Air changes per hour - the number of times in one hour that the entire volume of air inside a home leaks out and is replaced by outside air.

Air-to-air heat exchanger - a ventilation system or device that transfers a large percentage of the heat from outgoing air to incoming air, reducing the amount of heat that would be lost via ventilation in colder weather.

Annual Fuel Utilization Efficiency or AFUE - a measure of seasonal heating efficiency used with fossil-fuel furnaces and boilers; expressed as a decimal equivalent of a percent efficiency (e.g. .80 AFUE = 80% efficiency).

Ballast - an electrical device for starting and regulating fluorescent lamps.

Band joist - the board that covers the ends of the floor joists in a framed floor assembly.

Batts or blankets - a type of insulation, usually fiberglass, that comes in pre-cut rolls or sections and various thicknesses for easy installation.

Blower door - a tool used to identify and quantify air infiltration by depressurizing a home or building to accelerate air leakage; also used to test duct systems for leaks.

Boiling point - the temperature at which a substance will change state from a liquid to a vapor (also the temperature at which a vapor will condense back into a liquid).

Btu or British Thermal Unit \- a standard unit of heat, defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

Building envelope - all of the surfaces that separate the conditioned area of a home from the outside or from unconditioned areas - also called the thermal envelope.

Carbon monoxide - a colorless, odorless gas made up of one part oxygen and one part carbon; a potentially lethal poison that can be produced by burning a fuel with insufficient oxygen.

Coefficient of Performance or COP - a measure of heating energy efficiency used with heat pumps; expressed as a decimal equivalent of a percent efficiency (e.g. 2.5 COP = 250% efficiency).

Combustion - a reaction in which fuel combines with oxygen to produce heat.

Compressor - the component in a vapor compression or refrigerant cycle that increases the pressure on the refrigerant.

Condenser - the component in a vapor compression cycle where the refrigerant changes state from vapor to liquid, releasing heat in the process.

Continuous wall foundation - a type of foundation that encloses either a crawlspace or basement beneath the first floor of living space.

Convection - the movement of heat via a heated fluid (e.g. water or air).

Conditioned space - any enclosed space served by a heating or cooling system or both.

Cooling Degree Days - a common indicator of climate or weather used to represent the degree to which air conditioning or cooling would be required.

Cooling load calculation - the preferred method for sizing air conditioning equipment.

Cubic Feet per Minute or CFM - a measure of air flow often used to quantify either infiltration or air flow through heating or cooling ducts.

Door sweep - a flexible material attached to the bottom of a door to seal a gap between the bottom of the door and the threshold.

Double-glazed - a window with two panes of glass separated by an insulating air space (also called double-paned or thermal window).

Energy - the ability or capacity to do work or overcome resistance.

Energy Efficiency Ratio or EER - a measure of cooling energy efficiency used with heat pumps and air conditioners; represents BTU/hr. of cooling per Watt of electricity.

Energy-efficiency - a measure of useable energy output versus energy input.

Energy Factor (E.F.) - an efficiency rating used with all residential water heaters; expressed as a decimal equivalent of a percent efficiency (e.g. .90 E.F. = 90% efficient)

Energy-saving practice - a change in behavior that saves energy.

Evaporator - the component in a vapor compression cycle where the refrigerant changes state from liquid to vapor, absorbing heat in the process.

Fireplace damper- a device used in fireplaces to seal off the chimney when the fireplace is not being used.

Expansion device - the component in a vapor compression cycle that decreases the pressure on the refrigerant.

Flue- the vent pipe for a fossil-fuel heating system or water heater that exhausts the combustion byproducts outdoors.

Frame construction - a type of construction that uses a framework or "skeleton" to support the structure.

Geothermal - a term meaning "heat from the earth"; a very efficient type of heat pump that draws heat from the ground or from water.

Greenhouse effect - a phenomenon whereby solar energy is collected and trapped inside a glazed (or "glassed-in") enclosure.

Halogen - a type of incandescent lighting that produces a very bright and tightly focused light.

Heat exchanger - a component in a heating or cooling system that transfers heat from one medium to another.

Heat of vaporization - the heat energy involved in changing a substance from a liquid to a vapor.

Heat transfer - the movement of heat from a warmer to a colder substance or environment as a result of a temperature difference.

Heat trap - a device used in water heaters to prevent hot water from flowing back up the cold inlet line.

Heating Degree Days - a common indicator of climate or weather used to represent the degree to which space heating would be required.

Heating Season Performance Factor or HSPF - a seasonal heating efficiency rating used with heat pumps; compares total annual heating to total annual energy consumption.

Hung window - a window that slides up and down in its frame.

Hydroelectric - electricity generated by the force of running water.

Incandescent - a type of lighting in which a thin filament is heated, causing it to give off light; the traditional "light bulb".

Infiltration - the uncontrolled leakage of air into and out of a home or building.

Infrared - the longest wavelength of solar radiation ("before red"), sensed as heat.

Jalousie window - a window made of horizontal glass louvers or slats.

Joist - a horizontal framing member that supports either a floor or a ceiling.

Kilowatt-hour or kWh- the standard billing unit for electricity, represents one kilowatt of power used for one hour.

Law of Conservation of Energy - a law of physics that states that energy can neither be created nor destroyed.

Light Emitting Diode or LED - a very efficient type of solid-state lighting, initially used in instrument panels and electronics, but now available for broader applications.

Low E or "low emissivity" glass - a type of glass that incorporates a microscopic coating designed to reduce heat transfer.

Lumen - a unit of light.

Lumens per watt - an efficiency measurement used with lighting; the amount of light per unit of electric power.

Mastic - a thick paste-like material used to seal joints in heating and cooling ducts.

Orientation - a term used to describe the direction a window (or other surface) faces.

Packaged system - a type or configuration of air conditioner or heat pump with all the key components located in an outdoor unit.

Passive solar - a solar energy application that requires no additional energy to operate.

Programmable thermostat - an air conditioning or space heating thermostat that can be programmed to automatically adjust the indoor temperature at different times of day.

Radiant barrier - a highly reflective material designed to reflect some of the sun's radiant energy away from conditioned spaces.

Radiant heat transfer - movement of heat via electromagnetic waves; heat from the sun.

Rafters - framing members that support a sloped or pitched roof.

Register - a vent that delivers heated or cooled air into a room.

Renewable energy - energy from the sun, wind, running water and other sources that continually renew themselves (i.e. are not "used up" or depleted).

Reversing valve - the component in a heat pump that allows the system to switch between space heating and air conditioning.

R-value - a measure of thermal resistance; the reciprocal of a U-value.

Seasonal Energy Efficiency Ratio or SEER - a seasonal cooling efficiency rating used with heat pumps and air conditioners; compares total annual cooling to total annual energy consumption.

Solar Heat Gain Coefficient - a measure of the percentage of solar radiation that passes through a window or window treatment (also called Shading Coefficient).

Simple payback - an indicator of how quickly an energy improvement will pay for itself in energy cost savings; usually expressed in years.

Single glazed - a window with a single pane of glass (also called single-paned).

Slab at grade - a type of foundation with the floor slab at ground level (grade).

Soffit - the board covering the underside of a roof overhang.

Sole plate - the bottom plate of a framed wall assembly.

Split system - a type or configuration of air conditioner or heat pump with the key components split between an outdoor unit and an indoor air handler.

Standby losses - heat lost from a storage-type water heater when no hot water is being used.

Standby power - power drawn by electronic devices when turned off, often for the purpose of allowing the device to turn on instantaneously without the need to "warm up".

Static pressure - the force that causes air to flow through a duct system.

Temperature - a measure of the intensity of heat energy.

Temperature/pressure relief valve - a safety device on storage-type water heaters that relieves any build-up of pressure before it becomes hazardous.

Ton (of refrigeration or cooling) - the amount of heat required to melt one ton of ice in 24 hours; one Ton is equal to 12,000 BTUs per hour.

U- value - a measure of thermal conductivity; the reciprocal of R-value.

Ultraviolet - the shortest wavelength of solar radiation ("beyond violet").

Unconditioned space - a space or area within a building that is neither heated nor cooled.

Vapor retarder - a moisture resistant material used to prevent water vapor from contacting cold surfaces where it might condense and cause damage.

Vaporization - a change in state from liquid to vapor.

Watt - the standard unit of electric power, defined as one Volt of electric potential or "pressure" at a current flow of one Ampere.