flushing, using mechanical, natural, or mixed-mode ventilation to flush warm air out and pre-cool indoor air and thermal mass with cooler nighttime outdoor air. This free cooling allows a building to thermally “coast” through part of the day with minimal mechanical cooling demand. This strategy is less effective in hot, humid climates where the diurnal swing is smaller (<10 F).

Other heat island and air-conditioning demand reduction strategies include using light-colored paving (solar reflectance >0.33), paving alternatives (e.g., reinforced turf), de-paving, shade structures (e.g., solar arrays over EV charging stations), and shade trees adjacent to parking and buildings.

With climate change upon us, the increased need for air conditioning is inevitable, but there are things we can do to reduce heat gain in buildings and mitigate urban heat islands to limit the need for it. We must design new buildings and retrofit existing ones to increase the climate resilience of our built environment, including keeping building occupants safe and comfortable during extreme heat events, and reducing energy use and carbon emissions from cooling systems.

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Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 30 years of experience in sustainable building design. He is the director of sustainability with Intertek Building Science Solutions in Portland, Ore. To learn more, follow Scott on LinkedIn at www.linkedin.com/in/alanscottfaia/[3].

Source URL: https://www.metalarchitecture.com/articles/beating-the-heat/

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