Energy Efficient Glossary

General Terms - Windows - Heating and Cooling - Air Quality - Insulation
Water - Electric / Lighting - Solar

General Terms:

Alternative Energy - Energy from a source other than the conventional fossil-fuel sources of oil, natural gas and coal (i.e., wind, running water, the sun). Also referred to as "alternative fuel."

Certified or Certification - A process by which an independent agent verifies that the claims made by a product service, etc. are valid. Many certification programs exist through which products meeting independent standards may use a label or logo to indicate their claims have been verified.

Energy Efficient - Refers to the process of maximizing the ratio between energy use and product output.

Energy Efficiency - Ways and technology that can reduce the amount of electricity or fuel used to do the same work. Such as keeping a house warm using less energy.

Energy Star - A program developed by the Environmental Protection Agency (EPA) in order to reduce the nation's energy consumption. ENERGYSTAR® qualified heating equipment can be up to 15 percent more efficient than standard models. Energy Star-qualified cooling equipment can be up to 7 percent more efficient than minimum-standard equipment.

Energy Star Rating - Refers to the process of maximizing the ratio between energy use and product output.

Green Design - A design, usually architectural, conforming to environmentally sound principles of building, material and energy use. A green building, for example, might make use of solar panels, skylights and recycled building materials.

H.E.R.S. - For a new home, HERS, Inc.™ typically starts with a review of the plans and recommendations for making the home more efficient, affordable and comfortable. Intermediate inspections are provided during construction and a final verification inspection and commissioning of the house is provided at completion. Testing includes blower door testing for whole house leakage and duct leakage testing to measure duct leakage to the outside. With an existing house, going green should start with a whole house, diagnostic energy audit that looks at all aspects of the house "as a system." An audit includes prioritized recommendations with estimated dollar savings.

LEED - Refers to the U.S. Green Building Council's Leadership in Energy and Environmental Design. The Green Building Rating System has become an accepted benchmark for the design as well as the construction and operation of green buildings and homes. The system contains five critical areas; which are water savings, sustainable site development, materials selection, energy efficiency and indoor environmental quality.

Renewable Energy - Refers to the use of natural resources such as wind, sunlight, earth heat and tides in order to create a separate form of energy like electricity.

Sustainability - Meeting the needs of the present without compromising the ability of future generations to meet their needs.


Absorptance - The ratio of radiant energy absorbed to total incident radiant energy in a glazing system.

Air infiltration - The amount of air leaking in and out of a building through cracks in walls, windows and doors.

Air-leakage rating - A measure of the rate of air-leakage around a window, door, or skylight in the presence of a specific pressure difference. It is expressed in units of cubic feet per minute per square foot of frame area (cfm/sq ft). The lower a window's air-leakage rating, the better its air tightness.

Argon gas windows - Argon is inert and safe gas that occur naturally in the atmosphere. They are invisible and heavier than air, thus they are denser than normal air. These gases are added between the two panes of glass as they are manufactured into their sealed glass units. Once these gases are installed into a window, they are denser than regular air.

Condensation - The deposit of water vapor from the air on any cold surface whose temperature is below the dew point, such as a cold window glass or frame that is exposed to humid indoor air.

Convection - A heat transfer process involving motion in a fluid (such as air) caused by the difference in density of the fluid and the action of gravity. Convection affects heat transfer from the glass surface to room air, and between two panes of glass.

Double glazing - In general, two thicknesses of glass separated by an air space within an opening to improve insulation against heat transfer and/or sound transmission. In factory-made double glazing units, the air between the glass sheets is thoroughly dried and the space is sealed airtight, eliminating possible condensation and providing superior insulating properties.

Emittance - The ratio of the radiant flux emitted by a specimen to that emitted by a blackbody at the same temperature and under the same conditions.

Insulating glass - Two or more pieces of glass spaced apart and hermetically sealed to form a single glazed unit with one or more air spaces in between. Also called double glazing.

Insulating value - (See U-factor)

Low E-Windows - Low emissivity windows reflect heat, not light, keeping spaces warmer in winter and cooler in summer.

Low-emittance (Low-E) coating - Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. A typical type of low-E coating is transparent to the solar spectrum (visible light and short-wave infrared radiation) and reflective of long-wave infrared radiation.

Polyvinylchloride (PVC) - An extruded or molded plastic material used for window framing and as a thermal barrier for aluminum windows.

Pyrolytic low-E - A low- E coating which typically uses tin oxide with some additives deposited directly onto a glass surface while it is still hot. The result is a baked-on surface layer that is hard and durable and thus sometimes referred to as a "hard coat." Pyrolytic coatings are typically used in insulated glass units with the low-E surface inside the sealed air space, but can also be applied to single-pane glass and separate storm windows.

R-value - A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-sq ft-°F/Btu. A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value.

Reflective glass - Window glass coated to reflect radiation striking the surface of the glass.

Solar heat gain coefficient (SHGC) - The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window's shading ability. It is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability.

Thermal expansion - Change in dimension of a material as a result of temperature change.

Thermal mass - Mass in a building (furnishings or structure) that is used to absorb solar gain during the day and release the heat as the space cools in the evening.

U-factor (U-value) - A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-°F (US) The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value.

Ultraviolet light (UV) - The invisible rays of the spectrum that are outside of the visible spectrum at its short-wavelength violet end. Ultraviolet rays are found in everyday sunlight and can cause fading of paint finishes, carpets, and fabrics.

Weatherstripping - A strip of resilient material for covering the joint between the window sash and frame in order to reduce air leaks and prevent water from entering the structure.

Thermal Break - A non-conducting material positioned between metal components of windows to reduce thermal conduction. In building assemblies, a thermal break reduces conductive heat transfer through materials like wood or metal framing studs.

Heating and Cooling

Ambient Air - Open air, surrounding air, or outside air.

Air Handler - The indoor component of your air conditioner or heating system that moves air throughout your home.

British Thermal Unit (BTU) - In scientific terms, it represents the amount of energy required to raise one pound of water one degree Fahrenheit. One BTU is the equivalent of the heat given off by a single wooden kitchen match. For your home, it represents the measure of heat given off when fuel is burned for heating, or the measure of heat extracted from your home for cooling.

Capacity - The output or producing capability of a piece of cooling or heating equipment. Cooling and heating capacity are normally referred to in BTU's.

CFM (Cubic feet per minute) - A standard of airflow measurement. A typical system produces 400 CFM per ton of air conditioning.

Compressor - The heart of an air conditioning or heat pump system. It is part of the outdoor unit that pumps refrigerant. The compressor maintains adequate pressure to cause refrigerant to flow in sufficient quantities in order to meet the cooling requirements of the system and your home.

Condenser Coil or Outdoor Coil - Located in the outdoor unit, the coil dissipates heat from the refrigerant, changing the refrigerant.

Damper - Found in ductwork, this movable plate opens and closes to control airflow. Dampers are used effectively in zoning to regulate airflow to certain rooms.

Dehumidifier - A mechanical device which removes moisture vapor from the air.

Ductwork - Pipes or channels that carry air throughout your home.

Dual fuel heating/cooling system - A dual-fuel heat pump is an electric heat pump and a gas furnace all in one. When the temperature is above 35 degrees or so, the dual-fuel heat pump uses electricity to heat your home as necessary. This type of heat circulates evenly throughout your home, and isn't too dry. When it gets really cold outside (around 35 degrees or lower), the heat pump automatically switches to supplemental gas heat for better efficiency.

ERV (Energy Recovery Ventilators) - Energy recovery ventilation systems provide a controlled way of ventilating a home while minimizing energy loss. They reduce the costs of heating ventilated air in the winter by transferring heat from the warm inside air being exhausted to the fresh (but cold) supply air. In the summer, the inside air cools the warmer supply air to reduce ventilation cooling costs.

Geothermal - Referring to earth heat. This term is commonly used to describe alternative energy sources.

Gross Square Feet (GSF) - The total area occupied by a building when measured from exterior to exterior. This area included all mechanical areas.

HEPA (High Efficiency Particulate Absorbing) filter - A HEPA filter removes particles from the air by trapping them as air flows through.

Heat Recovery Systems - Building mechanical systems that capture waste heat from another system and use it to replace heat that would otherwise come from a primary energy source.

Hydronic HVAC - Water based HVAC

Hydronic System - Refers to a cooling or heating system that uses the circulation of water as a heat transferring medium.

Humidifier - A device that adds moisture to dry/indoor air during winter time and in hot, dry climates.

HVAC - The classic acronym for Heating, Ventilation and Air Conditioning. Today, an HVAC system also includes air cleaning and -moisture control.

Indoor Air Quality (IAQ) - The supply and introduction of adequate air for ventilation and control of airborne contaminants, acceptable temperatures and relative humidity.

Indoor Coil - The other, less visible half of your outdoor unit. It's attached to your furnace or air handler. As indoor air flows across it, heat and moisture are drawn out, leaving air that is cool, comfortable and conditioned.

Indoor/Outdoor System (also Split System) - Refers to a comfort system consisting of components in two locations. Common examples include an outside unit, such as an air conditioner, and an indoor unit, such as a furnace with a coil.

Mastic air duct work Economical water based duct mastic - This is a fiber reinforced and rated for high velocity HVAC Systems. It be used to seal all types of HVAC duct systems including metal, fiberglass, duct board and flex duct. Preferred by residential and commercial duct sealing professionals alike.

MERV - The Minimum Efficiency Reporting Value for air filtration.

Natural Ventilation - The process of supplying and removing air through an indoor space by natural means. There are two types of natural ventilation for buildings: wind driven ventilation and stack ventilation.

Passive Cooling - A building's structure (or an element of it) is designed to permit increased ventilation and retention of coolness with the intention of minimizing or eliminating the need for mechanical means of cooling.

Passive Ventilation - The introduction and/or removal of air that used both convective air flows resulting form the tendency of warm air to rise and cool air to sink, and takes advantage of prevailing winds.

Programmable Thermostats - A programmable thermostat helps make it easy for you to save by offering four pre-programmed settings to regulate your home's temperature in both summer and winter - when you are home, asleep, or away.

SEER - Seasonal Energy Efficiency Ratio, a rating that measures the cooling efficiency of a heat pump or air conditioner. The higher the number, the more efficient the unit.

Thermostat - A temperature-control device, typically found on a wall inside the home. It consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system.

Ton - unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTU's per hour.

Variable-Speed motor - The fan motor inside higher-efficiency indoor and outdoor units is designed to change its speed based on your home's heating and air conditioning requirements. Working in conjunction with your thermostat, it keeps the appropriate-temperature air (e.g., warm air on cold days) circulating throughout your home, reducing temperature variances. The variable-speed motor also increases dehumidification and is quiet because it runs at a lower speed most of the time.

Air Quality

Air Exchange Rate - The rate at which outside air replaces indoor air in a space. Expressed in one of two ways: the number of changes of outside air per unit of time air changes per hour (ACH); or the rate at which a volume of outside air enters per unit of time - cubic feet per minute (cfm).

Biological Contaminants - Agents derived from, or that are, living organisms (e.g., viruses, bacteria, fungi, and mammal and bird antigens) that can be inhaled and can cause many types of health effects including allergic reactions, respiratory disorders, hypersensitivity diseases, and infectious diseases. Also referred to as "microbiologicals" or "microbials."

Exhaust ventilation - Mechanical removal of air from a portion of a building (e.g., piece of equipment, room, or general area).

Indoor Air Quality (IAQ) - Pollution from gases or particles released into the air is the primary cause of indoor air quality problems. Inadequate ventilation can increase indoor pollutant levels by not bringing in enough outdoor air to dilute emissions.

Infiltration - Outside air that enters a structure through openings or cracks in the construction materials, especially windows and doors. "Design" infiltration in residences can range from one-half air change to three air changes per hour, depending on how well the houses are constructed, caulked, or weather-stripped. Average air changes over the heating season are lower. Infiltration is a major area of home heat loss.

Low-VOC - A term referring to reduced amounts of volatile organic compounds (VOCs) in paint and finishes Low-VOC paints do not off-gas as much as conventional paints and contain less toxins that are harmful to the environment.

Mechanically ventilated crawlspace system - A system designed to increase ventilation within a crawlspace, achieve higher air pressure in the crawlspace relative to air pressure in the soil beneath the crawlspace, or achieve lower air pressure in the crawlspace relative to air pressure in the living spaces, by use of a fan.

Outdoor air supply - Air brought into a building from the outdoors (often through the ventilation system) that has not been previously circulated through the system. Also known as "Make-Up Air."

Ventilation rate - The rate at which outdoor air enters and leaves a building. Expressed in one of two ways: the number of changes of outdoor air per unit of time (air changes per hour, or "ach") or the rate at which a volume of outdoor air enters per unit of time (cubic feet per minute, or "cfm").

Volatile Organic Compounds (VOCs) - Chemicals that contain carbon molecules and are volatile enough to evaporate from material surfaces into indoor air at normal room temperatures (referred to as off-gassing).

Zero-VOC - A term used to indicate paint containing no volatile organic compounds- a healthier alternative to conventional paints


Air sealing - A weatherization strategy that reduces air leaks in a home, measured by blower door test readings. "Major air sealing" includes sealing large openings between the heated/unheated spaces. "Minor air sealing" includes sealing small air gaps with caulk, weather stripping, and gaskets.

Baffles - Device to maintain a ventilation space between the insulation and roof deck, assuring air flow from the eave/soffit vents to ridge vent or other roof vents provided in attics and cathedral ceilings (Owens Corning product is Raft-R-Mate Batt Insulation that is pre-cut to a certain width.

Blow in loose - fill Material (usually mineral wool or cellulose) used for pouring or blowing into the space to be insulated. Air is usually the blowing agent. It air seals simultaneously as it insulates.

Building or Thermal Envelope - The entire perimeter of a building enclosed by its roof, walls and foundation. Properly designed the envelope can minimize temperature gain or loss and moisture infiltration. Five components of a building that ensure maximum retention and minimal loss of heat, minimal chemical fumes and other factors that contribute to a safe, secure living environment:

  • Weather Barrier - a building's outer skin that keeps it dry.
  • Air Barrier - limits air leakage or infiltration from outside.
  • Thermal Barrier - prevents movement of heat in or out.
  • Vapor Barrier - prevents movement of moisture in or out.

Interior finish-overall design, low-or no VOC paint, UL ratings. The Thermal envelope is becoming much more of a concern and certainly the first consideration for reaching our environmental and economical goals including more livable space and longer building life.

Fiber Glass Insulation - An energy-efficient glass fiber product manufactured by Owens Corning to ensure the best thermal and noise control performance available.

Icynene - is a spray-on foam insulation that you can use to insulate a home. Typically it would be used during new construction because it needs to be sprayed on like paint before the sheet rock goes on. The advantages of Icynene include: Higher R values (because it is uniform), much lower air infiltration from outside, better sound insulating properties, better moisture barrier. It can reduce heating and cooling costs by as much as 50%.

Insulation Density - Denser insulation products have more fibers per square inch and, therefore, give you greater insulating power through higher R-values.

Kraft-Faced Vapor Retarder - Created by coating kraft paper with a thin layer of asphalt adhesive. The coated side of the kraft paper is then applied to the unfaced insulation material. The asphalt adhesive bonds the kraft paper and the insulation together.

Polycel foam - is expanding Foam it's ideal for filling and sealing awkward gaps, hollows and cavities. It shoots liquid foam deep into cavities, creating a long lasting seal, which insulates against heat, sound and moisture. Suitable for inside or out, it can be easily trimmed and sanded ready to be painted, filled or plastered. Contains a CFC - free propellant.

R-Value - This measures a material's tested resistance to heat flow. Higher insulation R-value means greater resistance, and better insulation. R-values are quoted per inch, thus five one-inch layers of R-5 material yield a total insulation value of R-25.

Radiant barrier - A material that reflects radiant heat, typically a foil-faced or foil-like material. A common installation is to install in an attic to reduce solar heat gain. It requires an air space to work effectively.

Ridge Vents - A vent mounted along the entire ridge line of the roof to allow the passage of air through the attic or cathedral ceiling.

Spray foam - Includes multiple types, ensure protection from direct sunlight. Closed-cell (includes brands such as Versi-Foam and Heatlok).High density spray foam that is structural. Polyurethane foam made from petroleum products. Can contain CFCs or HCFCs and off- gases as it is applied. It air seals simultaneously as it insulates. Acts as a vapor barrier reducing water vapor transport.


Greywater - Wastewater that does not contain sewage or fecal contamination and can be reused for irrigation after simple filtration.

Recirculated Water - Rinse water that is reused before it is discarded, or water continually moving through a system, as in a fountain.

Hybrid hot water heater - A hybrid water heater combines tank type and tankless technology in one unit. A hybrid unit offers the best of both worlds: continuous hot water capabilities through high efficiency and the ability for homeowners to use multiple hot water-utilizing appliances simultaneously. Hybrid units offer optimum performance as they combine the positive attributes of both tank type and tankless water heaters.

Low flow plumbing fixtures - it's not just low flow, it's the law. In 1995, the National Energy Policy Act mandated the use of toilets that use no more than 1.6 gallons of water per flush. Since then, low-flow plumbing fixtures including toilets, faucet aerators and showerheads have been developed that save substantial amounts of water compared to conventional fixtures while providing the same utility. Different types of low-flow toilets use various technologies aimed at making the toilet more functional. Some toilets have large drain passages, redesigned bowls and tanks for easier wash down. Others supplement the gravity system with water supply line pressure, compressed air, or a vacuum pump.

Low-flow toilets use a maximum of 1.6 gallons of water per flush compared with about 3.5 gallons of water used by a standard toilet. Low-flow shower heads use about 2½ gallons of water per minute compared to between four and five gallons per minute used by conventional heads. Low-flow faucet aerators can cut the water usage of faucets by as much as 40% from 4 gallons per minute to 2½.

Electric / Lighting

Incandescent Light - An electric lamp in which a filament is heated to produce artificial light. Incandescent lighting consumes more energy and is less efficient than CFLs or LEDs.

LCD - Liquid crystal display.

LED - Light emitting diode.

Occupancy Sensors - Mechanisms that automatically turn off lighting, HVAC, and/or electricity once a room is vacant.

Kilowatt Hour (kWh) - A unit of energy measured at 1,000 watt hours.

Kilowatt Peak (kWp) - A measure of peak kilowatt output (e.g. of a photovoltaic system).


Absorber - The blackened surface in a collector that absorbs the solar radiation and converts it to heat energy.

Active Solar Heating Collection units - Absorb heat from the sun and transfer it through pumps to a storage unit. The fluid in the storage unit conveys its heat to the domestic hot water of the building through a heat hanger. The system requires controls to regulate its operation.

Active Solar Water Heater Collection units - Absorb heat from the sun and transfer it through pumps to a storage unit. The fluid in the storage unit conveys its heat to the domestic hot water of the building through a heat hanger. The system requires controls to regulate its operation.

Air System Solar - Domestic hot water systems employing air-type collectors are available. Hot air generated by these collectors is fan forced through an air-to-liquid heat exchanger with the potable water being pumped through the liquid section of the exchanger. The heated water is then circulated through the storage tank in a similar fashion to the liquid collector system. Air does not need to be protected from freezing or boiling, is non-corrosive, and is free. However, air ducts and air handling units require greater space than piping, and air leaks are difficult to detect.

Cell (solar) - A single unit of a photovoltaic panel capable of converting sunlight into direct electrical current

Collector Efficiency - The ratio of usable heat energy extracted from a collector to the solar energy striking the cover.

Cross Ventilation - Placement of vent openings so that air flows in one vent, over insulated space, and out the other. In attics, a combination of eaves and ridge or high gable vents is best for cross-ventilation, because this takes advantage of natural convection.

Dead Air - Air trapped between two infiltration-free spaces which serves as a good insulator. This insulating principle is usually applied in storm doors and windows.

Ground Mount - A solar power system that is built on a ground anchored frame instead of mounted to a roof.

Hybrid Solar Energy System - A system that uses both active and passive methods in its operation.

Inverter - An inverter is an electrical device designed to convert the direct current output of solar panels to usable alternating current.

Kilowatt Hour (kWh) - A kWh is the unit of energy used to quantify the production of a photovoltaic system. 1 kWh = 1,000 watts generated over a period of one hour.

Net Meter - An electricity meter that tracks how much electricity is drawn from/added to the utility grid.

Net Metering - An agreement between a solar system owner and the local electric utility that allows the system owner to buy and sell energy in the form of electric credits. When the solar system produces excess energy, it is sold back to the electric utility at peak prices, literally causing the electric meter to spin backwards. When the system is not producing energy, the system owner can use the credits to buy back energy at off-peak prices.

On/Off Grid System - A solar energy system that is interconnected with the utility grid is said to be an on-grid or grid-tied system, while a system with battery storage is not interconnected and is described as an off-grid system.

Orientation - The placement of a solar system in relation to the cardinal directions: N, S, E, W

Passive Solar Water Heater - A water heating system that does not require mechanical pumps or controls to create hot water for domestic use.

Photovoltaic Panels - Devices using semiconductor material to directly convert sunlight into electricity. Power is produced when sunlight strikes the semiconductor material and crates an electrical current.

Photovoltaic (PV) - A system that converts sunlight directly into electricity using cells made of silicon or other conductive material. When sunlight strikes the cells, a chemical reaction occurs, and this results in the release of electricity. Photovoltaic PV systems convert sunlight and ultra violet light directly into electricity. Solar thermal systems use a different technology that uses o sunlight to heat water or air, often used to heat swimming pools. SolarCity primarily designs and sells PV solar systems.

R-Value - Measure of resistance to heat flow. Insulation materials have tiny pockets of trapped air. These pockets resist the transfer of heat through material. The ability of insulation to slow the transfer of heat is measured in R-values. The higher the R-value, the better the insulation material's ability to resist the flow of heat through it.

Racking - The mechanical structuring that attaches a solar array to the roof of a building, or to the ground.

Solar Heat Gain coefficient (SHGC) - Measures how well a window blocks solar heat. SHGC is expressed as a number between O and 1, and lower numbers indicated better blocking ability. Limiting solar heat gain is desirable in hot climates, but not in cold ones.

Solar Panel - Refers to a device that is used for the collection of energy from sunlight that is then converted into heat or electricity.

Solar window screens - Refers to a mesh screen that can be used in order to block out insects, heat and light.

Sound Absorption - The process of dissipating or removing sound energy; the property possessed by materials, objects and structures (such as rooms) of absorbing sound energy; the measure of the magnitude of the absorptive property of a material, object or structure.

Stand-alone/Off Grid - A solar power system that is not connected to the local utility power grid and instead utilizes batteries to store excess power generated during the day for use at night.

Tilt Angle - The angle at which a solar array is set to face the sun and produce maximum power. Tilt angle may be flat or adjusted seasonally.

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