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Making the Most of Working and Living on Urban Heat Islands

08.10.10Dennis Roberts, President, Energy Efficiency Done Right

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Is it getting hotter in here, or is it just me? If you are in an urban area where cement, bricks, asphalt and other building materials are displacing lots, fields, and plants, you most likely are getting hotter. According to the U.S. Environmental Protection Agency (EPA) -- city growth where "surfaces that were once permeable and moist become impermeable and dry... cause urban regions to become warmer than their rural surroundings, forming an 'island' of higher temperatures in the landscape. Heat islands occur on the surface and in the atmosphere."

So how much extra heat is there in urban heat islands? The EPA has determined that exposed urban surfaces like roofs, and pavements can be fifty to ninety degrees hotter than the air! At the same time shaded, moist rural areas remain close to the air temperature. Of course this will lead to added heat discomfort for those of us working or living in cities. To be even more specific, the EPA calculated that the mean air temperature in a city of one million people or more can be almost five and a half degrees warmer than its surroundings. More profound is the fact that on a clear and calm night that difference can be as much as twenty-two degrees warmer!

No doubt these changes brought about by growth will impact a cities resident's quality of life. Among the most obvious impacts of the changes common to urban heat islands is increased energy consumption. According to the report, "Reducing Urban Heat Islands: Compendium of Strategies- Urban Heat Island Basics", most heat island impacts are negative and include:

Increased energy consumption: Higher temperatures in summer increase energy demand for cooling and add pressure to the electricity grid during peak periods of demand. One study estimates that the heat island effect is responsible for 5-10% of peak electricity demand for cooling buildings in cities.
Elevated emissions of air pollutants and greenhouse gases: Increasing energy demand generally results in greater emissions of air pollutants and greenhouse gas emissions from power plants. Higher air temperatures also promote the formation of ground-level ozone.
Compromised human health and comfort: Warmer days and nights, along with higher air pollution levels, can contribute to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and heat-related mortality.
Impaired water quality: Hot pavement and rooftop surfaces transfer their excess heat to storm water, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Rapid temperature changes can be stressful to aquatic ecosystems

Unfortunately there are additional complications of all this added heat. While the solar heat will be similar among rural or urban areas, the radiant heat will be more intense in the urban heat islands as evidenced by the fifty-ninety degrees difference between the roof or pavement and the air temperature. So in addition to the solar heat bombarding our homes and offices, urban island structures are also dealing with radiant heat coming from all directions. According to the study "Window Performance for Human Thermal Comfort" from the Center for the Built Environment-when it comes to summer thermal comfort the two major impacts are:

Inside window surface temperature
Transmitted solar radiation

Heat naturally goes toward cold so with the dynamics of the comparable environments, windows in structures in urban heat islands will be hotter than windows in rural areas. With inside window surface temperature being a major impact on human thermal comfort, people in homes and buildings in those urban heat islands will no doubt experience greater discomfort.

With the added heat on our urban heat island streets, in our offices and homes, and well into the night, what can we do for relief? A great place to look for relief is the source of much of that human thermal discomfort. Most of us by necessities spend most of our lives in buildings and homes. Windows and skylights are the only "enveloping" component designed to bring the outside in. That benefit comes at the cost of unequaled inefficiency relevant to the other envelope components.

A great way to improve the efficiency of existing windows while significantly reducing the temperature of the surface in contact with the controlled inside environment is to insulate the windows. Insulated windows will reduce the both the solar and radiant heat gain while significantly improving infiltration characteristics. In fact, insulated windows provide much more benefits than low e replacement windows year around, especially in extreme heat or cold.

There are many things that can be done to reduce urban heat island impact. Some of these things include increasing tree and vegetation and plant cover, installing cool roofing, and using cool pavements. Nothing though can have more impact on human comfort levels than reducing the unwanted heat gain where we work and live. Let's make life on the "island" more appealing.

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Sources

Heat Island Effect

Reducing Urban Heat Islands: Compendium of Strategies-Urban Heat Island Basics

Window Performance for Human Thermal Comfort Center for the Built Environment

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