The EMS Energy Encyclopedia

Your Guide to Everything Energy


Absolute Humidity: Air Moisture content expressed in grains (or pounds) of water vapor per pound of dry air. Also call humidity ratio

Absorptance: The ratio of a solar energy absorbed to incident solar 

Absorption: A solid material's ability to draw in and hold liquid or gas

Accent Lighting: Lighting that Illuminates walls, reducing brightness contrast between walls and ceilings or windows

Ambient Lighting: Lighting spread throughout the lighted space for safety, aesthetic, and security

Air Barrier: Any part of the building shell that offers resistance to air leakage. The air barrier is effective if it stops most air leakage. The primary barrier is the most efficient of a series of air barriers

Air Changes per hour at 50 pascals (ACH50):  The number of times that the complete volume of a home is exchanged for outside air each hour when a blower door depressurizes the home to 50 pascals. 

Air Conditioning: Cooling buildings with a refrigeration system. More generally means both heating and cooling. 

Air Exchange: The total building air exchanged with the outdoors through air leakage and ventilation

Air Handler: A steel cabinet containing a blower with cooling and/or heating coils connected to ducts

Asbestos: A material made of brittle mineral fibers that damages lungs and other bodily tissues

Audit: The Process of Identifying energy conversation opportunities in a building

Ampere: The unit of measurement of electrical current flow. A coulomb per second

Annual Fuel Utilization Efficiency:  A laboratory derived efficiency rating for heating appliances which accounts for chimney loses, jacket losses, and cycling losses

Annual Return:  The yearly savings divided by the the initial cost needed to achieve the savings expressed as a percent 

Approach Tempurature:  The temperature difference between the fluid inside a heat exchanger and the fluid outside it. 

Aquastat: A heating control that switches the burner or the circulator in a hydronic heating system 


Backdrafting: Continuous spillage of combustion gases from a combustion appliance

Backdraft Damper: A Damper installed near a fan, that allows air to flow in only one direction

Backer Rod: Polyethylene foam rope used as a backer for caulking

Baffle: A plate or strip designed to retard or redirect the flow of gases

Balance Point: The minimum outdoor temperature at which no heat is needed

Ballast: a coil of wire of electronic device that provides a high starting voltage for a lamp and limits the current from flowing through it

Band Joist: See Rim Joist

Batt: A narrow blanket of fiberglass insulation often 14.5 or 22.5 inches wide

Beam: A strong horizontal building support used to carry the weight of a floor or roof

Bimetal Element: A metal spring, lever, or disc made of two dissimilar metals that expand and contract at different rates as the temperature around them changes. This movement operates a switch in the control circuit of a heating or cooling device

Blown-down: Draining water from a boiler, evaporated cooler, water heater, etc. to remove sediment and suspended particulates 

Blower: The Squirrel cage fan in a furnace to air handler

Blower Door

Blower Door

Blower Door: A device that consists of a fan, a removaable panel and gauges used to measure and locate air leaks 

Boot: A duct section that connects between a duct and a register

Branch Circuit: An electrical circuit user to power outlets and lights within a home

Brightness:  The Intensity of sensation resulting from viewing a lit surface measured in footlamberts. Also called luminance or luminous intensity

British Thermal Unit (BTU): The quality of heat required to raise the temperature of one pound of water one degree Fahrenheit 

BTUH: British Thermal Unit per Hour

Building Cavities: The spaces inside walls, floors, and ceilings between the interior and exterior sheeting

Building Science: Branch of science dealing with construction, maintenance, safety, and energy efficiency of buildings

Building Tightness Limit: See minimum ventilation guideline

Burner:  A device that Facilitates the burning of a fossil fuel like gas or oil


Capillary Action: The ability of water to move through materials, even upward against gravity, through small tubes or spaces

Capillary Barrier: A material of air spec designed to stop capillary action from carrying water into a building

Carbon Dioxide:  One of two maing products of complete combustion of hydrocarbon (the other is water vapor)

Carbon Monoxide: An orderless and poisonous gas produced by incomplete combustion 

Caulking:  A mastic compound for filling joints and cracks

Celsius: A temperature scale on which water freezes at 0ºC and boils at 100ºC

Cellulose Insulation

Cellulose Insulation

Cellulose Insulation:  Insulation, packaged in bags from blowing, made from newspaper or wood waste and treated with a fire retardant. 

Centigrade: See Celsius

CFM50: The number of cubic feet per minute of air flowing through the fan housing of a blower door when the house pressure is 50 pascals. This figure is the most common and accurate way of comparing the airtightness of buildings that are tested using a blower door. 

CFMn: The number of cubic feet of air flowing through a house from indoors to outdoors during typical natural conditions. This figure can be roughly estimated using a blower door. 

Circuit Breaker: A device that disconnects an electrical circuit from electricity when it senses excessive current. 

Coefficient of Performance:  A heat pump or air conditioner's output in watt-hours of heat moved divided by watt-hours of electrical input.

Coil: A snakelike piece of copper tubbing surrounded by rows of aluminum fins that clamp tightly to the tubing in order to aid the heat transfer

Color-Rendering Index: A measurement of a light source's ability to render colors the same as sunlight. CRI has a scale of 0-100

Color Temperature: A measurement of the warmness of coolness of light source in the Kelvin Temperature scale. 

Column: A vertical building support usually made of wood or steel

Combustible: the ratting for a building materials that will burn under some conditions

Combustion Air: Air that provides oxygen for combustion

Combustion Analyzer: A device used to measure steady state efficiency of combustion heating units

Combustion Chamber: The area inside the heat exchanger where the flame burns 

Combustion Efficiency: Synonymous with fuel-burning efficiency or steady state efficiency. usually means the latter. 

Commissioning: The process of testing and adjusting building mechanical systems after building construction or as a retro fit measure

Compressor:  A motorized vapor pump that compresses the gaseous refrigerant and sends it to the condenser where collected head is released

Condense: When a gas turns into a liquid as it cools, we say it condenses. Condensation is the opposite of evaporation 

Condenser: the coil in an air conditioning system where the refrigerant condenses and releases heat

Condensate: Liquid formed by condensing vapor