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Stay Cool
July 2011
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Here’s How to Pay Less for AC

ThermostatRecord summer heat waves are already occurring more often and will be even hotter and more frequent over the next 30 years, according to a study by Stanford University scientists that have run climate simulations of temperatures across the United States. The study comes on the heels of a NASA report that concluded that 2000 through 2009 was the warmest post-industrial decade on record.

The hotter it gets, the more people run their conventional electric air conditioners (AC), releasing even more global-warming gas emissions from power plants into the atmosphere. Cooling accounts for nearly half the energy used by the average home during the summer, reports the Environmental Protection Agency’s Energy Star program. More than two-thirds of U.S. households have air conditioners, which set us back more than $10 billion each year in electricity bills, according to the American Council for an Energy-Efficient Economy.

Passive Cooling a Priority

There is a better way to stay comfortable using both active and passive strategies. The first requires specialized equipment, while the second uses the windows, walls, floors and roof to collect, store and distribute heat that is naturally available from the local environment.

The basic principles of passive solar design have been understood for millennia. From Mexico to the Middle East, people have built homes with thick walls to slow heat transfer, observes Doron Amiran, former development director of the Solar Living Institute. The Pueblo Indians constructed their cities to maximize solar warming in winter and screen the strongest rays in summer.

Many of these ancient techniques were abandoned in the age of cheap fossil fuels. “We build our houses for curb appeal or for the view, not thinking that all those windows facing south in the summer are going to cook the inside of the house,” says Amiran.

Daniel Aiello, chair of the nonprofit Arizona Solar Center and a principal of Janus II Environmental Architects & Planners, helps homeowners create vertical shading on east and west exposures with manmade screens or shrubs, trellises and vines, which have the added benefit of letting light and heat in during the winter, if they are deciduous. “Each side of the building is going to look different,” notes Aiello, who uses overhangs or awnings over south-facing windows in warm climates.

Aiello also points out that, on a home’s exterior, light-colored surfaces reflect more heat than dark-colored ones. He adds that textured surfaces stay cooler than flat ones, due to small-scale shading and the breakup of the interface between warm air and the surface. Inset windows are cooler as well.

It’s all important, because 35 percent of a building’s potential heat gain stems from the direct action of solar rays striking surfaces, according to Aiello. Incorporating such passive solar design elements into buildings can reduce heating bills by as much as 50 percent, according to the U.S. Department of Energy.

Another passive technique is to use cross ventilation by opening opposing windows. Take this a step further by installing vents to allow hot air to escape from high spaces and cool air to enter at lower ones. Also, make sure walls and windows are well insulated against outdoor air. Inside, shutters, light-colored blinds and curtains can also make a big difference.

Also consider glass with low-emittance (low-E) coating, which reduces heat transfer. The position of light-colored gravel, pools and other reflective surfaces are important because they can bounce heat; consider putting up a screen to block the energy.

Effective Active Cooling Comes Next

Alex Wilson, editor of Environmental Building News and author of Your Green Home, says the easiest and most efficient option is to use portable floor fans or install ceiling fans, which use 90 percent less energy than air conditioning. Fans can cool a room by a perceived seven to 10 degrees simply by moving air, which effects greater evaporation of perspiration.

The next step in terms of low price and high efficiency would be to use a whole-house attic fan, which blows hot air from inside the entire structure outside. However, Wilson points out that such devices are only able to provide substantial heat relief under certain conditions—usually at night and when the humidity isn’t too high.

A less comprehensive solution is simply to push hot air out of the attic, which will also help cool the house. According to the utility Austin Energy, reducing the attic temperature by 10 degrees or more saves up to 10 percent on AC costs; solar-powered attic fans are available.

Some other alternatives to conventional compression-cycle central and room AC units are emerging, such as evaporative coolers, often called “swamp coolers.” These draw air over wet pads, and the resulting evaporation causes cooling. Wilson says they only make sense in dry climates, because they add moisture to the air. They typically cost 50 percent less than traditional AC and use 75 percent less energy, although they do require more maintenance.

The most energy-efficient and initially expensive way to cool your home and heat it in the winter is with a geothermal heat pump that takes advantage of the Earth’s subterranean heat gradient. Although they have a hefty upfront installation cost, operating costs are much less than conventional AC.

Finally, don’t set the home’s thermostat below 78 degrees Fahrenheit, and install a programmable model. Utilize dehumidifiers, bathroom fans and heat-producing appliances sparingly; switch to compact fluorescent and LED lighting instead of heat-emitting incandescent bulbs; and keep those AC screens clean.


Brian Clark Howard is a New York City-based multimedia journalist and the co-author of
Green Lighting and Geothermal HVAC. Build Your Own Wind Power System will be released in 2011. Connect at BrianClarkHoward.com.