Understanding the Green and Energy Efficiency language can be just like learning a foreigh language.  We hope this list of definitions helps!


Absorptance – a measure of how much heat from the sun is absorbed by the roof. The number falls between zero and one. The closer the number is to 1.0, the more energy is absorbed. In climates that use more air conditioning than heating, it can be beneficial to have a roof with a number less than 0.5.

Air Barrier – the building components that reduce or stop conditioned air from moving between inside and outside. This is typically the ceiling, the lowest floor, and the walls (but can also be the roof). To be effective the air barrier must be continuous and must be in contact with the thermal boundary.

AFUE (Annual Fuel Utilization Efficiency) – the estimated energy efficiency of heating systems that use fossil fuels. It accounts for all losses related to the actual heating unit.

Base Load – the energy used by the equipment in your home that runs year round. This includes the water heater, lights, appliances and entertainment equipment. Air conditioners and heaters are not part of the base load.

Batt – a blanket of insulation that is provided in standard widths and thicknesses.

Blower Door – a large fan, removable door panel and gauges used to measure the degree of air leakage in a building’s envelope, and also used to locate the leaks in the air barrier.

BPI (Building Performance Institute) - BPI develops standards for energy efficiency retrofit work using an open, transparent, consensus-based process built on sound building science.

Btu (British thermal unit) – an international standard unit of measuring energy content or power; one Btu is the amount of heat needed to raise one pound of water one degree Fahrenheit. An MBtu is one million Btu’s.

CAZ (Combustion Appliance Zone) – A contiguous air volume within a building that contains a combustion appliance; the zone may include, but is not limited to, a mechanical closet, mechanical room, or the main body of a house, as applicable

Cellulose Insulation – insulation made from recycled newspapers, cardboard and/or wood waste that is treated with a fire retardant.

CFL (Compact Fluorescent Light bulb) – A smaller version of standard fluorescent lamps which can directly replace standard incandescent lights.  These highly efficient lights consist of a gas tube , and a magnetic or electronic ballast.

CFM (Cubic Feet per Minute) – A measurement of airflow that indicates how many cubic feet of air pass by a stationary point in one minute

CFM 50 (Cubic Feet per minute at 50 Pascals) – The typical measurement rate of a blower door test.  This indicates depressurizing the house to 50 Pascals with reference to the outdoors.  This simulates a 20 mph wind acting on all sides of the house equally. 

CO (Carbon Monoxide) – a colorless, odorless, deadly gas that can be produced by any equipment that burns fossil fuels.  CO is a byproduct of incomplete combustion

COP (Coefficient of Performance) – a measure of the efficiency of a heating or cooling system that uses electricity as its fuel. The number typically falls between 1 and 10 with 10 being the most efficient. Typically used only for ground source heat pumps.

Cooling Load – the maximum amount of heat removal required from the air conditioner when the outside temperature and humidity are at design conditions.

Dense Pack Insulation – a method of using loose fill insulation (cellulose or fiberglass) to completely fill a wall, floor, or ceiling cavity enough to significantly reduce air movement. Fibrous insulation (e.g. fiberglass or cellulose) will not stop air flow, but can reduce it when applied under pressure in an enclosed cavity.

Design Conditions – the temperature and humidity measurements used to calculate the amount of heating or cooling capacity needed by the house to maintain comfort. The temperature values are dependent on how severe the weather typically is in that location.

EER (Energy Efficiency Ratio) – a measurement of the energy efficiency of air conditioners. Typically it is only applied to window or through the wall air conditioners. The number typically falls between 5 and 10 - the higher the number, the better.

Energy Factor (EF) – a measure of the efficiency of a water heater. The value typically falls between 0.5 and 1, although the most efficient systems can have an energy factor greater than 2.

Envelope – the part of the house that separates inside from outside. Typically the envelope is comprised of the walls and windows, the lowest floor, and the ceiling.

Exfiltration – Uncontrolled flow of air out of the house. This can be caused by wind, fans and/or the stack effect.

Fiberglass – an insulation material made by spinning molten glass.

Heating Load – the maximum amount of heat needed by the house during design conditions.

HERS Index (Home Energy Rating System) - The HERS Index measures a home’s energy efficiency as compared to a geometric copy of your home build to current standards.  The lower the score, the more energy efficient. 

HRV (Heat Recovery Ventilator)/ ERV (Energy Recovery Ventilation) – A device that captures the heat or energy from the exhaust air from a building and transfers it to the supply/fresh air entering the building to preheat the air and increase overall heating efficiency while providing consistent air.

HSPF (Heating Seasonal Performance Factor) – a measure of the efficiency of an air source heat pump during the heating season. The number typically falls between 6 and 12 - the higher the number, the more efficient the equipment.

HVAC – Heating Ventilation and Air Conditioning

Infiltration – the uncontrolled flow of air into the house. This can be caused by the wind, fans and/or the stack effect.

Insulation – any material with a relatively high thermal resistance or R-value.

IR Imaging (Infrared Imaging)/ Thermography – By using an Infrared camera, this allows the operator to view radiant thermal image of an object.  This is not night vision.

kWh (kilowatt hour) – a unit of electric energy equal to 3412 Btu’s

LED (Light Emitting Diode lighting) – An extremely efficient semiconductor light source.  LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, and smaller size

N-Factor – A factor of how susceptible your house is to wind.  Used in the calculations of NACH.

NACH (Natural Air Changes per Hour) – The number of times in one hour the entire volume of air inside the building leaks to the outside naturally.

Mastic – a thick, gooey material used to seal duct joints.

Mechanical Ventilation – the use of a fan to control the amount of air that moves into and out of a building.

Pascal – A unit of pressure measurement in the metric system.  A pascal is equal to the weight of a post it note.  6895 pascals = 1 p.s.i.

R-value – a measure of a material’s resistance to heat flow - the higher the number, the better. R-value is the inverse of U-factor.

Radiant Barrier – typically a shiny material that reflects radiant heat waves (e.g. heat from the sun) away from the house. This can be beneficial in climates where air conditioning requires more energy than heating on an annual basis.

Radon –A naturally occurring radioactive gas found in the U.S. in nearly all types of soil, rock, and water.  It can migrate into most buildings.  Studies have linked high concentrations of radon to lung cancer.

Rim Joist – In the framing of a deck or building, a rim joist is the final joist that caps the of the row of joists that support a floor or ceiling. A rim joist makes up the end of the box that comprises the floor system.

SEER (Seasonal Energy Efficiency Ratio) – a measurement of the efficiency of a central air conditioner or the cooling efficiency of a heat pump. The number usually falls between 8 and 20, but can be higher. The higher the number, the more efficient the equipment.

SIR (Savings to Investment Ratio) -  A ratio used to determine whether a project that aims to save money in the future is worth doing.  The ratio compares the investment that is put in now with the amount of savings from the project.

SHGC (Solar Heat Gain Coefficient) – a measure of how effective a window is at preventing radiant heat from passing through the glass. The number falls between 0.0 and 1.0 - the lower the number the more effective the window is.

Thermal boundary – the surface of a house that reduces heat movement between inside and outside by the application of insulation. The thermal boundary should be continuous and be in direct contact with (or be part of) the air barrier.

U-factor – a measure of the ability of a material or combination of materials to transfer heat. Window efficiency is typically expressed as a U-value or U-factor. The U-factor is the inverse of the R-value. The U-factor is usually less than 1 for windows. The smaller the number, the better the window performs.

Jon Howes
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