Heat Island Mitigation
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Decreases load on heating, ventilating, and air conditioning (HVAC) systems, thereby reducing associated utility charges; provides shade for comfort; promotes human health; promotes biodiversity; and helps control stormwater runoff. |
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One to two years for plant establishment; major retrofits or structural additions should be part of a larger capital project. |
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Installation of plantings or architectural shade structures, and replacement of existing roof or paved areas |
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Heat island effect is the phenomenon whereby metropolitan areas are significantly warmer than their rural surroundings due to the amount of paved area and buildings that have replaced vegetated and permeable surfaces. Dry and unshaded urban surfaces absorb solar gain and re-radiate heat into the atmosphere, elevating air temperatures. For example, low reflectance building rooftops and pavement can reach temperatures of 66–88 degrees Celsius (⁰ C) (150–190 degrees Fahrenheit (⁰ F)),11 warming the local area (see Figure 1). As urban densities, areas, and populations increase, this issue may become of greater concern.
Heat islands can contribute to the following:
- Risks to human health, including heat-related illness and mortality12, as well as species loss
- Reduced indoor and outdoor comfort
- Increased cooling energy use and higher utility bills
- Higher peak electricity demand and raised electricity production costs
- Higher intake air temperatures for building conditioning systems
- Increased air pollution from increased energy to cool building interiors
- Accelerated deterioration of roofing materials, increased roof maintenance costs, and higher levels of roofing waste sent to landfills13
Recommended design criteria for new projects include shading and high-albedo (reflective or ‘cool’) materials on site surfaces and roofs. Various heat island mitigation strategies can also be implemented around existing buildings and are particularly beneficial in locations with extended cooling seasons.
Consider the following strategies:
- Installing native, drought-tolerant species, with shade provided within five years (see Site: Plant Selection)
- Working with landscape architects to identify appropriate locations for planting additional trees, taking care not to breach anti-climb walls and building conditions and without encroaching into the perimeter ‘clear zone’ (see Site: Plant Selection and Original Design Intent)
- Where energy costs and solar insolation values are high, installing solar energy generating systems, such as photovoltaics (PV) or solar thermal, at shade structures and canopies (see Energy: Photovoltaics)
- Resurfacing with high albedo materials (see Resources: Energy Star®)
- Installing green roofs (see Resources: Cool Roofs and Emissivity and Vegetated Roof Study)
- Replacing asphalt or other dark surfaces with permeable, open-grid pavers or light-colored materials