Control Air Leakage, Save Energy

March 4, 2007
The building envelope affects the sustainability of your buildings on a number of fronts, burdening your HVAC systems and affecting IAQ

By Anne K. Goedken

Air leakage through the walls and windows, as well as through façades and curtainwall, will put unnecessary burden on your HVAC systems, draining your buildings of valuable energy. Infiltration can also decrease the quality of your building's IAQ. And, numerous studies have shown that even the newest high-performance buildings leak air (and let air in).

How is air getting in?
Over the course of a building's life, some air will unavoidably find its way inside. Davor Novosel, chief technology officer at the Alexandria, VA-based National Center for Energy Management and Building Technologies (NCEMBT), explains, "The building is alive - it doesn't go dormant. There are temperature expansions: One side is cold and one side is warm, so one side expands and one side contracts. All of that can lead to miniscule gaps that produce leaks." Once these gaps have formed, three main forces change the pressure inside a building and allow air to penetrate the building envelope: wind, stack effect (the flow of air that results from warm air rising), and mechanical heating and ventilation systems.

How does air leakage affect energy consumption?
A 2005 study sponsored by the Washington, D.C.-based U.S. Department of Energy (DOE) Office of Building Technologies (Impact of Infiltration on Heating and Cooling Loads in U.S. Office Buildings) brought even more clarification to the issue. According to its estimates, infiltration is responsible for 33 percent of total heating energy use, but only saves 3.3 percent of total cooling energy use in U.S. office buildings. Novosel describes how this infiltration increases a building's energy use: "Generally, if you have a leaky building, your HVAC system will not perform as well because you have a certain amount of controlled exhaust [and] outside air being brought in."

How does air leakage affect IAQ?
A leaky building envelope can have severe IAQ consequences. Although some external air is necessary to ventilate a building, too much of the wrong kind of air can create moisture problems. When hot and humid air enters the building through gaps in the building envelope, it comes in contact with cooler surfaces and creates condensation inside the wall. "If that condensation goes unchecked for a long time, then you can have serious IAQ problems, which [can lead to] microbial growth," says Novosel. Leakage can also allow dust and other contaminants to get inside your building, deteriorating the quality of the air that occupants and tenants breathe.

How can you control air leakage and reduce energy?
The first step in controlling infiltration is to find the source of the problem. Areas to pay close attention to include spaces around windows, entry points of utilities and exhaust, between tilt-up concrete panels, and entryways. "Little things can go a long way toward making sure that a building operates as it was designed," Novosel advises. But, when caulking and weather-stripping around windows are not enough, a building-envelope assessment will help identify larger problems.

To solve the problem of air leakage before it starts, take preventive measures in the original design and construction of a building. An air-barrier system may be an effective way to reduce a building's energy consumption, especially in cooler climates. According to the Air Barrier Association of America (ABAA), Walpole, MA, an air-barrier system is comprised of a number of materials assembled together to "wrap" a building shell and protect it from leakage. These materials, and the entire assembly, must be tested to ensure they do not exceed a predetermined level of air leakage.

Another 2005 study, Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use, also sponsored by the U.S. DOE Office of Building Technologies, tested the effectiveness of air barriers. It prepared energy simulations for three types of commercial buildings in five types of climates (Bismark, ND; Minneapolis; St. Louis; Phoenix; and Miami). The study predicted that the potential for annual heating and cooling energy-cost savings in buildings with an air barrier ranged from 3 to 36 percent.

Tax incentives for constructing high-performance building envelopes are available. As part of the Energy Policy Act of 2005, building owners can receive deductions of up to 60 cents per square foot for energy-efficient upgrades to the building envelope that achieve a 50-percent reduction in annual energy costs (when combined with upgrades to lighting systems and HVAC and water-heating systems). In addition to controlling air leakage, other energy-reduction strategies for the building envelope include the control of heat transfer, solar gain, and moisture migration. Novosel maintains that, although it is difficult to obtain a 50-percent reduction in energy costs in any commercial building, a holistic approach (including the building envelope) is necessary to improve its sustainability.

Anne K. Goedken ([email protected]) is new products editor at Buildings magazine.

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