Glossary of Green Building Terms


BPC Green Builders prepared this glossary of green building and related terms to help consumers who are researching, considering, or building a green home to better understand green building terms.

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BPC and organizations working with BPC  used their expertise and information from the online sources whose links are at the bottom of the glossary when preparing this glossary. Not all sources agreed on the meanings of some terms. Should you believe any definition in this glossary is inaccurate, or have comments, suggestions, or questions, please let us know at:

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The portion of a solar heating system that soaks up heat energy from the sun which is then sent to the thermal solar system to heat hot water or provide home heating.

Active Solar

This is the use of solar energy that is first converted into another type of energy such as a solar photovoltaic panel, or a solar thermal system that concentrates solar energy to heat water (or other medium) that then generates steam which is converted into electrical power.

Active Solar Heating

This kind of heating is accomplished by using mechanical systems that collect solar thermal radiation and transport that heat via a liquid or air to heat the interior of the home.

Active Solar Power

This method uses a photovoltaic solar (aka PV or PV solar) panel system which converts sunlight into electrical energy.

See PV Solar Panels & Photovoltaic

Active System

In a home, an active system is one that requires mechanical energy to work. Heaters and air conditioners are part of an active HVAC system. The opposite of an active system is a passive system.

See Passive System

Advanced Framing

Framing consists of the structural elements of your home like studs, trusses, and so on that make up the skeleton of a home.

Advanced Framing (aka “Optimum Value Engineering” or “OVE”) are methods that optimize the use of lumber and other framing materials while simultaneously improving the home’s energy efficiency.

For example, an outside wall in a conventional home would normally have vertical 2 x 4 wood studs at 16” intervals that are in contact with the exterior and interior walls of a home. Each stud becomes a thermal bridge from the inside to the outside of your home, reducing energy efficiency. A wall that uses larger (2 x 6 for example) but fewer studs (studs placed at 24” intervals) minimizes the number of thermal bridges. Fewer and deeper studs increase the distance energy must travel to be lost and allows for 6” of insulation instead of 4” of insulation between the inner and outer walls.

Walls can also be built as “double walls” with a gap between the inner and outer wall. This eliminates the studs from being thermal bridges and adds yet more room for insulation.

double wall construction diagram 1

Example of on type of Advanced Framing – Double Wall Construction. Source BPC Green Builders, Inc.

Energy Truss photo and diagram

Example of Energy Truss, an advanced framing approach


AFUE stands for Annual Fuel Utilization Efficiency.

See Annual Fuel Utilization Efficiency

Aging in Place (AIP)

Also called living in place, this is an approach to home design that makes a home safer and easier to use for older homeowners, such as wider doors and halls to accommodate wheelchairs. The objective is to allow homeowners to stay in their homes longer and be safer.

Air Barrier

Air barriers (aka wind barriers) are the parts of a home’s envelope that work to restrict airflow and air infiltration. Air barriers are always on the outside of a home’s sheathing and under the home’s cladding (siding, etc.) and water plane.

Air barriers are different from vapor barriers and moisture barriers, although some building products can function as more than one type of barrier.

See Vapor Barrier

Air Changes per Hour (ACH & ACPH)

ACH and ACPH both stand for air changes per hour. This is how many times the entire volume of air in your home is replaced by an equal volume of air from outside.

The term ACH relates to both intentional and unintentional air changes per hour.

Intentional ACH

Because green homes being built today are almost airtight, fresh air is brought in and filtered while the stale air is exhausted in a controlled manner. The ACH are controlled using an ERV or HRV system (energy recovery ventilation or heat recovery ventilation system), something conventional homes don’t usually have. These systems usually also filter incoming air so the air coming into the home is both fresh and clean. These systems control the number of air changes per hour while at the same time recovering the energy used to heat or cool the inside air being exhausted.

Unintentional ACH (aka Natural Air Changes Per Hour or NACH ACHn)

Unintentional ACH occur and change based on many factors.

One factor is the air tightness of your home’s envelope. Then temperature inside vs outside your home, wind speed and direction, the barometric pressure and other factors can change the unintentional ACH of your home. This type of air change per hour is also called the Natural Air Changes Per Hour.

To measure the air tightness of your home, all the windows and doors in your home are close and one door is replaced with a special “blower door” for a blower door test. The blower door causes the pressure inside vs outside the home to be different by a set pressure differential.

By doing this the technician can calculate how much air is leaking into or out of your home as a base line.

For green homes the unintentional air changes per hour must be at or below the ACH set for the home’s target green building standard. In conventional homes, especially older homes blower door tests find excessive uncontrolled ACH.

See Building Envelope, ERV, and HRV

Air Leakage (AL)

Air leakage, typically referred to on energy performance charts as AL, means the amount of air that gets through or around the building product or building. This may be the air that gets through a window or door frame or the amount of air that is getting through the home’s envelope.

Air leakage lowers energy efficiency and can cause air infiltration into the home. The lower the air leakage is the better.

Some types of products, like windows and doors, have an AL rating.

Air-Source Heat Pump

An air-source heat pump is the most common type of heat pump used by homes in the US. It uses the latent heat in the air (there is latent heat in the air even in the winter) and transfers that latent heat to help heat the home.

Air-source heat pumps vary in their performance in several ways. The most important ways include the lowest outdoor air temperature from which they can still extract heat and the efficiency with which they can convert the heat they extract into heat for the home.

Technically, there is latent heat in even the coldest atmospheric temperatures on earth. However, due to the current level of technology, heat pumps can only extract and transfer latent heat from certain minimum outside temperatures. Therefore, heat pumps are viable options for home heating in all but the coldest climates. High-performance air-source heat pumps can provide all the heat a home needs most of the time; when it is extremely cold, they need some electrical heating to work with, or in place of, the heat pump system.

air source heat pump in home

Air Source Heat Pump Illustration from US DOE

Also see Heat Pump and Ground-Source Heat Pump


Albedo describes how much solar radiation a surface reflects. The scale used is a fraction from 0 to 1, where 0 represents a black surface which reflects no solar radiation and 1 is the albedo of a surface that reflects all the solar radiation. A colored surface albedo might be 0.6 for example.

Relative to green building, the albedo of the materials and colors you use for your home affect how much solar radiation that material would reflect or absorb. Not accounting for insulation, a white roof would reflect more heat, keeping the attic cooler, whereas a black roof would heat the attic.

albedo illustration

Example of colors and approximate Albedos. Source BPC Green Builders, Inc.

Alternative Energy

As more and more energy comes from clean energy sources, this type of clean energy will no longer be considered alternative energy. The important issue here is that the energy is clean and has little to no negative environmental downsides. Relative to green building and climate change, alternative energy consists of those sources which are:

  1. Newer and used less often than conventional energy sources, like coal, gas, and nuclear power.
  2. Clean energy sources such as wind, solar, geothermal, tidal, and hydrogen fuel cells.
  3. Renewable, meaning use of the energy does not diminish the source of the energy.

Alternative energy for other uses such as transportation includes electric, hydrogen, natural, or liquified gas and biofuels. Of these, gas and biofuels are alternative and better than gasoline and diesel, but they are not truly clean sources of energy.

Electricity is only as clean as the way it was generated. An electric car charged with energy from a coal-fired power station is no longer a clean energy vehicle; an electric car charged via solar panels or wind energy is a using clean alternative energy.

Ambient Heat

The heat that is around you or an object, whether inside or outside. Even when it is cold outside, there is still ambient heat in the air that can be used by a heat pump to transfer heat into your home to heat it.

Also see Conductive Heat and Radiant Heat.

Angle of Incidence

This is the angle the sun’s rays hit a surface relative to a perpendicular line from the angle or pitch of that surface. When the sun’s rays hit a surface perpendicularly, they transmit the most solar energy to that surface. Any variation from the perpendicular reduces the potential transferable energy.

Angle of Incidence illustration

Angle of Incidence. Source BPC Green Builders, Inc.

Relative to green building, the angle of incidence is important because it impacts the efficiency of both passive solar thermal heating and active solar systems such as PV solar panels.

The sun’s position relative to a given surface on a home varies over the course of a day and over the course of a year.

Daily, the sun’s angle of incidence changes as the earth rotates and the sun’s position appears to move from east to west.

Annually, the sun appears  higher or lower in the sky, depending on the season, which also alters the angle at which the sun’s rays are to a given surface on the home.

PV solar panel systems have either fixed or movable panels. Movable systems can maintain an optimal angle of incidence for a longer time than fixed panel systems. Some movable systems can track the sun seasonally, and some can track the sun both seasonally and over the course of a day. Movable solar panel systems are more expensive than fixed panel systems and the added performance of a movable panel system can be offset by adding more panels to a fixed system, sometimes at a lower cost than a moveable system. PV solar panel technology is improving, and costs are reducing.

Passive solar heating strategies using the sun’s radiant energy optimize the angle of incidence by allowing the most radiant energy into the home in the colder months and using shade trees, awnings, and roof overhangs in the summer so that when the sun is higher in the sky, less radiant heat can enter the home.

Annual Fuel Utilization Efficiency (AFUE)

As a metric, AFUE is used to measure the fuel efficiency of a heating system. It measures all aspects of real-world use such as starting up, operating, and cooling down. AFUE is different from combustion efficiency.


Argon is one of the gases commonly used in double and triple pane windows to make them even more energy efficient. Argon has less thermal conductivity than normal air used between glass panes in less energy efficient windows.

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Balanced Ventilation

Balanced ventilation is accomplished by having stale air from a home exhausted at the same rate as outside fresh air is brought in. ERV and HRV systems provide balanced ventilation in green homes while greatly minimizing energy loss.

Also see ERV and HRV

Batt Insulation

Batt insulation is typically made from rock wool or fiberglass that is backed by a paper or composite sheet. Batt insulation typically comes in rolls and is usually used between wall studs or beams along the home’s envelope. It can also be used to insulate an attic.

The insulation’s backing can simply be paper, or a composite material engineered to reflect heat and/or be a barrier to vapor and air. However, alone, batt insulation does not create an airtight air or vapor barrier. Batt insulation must be properly installed and of the proper R-value to correctly insulate a home.

batt insulation being installed

Batt Insulation being installed

See R-value

Bidirectional Meter (aka Net Meter)

A bidirectional meter is a type of electric meter used with homes that have photovoltaic (PV) solar panels or some other form of clean energy generation system such as wind power. The meter that runs forward, showing how much electricity a home is using from the power grid when the PV solar panels are not producing all the electricity the home needs at the moment and runs backward, showing how much electricity the home’s photovoltaic solar panel system is feeding into the grid when the solar panels are making more energy than the home is using at that moment.

bidirectional net meter

Blower Door Test

A blower door test is a method to determine how airtight a home is. A specialized door with a powerful fan or blower is installed in one of the home’s exterior doorways (one that is part of the home’s envelope). All the home’s other doors and windows are closed, and if you have wood burning stoves or fireplaces, the flue vents are closed.

The fan either overpressures or depressurizes your home’s interior, and by doing so, a trained technician can measure how much air infiltration your home’s envelope has. This test can be used to identify areas where your home has air leakage so you can weatherize your home. With green homes, a blower door test is used to determine if the air leakage is within the permitted limits of the green building certification.

blower door test in progress

Blower Door testing in progress. Source BPC Green Builders, Inc.

Blowing Agent

Foam insulation is essentially made of a building material, such as polystyrene, that contains lots of small bubbles. To create the foam, such as expanded polystyrene foam (EPS), a gas is essentially blown into polystyrene to create the bubbles. The gas used to create the bubbles is the called a “blowing agent.” Different types of foam use different types of blowing agents.

During the manufacturing or installation of foam insulation some of the blowing agent can escape into the atmosphere. Unfortunately, most of the blowing agents used are greenhouse gases that are many times worst for climate change than CO2 or have other negative environmental impacts. While the blowing agents used today are less harmful than in the past, they still have a way to go before they are carbon neutral.

Also see Embodied Carbon, Greenhouse Gas and Global Warming Potential.


BTU is the acronym for British Thermal Unit. One BTU is the amount of heat energy needed to increase the temperate of one pound of water one degree Fahrenheit.

BTUs are used as a common metric to gauge and compare the amount of heat they generate or the amount of electricity they use. Home heating systems and cooking appliances, such as stove tops commonly use BTUs to describe their heat output.

Building Code

A building code is a set of minimum standards for construction of a building. Building codes often pertain to structural and fire safety. Building codes are generally set at the state level and help ensure that all construction meets an agreed-upon minimum standard of safety and comfort.

The term building code, as it is commonly used, rarely, if ever, pertains to green building standards, as those are always higher than building codes.

Building Envelope

NOTE: A building envelope is one of the most vital parts of an energy efficient home and is important to understand.

A building envelope refers to those elements that divide the home’s inside, conditioned spaces from the home’s non-conditioned spaces and the outdoors. Conditioned spaces are the parts of your home you use energy to heat and/or cool.

Non-conditioned areas of the home usually include the garage, porch, deck, and sometimes the attic.

Elements of a home’s envelope include the foundation, exterior walls and framing, insulation, air sealing, vapor barriers, wind barriers, drainage planes, exterior windows and doors, attic floor, and/or roof.

The quality of a home’s envelope, including its strength, insulative qualities, it being airtight, and acting as a vapor/moisture barrier are critical to a green home’s energy performance, indoor air quality, comfort, and health.

Green homes have envelopes with superior insulation and that are essentially airtight. The insulation and airtight nature of the envelope is, in large part, what makes your home energy efficient.

home envelope illustration

Red Outline is Home’s Building Envelope

Building Science

Building science is a scientific approach to the design and construction of buildings. It includes the physics, chemistry, math, engineering, materials, construction methods, and testing that go into building higher performing homes and commercial buildings.

It is important to have a clear understanding of building science to build high-performance, green, LEED-rated, Passive House-certified, sustainable homes.

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Carbon Dioxide (CO2)

Carbon dioxide or CO2 is the most prevalent of the greenhouse gases. It is primarily caused by burning fossil fuels, but it can also come from other sources such as burning biomaterials like wood and naturally occurs from sources including animal respiration, ocean-atmosphere exchange, and volcanic activities.

Also see CO2E and Greenhouse Gases

Carbon Footprint

A person’s or family’s carbon footprint is the sum total of all the carbon and other greenhouse gases caused by how you live your life.

It is well known that carbon is released any time a fossil fuel is burned to generate energy. Coal- and gas-fired electric plants, manufacturing facilities, gas- and diesel-powered cars and trucks all contribute to your carbon footprint. If you use gas to heat hot water or your home, your hot water heater and furnace emissions contribute to your carbon footprint.

Less known is that your carbon footprint also includes the greenhouse gas emissions created by