Acid Rain - Rain that has become acidic due to the emission of sulfur dioxide and nitrogen oxides. To learn more, see the U.S. Environmental Protection Agency's Acid Rain Home Page.

Air Leakage Rating - The air leakage rating is a measure of how much air leaks through the crack between the window sash and frame. The rating reflects the leakage from a window exposed to a 25-mile-per-hour wind, and is measured in cubic feet per minute per linear foot of sash crack.

Annual Fuel Utilization Efficiency (AFUE) - A rating that denotes the efficiency of gas heating equipment. It is the amount of heating your equipment delivers for every dollar spent on fuel. An indication of how well a furnace converts energy into usable heat. A higher rating indicates more efficient equipment. The rating is expressed as a percentage of the annual output of heat to the annual energy input to the furnace. This rating is calculated in accordance with the Department of Energy test procedures.

Blower Doors - Energy contractors use blower doors to see how much air leaks through windows, doors, and other places in your house. The blower door is a large board that blocks the front door of your house. A powerful fan installed in the door draws the air out of your house and causes a strong draft inside where ever the air is leaking in. This can help the contractor locate the air leaks, and gives a good overall indication of how "leaky" your house is.

British thermal unit (Btu) - One British thermal unit, or Btu, is roughly equivalent to burning one kitchen match. The standard of measurement used for measuring the amount of heat required to raise the temperature of one pound of water by one degree (Fahrenheit). That may not sound like much, but a typical home consumes about 100 million Btus per year. Approximately one-half for the total is used for space heating.

Energy Efficiency Ratio (EER) - The ratio of the cooling capacity of the air conditioner, in Btu per hour, to the total electrical input in watts under test conditions specified by the Air-Conditioning and Refrigeration Institute.

Global Warming - Global warming is the gradual increase in global temperatures caused by the emission of gases that trap the sun's heat in the Earth's atmosphere. Gases that contribute to global warming include carbon dioxide, methane, nitrous oxides, chlorofluorocarbons (CFCs), and halocarbons (the replacements for CFCs). The carbon dioxide emissions are primarily caused by the use of fossil fuels for energy.

Heat Exchanger - A device used to transfer hear from a fluid (liquid or gas) to another fluid, where the two fluids are physically separated (usually by metal tubing). Household examples of heat exchangers are heating radiators and the coils on your refrigerator and room air conditioner.

Heat Pump - Devices that extract available heat from one area (the heat source) and transfers it to another (the heat sink) to either heat or cool an interior space. For instance, in heating climates, during the winter the heat pump extracts heat from the air outside and transfers it to the inside of the house to heat the house. In cooling climates, during the summer the heat pump extracts heat from the air inside the house, cooling it, and transfers it outside. Heat pumps work very much like your refrigerator: heat is released from the back of your refrigerator as it grows cooler inside. This is exactly like cooling your house during the summer.

Heat pumps can be very energy efficient, because instead of actually generating heat like a furnace, they just draw heat from the outside. But because the efficiency drops as the air outside gets very cold, many builders are turning instead to ground-loop or geothermal heat pumps. These heat pumps operate more efficiently than the standard air-source heat pumps, because the ground doesn't get as cold as the outside air (and during the summer, it doesn't heat up as much).

Heat Transfer - The flow of heat from one substance to another, for instance, the flow of heat from your water heating element to the water that surrounds it.

Heating Seasonal Performance Factor (HSPF) - The total heating output of a heat pump in Btu during its normal usage period for heating divided by the total electrical energy input in watt-hours during the same period.

Housewrap - Housewrap is a sheet of plastic, often fiber-reinforced, that is used to reduce air leakage in new homes. These sheets are wrapped around the outside of a house during construction. Builders must seal the housewrap at all joints and seams to create a truly continuous, effective air retarder. .

Infrared Cameras - Energy contractors use infrared cameras to look at the heat leaking into or out of your house. The infrared camera "sees" the heat and can show "hot spots" where a lot of heat is being lost. This helps to identify the places where your home's energy efficiency can be improved. Kilowatt-Hour (kWh) One kilowatt-hour (kWh) is equal to using 1000 watts of electricity for one hour. This is equal to burning a 50-watt light bulb for 20 hours, or roughly equivalent to cooking a pot of rice for an hour. Your utility bill usually shows what you are charged for the kilowatt-hours you use. The average residential rate is 8.3 cents per kWh. A typical U.S. household consumes about 10,000 kWh per year, costing an average of $830 annually.

Seasonal Energy Efficiency Ratio (SEER) - The total cooling output of a central air conditioner in British thermal units during its normal usage period for cooling divided by the total electrical energy input in watt-hours during the same period. The test procedure is determined by the Air-Conditioning and Refrigeration Institute.

Solar Heat Gain Coefficient (SHGC) - The solar heat gain coefficient, also called a shading coefficient, is a measure of how well a window absorbs or reflects heat from the sun. The lower the coefficient, the better the window is at blocking the sun's heat. Windows in hot or temperate climates should have a low SHGC; south-facing windows in cold climates should have a high SHGC.

Storm Windows - An extra pane of glass or plastic added to a window to reduce air infiltration and boost the insulation value of a window. If you are considering adding storm windows, you should compare the costs to installing new energy-efficient windows

Surface thermometers - As the name implies, surface thermometers have a temperature probe that can be placed directly on a surface to see what temperature it is. This can help energy contractors evaluate how well heat is passing through your doors, windows, walls, floor, and ceiling. Placed on a window, for instance, it can tell you if the window is close to the room temperature (indicating that it insulates well) or closer to the outside temperature (indicating that it insulates poorly).

Vapor Barrier - Also called a vapor retarder, this is a material that retards the movement of water vapor through a building element (such as walls, floors, and ceilings) and prevents metals from corroding and insulation and structural wood from becoming damp.

Whole-House Fan - A large fan used to ventilate your entire house. This is usually located in the highest ceiling in the house, and vents to the attic or the outside. Although whole-house fans are a good way to draw hot air from the house, you must be careful to cover and insulate them during the winter, when they often continue to draw hot air from people's houses.