Cooling Buildings With Green Urban Infrastructure

Cooling Buildings With Green Urban Infrastructure

The Prudential Tower Living Wall in Newark, New Jersey. Credit: Prudential Financial, Inc.

Summer is here, and the air is filled with the hum of air conditioning units cooling buildings and homes. Although the season’s high temperatures are felt throughout the country, these effects are intensified in urban areas, home to more than 71 percent of the U.S. population, due to the urban heat island (UHI) effect.¹ Higher temperatures increase energy demand for air conditioning (AC), leading to higher energy costs. Although AC was once considered a luxury, we now rely on it not only to ensure the thermal comfort of people in their homes and workplaces but also to keep essential information technology equipment, such as servers, cool. As researchers work to improve the energy performance of heating, ventilation, and AC systems, a growing body of evidence suggests that incorporating passive cooling methods through green urban infrastructure effectively lowers indoor air temperatures and reduces energy demands on active cooling systems.

Heat Risk

In urban areas, building materials and covered land surface areas absorb heat during the day and emit it at night, resulting in air temperatures that are 1.8 to 5.4ºF warmer during the day and as much as 22ºF warmer at night than those in rural areas.^2,3 Because outdoor air temperatures remain high, residents are less likely to experience relief from cooler overnight temperatures, and this extra heat transfers through building walls, raising indoor temperatures. Warmer seasonal air temperatures result in increased energy consumption because energy demand for AC is maintained throughout the night. The unrelenting high temperatures caused by the UHI effect also increase the risk of heat-related illnesses (HRI) among vulnerable members of the population who have a more difficult time achieving thermal regulation through natural bodily processes, namely children, the elderly, and those with disabilities. Low-income households and the homeless are also affected because they lack the shelter or amenities to protect them from grueling temperatures. Potential dangers of HRI include heat stroke and dehydration, and during a heat wave, the relative risk of mortality in Northeast cities increases 4.39 percent for each degree Fahrenheit rise in air temperature.⁴

Green Urban Infrastructure

Green urban infrastructure (GUI) is broadly defined “as an interconnected network of natural areas and other open spaces that conserves natural ecosystem values and function, sustains clean air and water, and provides a wide array of benefits to people and wildlife.”⁵ Perhaps GUI can be more narrowly defined as the incorporation and management of natural features such as trees, plants, and exposed soil into the built environment to enhance or improve the grey infrastructure performance. It is more commonly associated with stormwater management practices as it is proven to help prevent the flooding and pollution of local waterways when built stormwater systems are inundated⁶ , but GUI consists of elements, such as ground vegetation, trees, green façades, green walls, and green roofs, that also help to reduce surface air temperatures in two ways. First, natural ground cover that is free of concrete or asphalt absorbs heat from sunlight without re-emitting it, which helps surface and air temperatures remain cooler. Second, plants that shade the soil, which effectively blocks heat from being absorbed, help to offset the heat through evapotranspiration, in which moisture transpired from plant leaves evaporates into the air and cools it. Although green spaces in cities, such as parks, are generally cooler than other urban spaces⁷, GUI directly cools buildings through shade and humidity.

One GUI element, vertical greenery systems (VGS), offers a combined benefit of cooling and requiring minimal ground space — ideal for buildings with little or no available land for green features. There are two main types of VGS — green façades and green walls — and they vary in their upkeep and maintenance requirements. Green walls, also known as vertical gardens or living walls, consist of rooted plants in modules that are attached to the side of a building’s structure, which requires a system for watering and fertilizing the plants to maintain their health. A green façade involves a trellis structure with vine plants that are rooted at the ground level. Both methods avoid direct contact with the structure to prevent damage from trapped moisture. These VGS methods also have aesthetic benefits, and the level of ornateness, such as decorative patterns and shapes made by plants of differing colors, ranges widely. These structures are also used indoors, as at the Birmingham-Shuttlesworth International Airport in Birmingham, Alabama.

A vertical garden covers the Issaquah Transit Center in Issaquah, WA. Credit: greenscreen

Supporting Research

Researchers have conducted field experiments and computer modeling studies to measure the effects of VGS on the indoor and surface temperatures of buildings.^{8,9,10,11,12,13,14,15} Temperature ranges vary based on location, but the results consistently show that the protective insulation provided by the plants keeps the indoor temperatures of buildings with VGS cooler than uncovered buildings.¹⁶ Uncovered walls tend to cause a “continuous inflow of heat at night,” making occupants uncomfortable and, in severe cases, interfering with the important bodily process of thermoregulation.¹⁷ In general, these studies show that, at their maximum daily temperature, indoor temperatures in structures with VGS were 10ºF cooler¹⁸ and building surface temperatures were 5.4 to 8.1ºF cooler¹⁹ than in the same structures without VGS. One public housing building had surface temperatures 28.8ºF cooler, significantly reducing heat transfer from the building’s exterior to the interior.²⁰ The studies also note a wintertime benefit; just as VGS methods slow the transfer of heat from outside the building to the inside during warm weather, they also slow the transfer of heat from inside the building outward during cold weather.^21,22,23

Builders can improve indoor temperatures and residents’ comfort by installing insulation materials with a higher R-value. In general, using high-grade insulation to block heat transfer is a practical first step toward improving the energy performance of any building. Unlike insulation, however, VGS provides several benefits with an impact at the neighborhood and city levels, such as improving air pollution through UHI mitigation,²⁴ enhancing aesthetic value, and reducing noise.²⁵,²⁶ VGS both insulates buildings to maintain cool indoor temperatures and cools building surface temperatures, which affects the ambient air temperature and local climate conditions. VGS also has a greater cooling effect on hotter days,²⁷ and buildings partially covered by VGS have a reduced internal heat transfer from their envelope.²⁸

Options

Numerous VGS options are commercially available, with products ranging from small wall kits requiring minimal upkeep that can be purchased and installed by a homeowner to professionally installed and maintained systems at large-scale properties such as Studios 5c in Tempe, Arizona, or Universal CityWalk in Universal City, California. Costs reflect the complexity of the system; green facades, which cost between $25 and $30 per square foot, require fewer materials and less labor to install than green walls, which cost between $95 and $165 per square foot. VGS and other GUI features help protect urban utility networks from disruptions and minimize the risk to residents’ productivity and health by sharing performance responsibilities across many resources. A system that relies heavily on energy to maintain temperature control is vulnerable to power failures.

VGS and other GUI features enhance the aesthetics of a building’s interior and exterior, are economically sustainable,²⁹ and reduce energy consumption while maintaining residents’ comfort. VGS offers multiple benefits that both complement and enhance the performance of current thermal regulation technologies, especially for structures with limited space and older infrastructure. With an estimated $11 billion spent annually by Americans to cool their homes,³⁰ the potential cost savings gained from technologies such as VGS and other GUI features can be considerable.