The following terms may be useful as you review the results of your Home Performance with ENERGY STAR ® Home Energy Audit and its detailed recommendations for energy improvements. If you have any questions throughout the audit process, please ask your Home Audit Professional.


AFUE: Annual Fuel Utilization Efficiency . It is a measure of the amount of heat actually delivered to your house compared to the amount of fuel that you must supply to the furnace.

Air Barrier: The barrier that prevents infiltration of outdoor air into the conditioned space and exfiltration of indoor air to the outside. Also known as “pressure boundary”, it should be continuous and aligned with the thermal boundary (the part of the home that physically separates the conditioned spaces from the unconditioned spaces).

Air Duct: A hollow conduit or tube (square or round) that circulates air from a forced-air heating and/or cooling system to a room (supply duct) or returns air back to the main system from a room (return duct).

Air Leak: A hole, crack, or gap where air can leak in or out of a house. Air leaks can make a home feel drafty or uncomfortable and waste energy.

Air Sealing: The process of sealing bypass ducts in the pressure boundary to prevent air leakage. Air sealing reduces heat flow from air movement and prevents water vapor from entering the wall.

Asbestos: Heat-resistant material used previously in many building products; represents serious health hazard as an airborne particulate.

Atmospheric Venting System: Negative vent pressure that uses standard chimney to remove combustion by-products from the home.

Attic Bypass: An air connection between the living space and the attic.

Attic Ventilation: Intended to remove heat and moisture from attic areas to the outside.

Back Draft: Spillage of combustion by-products into the home (after running the appliance for one minute). Instead of venting out through the chimney or flue, the combustion by-products spill into the home.

Balanced Mechanical Ventilation System: A system of inlets and outlets that prevents pressure differences while ventilating the home.

Balloon Framing: A construction method where the walls are framed before the floor is built, and the wall cavity is often open to both the attic and the basement.

Band Joist: The framing ribbon between first and second floors of a structure.

Baseload: The amount of energy (electric, natural gas, propane or oil if these are used for heating) used to operate lighting and appliances year round. The minimum energy you use.

Beam: Structural element that is capable of withstanding load primarily by resisting bending.

Blower Door: A diagnostic tool used to test for house leakage.

Bottom Plate: The bottom horizontal wall component.

Building Envelope / Building Shell: Any part of the building that creates a boundary between indoor and outdoor space.

Capillary Action: Moisture transfer through which water is sucked into tiny spaces in and between building materials, caused by the attraction of water molecules to each other and to other substances.

CFM: Cubic Feet Per Minute . This is a measurement of the flow of a gas or liquid that indicates how much volume in cubic feet pass by a stationary point in one minute. The higher the CFM the better the suction.

CAZ: Combustion Appliance Zone .

CCF: Cubic Foot .  May be used as a measurement of natural gas usage and roughly equivalent to 1 Therm or 1020 BTUs.

Cellulose: A form of insulation made from recycled material and treated to be flame- and insect-retardant.

CO: Carbon Monoxide .  An odorless, tasteless, poisonous combustion by-product lighter than air.

CO2: Carbon Dioxide .  A combustion by-product heavier than air.

Compact Fluorescent Lamps (CFLs): Re-designed lights for use in most conventional light fixtures. CFLs use about 75% less energy when compared to standard (incandescent) bulbs.

Condensation: The process by which moisture vapor changes to liquid form.

Conditioned Space: Areas or zones within the house that are heated or cooled for comfort.

Conduction: The transfer of heat between objects that are in contact. (Heat is transferred from molecule to molecule, through solid materials.)

Consumption Analysis: An analysis of energy usage. This could be an in-depth review of month-to-month usage, or a comparative analysis before and after work is completed.

Convection: The transfer of heat by circulation or movement of the heated parts of a liquid or gas.

Convective Loop: When air (or another medium) continuously circulates around in an enclosed space as it is heated and cooled.

Cooling Load: The amount of energy consumed to provide seasonal cooling.

Cost-Effectiveness: An indicator of how worthwhile an investment is.

Crawlspace: The space under a house often used to distribute mechanical/ electrical and plumbing systems. It is usually unconditioned.

DHW: Domestic Hot Water system .

Diffusion: A moisture transport mechanism; the way in which water vapor moves through materials such as sheetrock and plywood, working its way from high concentrations of moisture to low concentrations.

Distribution System: When used in the context of an audit, it is the system through which heat/cooling is distributed throughout the house.

Draft: The movement of combustion by-products through the flue and chimney and out of a building.

Draft Test: A way to determine whether venting systems are effectively moving combustion gases out of the house, even under worst-case conditions.

Duct Blaster: Duct leakage testing equipment.

EER: Energy Efficiency Ratio . This is a measure of how efficiently a cooling system operates. The higher the EER, the more efficient the air conditioner.

EPA: Environmental Protection Agency .

Exfiltration: Air leakage. Air leaving the building through the building envelope, caused by pressurization with reference to outside. (Replaced by an equal amount of air entering the building).

Exhaust-Only Mechanical Ventilation System: This system uses a fan assembly to remove air, moisture, and contaminants from the home, but relies on building leakage or planned inlet dampers to provide incoming air.

Fenestration: Envelope penetration used for access and egress or lighting. Examples include windows, doors, and skylights.

Fiberglass: Common insulation material made from glass fibers.

Firestop: Material used to stop smoke, toxic fumes, and fire from migrating from one floor to another.

Flame Impingement: Flame interference or obstruction.

Floor Joist: A horizontal support made of wood, steel or concrete usually smaller than a beam.

Forced Air Distribution System: A forced warm-air system that uses a blower and ductwork to distribute heated air from a source (furnace or air handler) to each room.

Heating Degree Days: A value reflecting the amount of energy needed to heat a building. Annual Heating Degree Days (HDD) is the total HDD in a year based on 30-year averages for specific climate regions. Example: The amount of energy required to heat a building based on the average daily temperature over a 30 year average for a specific climate region.

Heating Load: The number of Btu’s per hour that must be added to provide indoor comfort. Heating load is based on worst-case Winter Design temperatures determined by local weather statistics.

HSPF: Heating Seasonal Performance Factor . It is a commonly used measure of the heating efficiency of heat pumps. The cooling efficiency of a heat pump is measured by its SEER.

IAQ: Indoor Air Quality .

Infiltration: Air leakage. The air that enters a structure through the envelope; caused by depressurization with respect to outside. (Replaced by an equal amount of air leaving the home).

Insulation: Any material that slows heat transfer.

Interacting Relationships: Changing conditions that are inter-related and therefore affect each other.

Internal Gains: Heat gains from internal sources. An example would be cooking in a home, which increases the amount of heat in the internal environment.

Kilowatt Hour (kWh): 1000 watts; 1 kWh = 3414 Btu’s.

Knee Wall: A short wall, usually about 3 to 4 feet high, in the attic of a home, anchored with plates between the attic floor joists and the roof joist. Sheathing can be attached to these walls to enclose an attic space. Commonly found in cape-style homes.

Low-Flow Aerators: A devise that affixes to a sink faucet to limit flow. Low-flow aerators can generally range in size from approximately 1.0 to 1.5 gallons/minute.

Low-Flow Showerhead: Restrictive showerhead that limits water flow to a lower gallon/minute flow that usually ranges between 1.5-2.0 gallons/minute.

Makeup Air: Air brought in to replace air displaced by combustion or exhaust appliances.

Mastic: A non-toxic, low volatile organic compound (VOC), air duct sealant used for permanently sealing the fabricated joints and seams of sheet metal air ducts, rigid fiberglass air ducts, flexible air ducts and thermal insulation.

Mechanical Ventilation: Intentional ventilation that allows the homeowner some control in the amount and timing of air exchange in the home.

Permeability: A measure or rating of a material’s ability to permit moisture to pass through it.

Polystyrene: A type of foam insulation; available in sheet or spray form.

Power-Vented System: System that uses positive vent pressure to remove combustion by-products to the outside. A power vent located on the appliance pulls air through the heat exchanger and pushes byproducts out through the vent pipe.

Pressure Boundary: Refers to the location of the barrier that separates indoor air from outdoor air. It should be continuous and aligned with the thermal boundary.

Radiation: Heat transfer from a warm object to a cold object, where the objects are not in contact with each other. Unlike convection or conduction, radiation does not require a “medium” to carry the heat. The objects must be “in sight” of one another.

Radon: Naturally occurring radioactive soil gas; a common indoor air contaminant.

Rafter: A type of beam that supports the roof of a building.

Rate of Return: The percentage of savings from an energy investment accrued each year.

Relative Humidity: The percentage of moisture vapor present in the air, relative to the total amount of moisture the air could hold at a specific temperature and/or pressure, expressed as a percentage.

Respirator: A personal safety device designed to protect against inhalation of airborne particulates.

Rim Joist: The external frame for a floor platform of a house. Also called box sill.

Rock Wool: A type of insulation made from glass fiber.

R-Value: The measured resistance of a material to heat transfer. Every material used for construction has an R-Value and the R-Value is the amount of resistance to heat flow through any given construction material.

Savings-to-Investment Ratio (SIR): The life-cycle savings of an energy improvement divided by the initial investment. (For every dollar invested in an energy improvement, how many dollars will be saved over the life of the improvement in today’s dollars)

Sealed Combustion Venting System: Uses either a concentric or a two-pipe vent. (A concentric vent is one in which there is a small pipe inside a larger pipe.) One pipe is designated to bring in combustion air, and the other is designated for removal of combustion by-products out of the home. No combustion air comes from inside the home.

Seasonal Efficiency: Total system efficiency. Includes Annual Fuel Utilization Efficiency (AFUE) and distribution losses. Average efficiency over the course of a heating season.

SEER: Seasonal Energy Efficiency Ratio . This is a measure of the energy efficiency of the air conditioning system. SEER ratings permit consumers to compare operating costs of various cooling systems and products. The higher the SEER, the more efficient the system.

Simple Payback: The number of years it takes for annual savings from an energy improvement to equal the initial investment.

Soffit: The underside of a roof overhang. A dropped soffit is a small lowered ceiling often found above cabinets.

Solar Heat Gain Coefficient: The percentage of solar heat that is absorbed vs. reflected when it contacts a window.

Stack Effect: Occurs when a building is heated, and the warm air inside the building is less dense than the colder air outside. The inside air rises up and out of any holes in the upper portions of the envelope. This escaping air is replaced with outside air that enters through holes in the lower portions.

Supply-Only: A type of mechanical ventilation that uses a fan assembly to bring air into the home, but which relies on building leakage or planned outlet dampers to remove moisture and contaminants.

Temperature: A measurement related to the amount of kinetic energy within a material or substance. The greater the kinetic energy, the higher the temperature.

Therm: A unit of heat equal to 100,000 British thermal units.

Thermal Boundary: The insulation boundary that separates conditioned from non-conditioned spaces in a building. It should be continuous and aligned with the pressure boundary.

Thermal Bypass: A material or component piece that allows more heat transfer when compared to adjoining component pieces.

Thermodynamics: A branch of physics that explains the effect of temperature and heat, and the conversion of energy from one form to another.

Thermostat: Control for maintaining heat at a prescribed temperature.

Top Plate: A horizontal framing member at the top of a wall.

Unconditioned Space: An area or zone in a building that is not intentionally conditioned.

Urethane: A common foam insulation; available in sheet or spray form.

U-Value: A measure of thermal transmittance (how fast heat moves through a material).

Vapor Barrier / Vapor Diffusion Retarder: A material with low permeability, used to reduce moisture migration through building components.

Ventilation: Air turnover (intentional and unintentional) in the home.

Water Vapor: Water in the form of gas. Water vapor is transported by air movement caused by pressure differences.

Windwashing: Reductions in R-value of insulation caused by air movement through insulation, commonly occurring in attics where baffles have not been installed.

Winter Design Temperature: A regional calculated average indoor temperature designed to determine the heating and cooling load for a specific home.