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 trucks, trains, planes, and ships that transport the things you use from where they are made or grown to you where you use them. So, if you buy a piece of lumber or food grown locally, it contributes to your carbon footprint less than if the lumber or food you buy was grown or made thousands of miles away.

Note: The word “carbon” in carbon footprint is often used to mean any greenhouse gas. Some greenhouse gases like methane trap heat at a much greater rate than carbon. For example, pound for pound methane emissions are 25 times worse as a greenhouse gas than carbon.

CO2 is used to indicate emissions specifically from carbon. CO2E means CO2 equivalent and 1 unit of methane emissions is equal to 25 units of CO2. So, each unit of methane emissions you cause increases your carbon footprint 25 times more than if you caused 1 unit of CO2 emissions.

Also see CO2, CO2E, and Global Warming Potential (GWP).

Carbon Neutral

Carbon neutral refers to a process, energy source, materials, or product that when factoring everything that goes into it neither adds to nor reduces the amount of CO2 or CO2E (aka greenhouse gases) in the atmosphere.

Carbon Sequestration

Carbon sequestration is the trapping and storing of carbon and carbon emissions so that they don’t escape into the atmosphere.

Traditionally, this term has been used in the energy and manufacturing fields to refer to pumping carbon emissions (generated by burning or extraction of fossil fuels) into deep underground storage.

Carbon sequestration is now starting to be used within the context of green building, although not in the traditional way. Wood and other types of natural building materials, such as cellulose insulation, contain carbon. By using these materials to build a home, especially those that include what otherwise would be waste products, such as sawdust and wood chips and would otherwise be burned to generate energy, the home is trapping or sequestering the carbon in these materials for the life of the home. This also reduces the supply of and increasing the cost of the carbon-based waste (like wood chips) that would otherwise be burned.

Carbon Smart

Carbon Smart building materials have some embodied carbon but even more sequestered carbon, which is atmospheric carbon locked up in the material itself for the life of the home. These are typically fiber-based, renewable materials.

Also see Embodied Carbon


See Closed Cell (spray) Foam Insulation (CCF)


CDD means Cooling Degree-Days.

See Degree Days

Cellulose Insulation

Cellulose insulation is plant fiber-based insulation and is generally made from recycled paper or by-products of manufacturing wood products, like sawdust. Cellulose insulation is generally treated with borates or other materials to make them fire and insect resistant.

Cellulose insulation comes in several forms including dry, spray applied, stabilized, and low dust. It can be installed damp and sprayed on, or loose – either “tight-packed” meaning densely packed in walls and other cavities or “loose-packed” as it is typically used in attics.

Cellulose insulation offers several key advantages over other forms of insulation.

  1. It is among the cheapest insulation materials.
  2. It is an efficient insulator, equal to or better than many other types of insulation materials.
  3. It can be used in new construction and can often be used to improve the insulation qualities of existing homes.
  4. It can be added to existing homes without having to remove the inner or outer walls completely.
  5. It has good moisture handling properties.
  6. It sequesters carbon in your home helping reduce CO2
  7. Its manufacturing and installation have a low carbon footprint compared to most other forms of insulation.


The meaning of certified depends on the context. In green building, people, building products, home products, and homes themselves can be certified.

People can be certified as having been trained or trained and passed exams.

Certified products and building materials can mean they were built or made in a manner that has been prescribed or they have been tested and proved to meet specific certification benchmarks.

Homes can be built to meet certain green building certification standards. In some cases, simply building to the prescribed way and using the prescribed materials makes the home certified. In other cases, to be certified the home must be built according to prescribed approaches and be tested to ensure it has achieved the certification performance requirements.

There are self-certifications and those that require testing and certification by a third party.

A home built to meet certification standards may have achieved the performance benchmarks, but if the homeowner opts not to have the home tested and go through the certification process, the home is not considered certified.

Chimney Effect

See Stack Effect


Cladding refers to the building products used on the exterior sides of roofs and walls. Siding, brick, stone, shingles, and metal are examples of cladding.

Some types of cladding are more eco-friendly, last longer, require less maintenance, and/or are made from recycled materials or can be recycled when removed.

Clean Energy

Clean energy is the term for sources of energy that do not emit pollution to produce that energy. Geothermal, solar, hydro, and wind are all considered clean energy sources.

Nuclear-generated energy is sometimes called clean energy in that it doesn’t emit greenhouse gases and exacerbate climate change. However, it being a clean energy is debatable in a wider sense as current nuclear power generation technologies produce radioactive waste which is dangerous and not environmentally friendly. Nuclear reactors today also generate nuclear waste that can be weaponized.

Also see Foam Insulation

Climate Change/Climate Crisis

Climate change (or more accurately, climate crisis) is arguably the single most critical danger to life on our planet. Climate change is the impact that human activities are having and will have on the world’s climates and weather patterns now and in the future.

Climate crisis, climate destabilization and greenhouse effect are alternative terms for climate change. Climate change is increasing average global temperatures, but more importantly it is shifting climate zones and weather patterns, making weather patterns less stable and more severe. This means more flooding and more droughts, more storms, more severe storms, and more extreme temperatures, all of which can be deadly. Climate change affects rainfall patterns and is causing increasing and more severe wildfires, water shortages, and flooding.

The increase in “global” temperature does not mean increasing temperatures everywhere or equally in all areas. Because the most northern and southern areas of the globe are warming the most, polar icecaps and glaciers are melting, and the melt water is raising sea levels. This exacerbates the damage to coastal areas during storms and is already causing low-lying areas like Miami, Florida to flood at high tide even without rain or storms.

Within decades coastal areas around the world, where most of the world’s population live, will experience severe flooding and eventually be below sea level. This will cause massive numbers of people around the globe to become climate change refugees and will cost trillions of dollars and cause loss of life.

Closed Cell (spray) Foam Insulation (CCF)

Closed cell (spray) foam insulation (CCF) is a very energy efficient type of insulation that also acts as a moisture and air barrier, but which is more expensive than traditional types of insulation. Sometimes a small amount of CCF is used in combination with other insulation to get the most cost-efficient and energy-efficient insulation in a given space.

spray closed cell foam insulation being installed

Closed Cell (spray) Foam insulation being applied

Also see Foam Insulation


CO2 stands for carbon dioxide, the most common greenhouse gas.

See Greenhouse Gas


CO2E stands for carbon dioxide equivalent. Greenhouse gases vary in their ability to trap heat and persist in the atmosphere. CO2 is equal to a CO2E of 1. Other greenhouse gases have a higher CO2E.

See Greenhouse Gas


Commissioning is the testing of a home after its construction or renovation to verify that the home’s systems are working at maximum efficiency. Typically, minor adjustments to the home’s operating systems occur at this time.

Commissioning is different from certification although a certification process can be the method of commissioning.

Also see Certified

Composite Lumber

Composite lumber refers to building materials that are made from plastic mixed with wood fiber or other agricultural by-products. Composite lumber often contains recycled material and is generally used for decking; hence it is often referred to as composite decking.

Conditioned Space

Conditioned space is the portion of your home that you heat and/or cool, such as the kitchen, bedrooms, bathrooms, and family rooms. Ideally and in green homes, these spaces are well insulated and air sealed. Porches, decks, patios, and often garages and attics are not part of the conditioned spaces of your home. In green homes crawlspaces that have HVAC systems or water and sewer pipes are conditioned spaces.

It is the conditioned space portion of your home that is measured for verification and certification of green standards.

Conditioned space of home

Conditioned space is the Space inside of the home’s envelope 

Also see Building Envelope


Conduction is the movement or transfer of heat energy through a material. In homes the materials are usually glass, metal, wood, vinyl, sheathing, and drywall.

Green homes are designed to minimize movement of heat energy through the home’s envelope, so the home doesn’t lose energy either by it leaving or entering the home via conduction. Insulation and reducing the physical connections between the inside and outside of your home help reduce conduction.

The R-value is a metric used to show insulation’s resistance properties to reduce conduction and the U-value is the metric used for windows for the same purpose.

Also see Thermal Bridge

Conductive Heat

Conductive heat is heat energy that moves through a solid. This comes into play with a green home when building materials like wall studs are in direct contact with an inside and outside wall. In the winter, the stud can conduct heat energy from inside your home to outside causing your home to be less energy efficient. When it is hot outside, heat energy from outside your home can be conducted to inside, again making your home less energy efficient because it increases your need to cool your home.

Even if the space between a home’s wall studs are well insulated, there can be conductive heat energy moving through the wall studs that diminishes the insulation value of your home. Therefore, there are building approaches, such as double wall construction and the use of sheet foam, that help minimize or eliminate any part of an outside wall, floor, or ceiling from being able to conduct heat into or out of your home.

Also see Thermal Bridge

Construction Waste Management

Construction waste management involves strategies for the control and reduction of the amount of waste generated by construction through careful planning, sizing, and ordering of construction materials.

The objectives include:

  • lowering construction costs
  • using fewer non-renewable natural resources
  • reducing the amount of materials going into landfills
  • increasing the use of salvaged and recycled building products and recycling more post-construction building materials.

Conventional Home

A conventional home is one that is built to meet building code standards. It meets the minimum requirements for safety and comfort. In contrast, a green, high-performance home, such as a net zero or zero energy ready home, is one that meets much higher performance standards.

Cooling Degree-Days (CDD)

See Degree Days

Cost of (Home) Ownership

Cost of home ownership includes the first cost (cost to design and build the home or to buy the home) combined with the operational costs (costs of living in the home such as energy and maintenance).

Relative to green homes the first cost (also called prime cost) may be marginally higher than the that of a conventional home, but the operational costs of the home are often considerably lower than a conventional home.

Without a Mortgage:

After a certain period of time, the lower operational costs provide enough savings to recover the marginal cost of building a green home. After that is achieved, the operational costs remain lower for a green home and so the cost of home ownership is lower for a green home.

For example, if the marginal first cost of a green home is $15,000 more than a conventional home but the green home uses $5,000 less energy per year, after 3 years the marginal cost of $15,000 is fully recovered. Each year going forward the green home owner saves $5000. By year 5 of living in the home, the cost of green home ownership is $10,000 less, after 7 years, $20,000 less and so on.

With a Mortgage:

If you have a mortgage on your home, the cost of home ownership can also be looked at in terms of PITI vs PITI-E.

For a conventional home PITI is the cost of your mortgage based on what you pay each month for principle, interest, taxes, and insurance.

For a green home, PITI-E should be used to calculate cost of ownership because while the monthly principle and interest may be slightly higher because the green home is slightly more expensive, the E or Energy of PITI-E savings is greater than the marginal increase in the principle and interest so your ownership costs (or monthly costs) are lower.

For example, if a conventional home mortgage’s PITI is $2000 a month, a green home’s PITI may be $2050 (higher to cover the marginal cost increase of building a green home) but the green home’s monthly energy savings is $350. So, the PITI-E per month for the home is $1,700 and not $2000 as its $300 less per month in mortgage and energy costs.

Cost Per kWh

Cost per kWh (kilowatt hour) the cost paid for or cost to generate 1 kilowatt of energy for an hour. kWh is a standard measure of electrical use. Cost per kWh on home energy bills may or may not include a transmission fee, depending on your energy provider.

Cradle to Cradle

Cradle to cradle describes the process in which a material is created, used, and then recycled so it can be used again, rather than becoming waste and no longer useable. This is in contrast to the phrase cradle to grave.

See Cradle to Grave

Cradle to Grave

Cradle to grave, in building and engineering parlance, describes the creation, use, and then disposal of a product or material which then becomes waste or is no longer usable. While some portions of a green home have a cradle to grave life cycle, green building aspires to use products that have a cradle to cradle lifecycle.

Also see Cradle to Cradle

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Daylighting is optimizing the use of natural light, such as through windows, to reduce the energy needed for artificial lighting. At the same time, it helps create environments that are healthy, stimulating, and productive.

Deep Green

Relative to green building, Deep Green refers to a building being very green vs somewhat green. An ENERGY STAR® Home is light to somewhat green. A LEED for Home’s Platinum certified net zero energy home is Deep Green.

See Green


Degree-day is the metric used to show how cold or warm a location is over a given time period relative to a specific base (room) temperature which may vary, but is typically 65°F when used for heating (and sometimes cooling) and typically 75°F when used for cooling.

When degree-days is used relative to heating it is called heating degree-days, or HDD.

Likewise, cooling degree-days or CDD is used to measure the air conditioning requirements of a home.


Delta-T is the temperature difference across a specific divider. Delta-T is frequently used to measure the difference between a home’s indoor and outdoor temperatures.

Department of Energy, US (DOE)

The US Department of Energy is responsible for a broad range of issues related to energy as well as some other areas such as nuclear weapons.

Relative to Green Homes, the DOE created the Zero Energy Ready Home (ZERH) program including its standards for certification and other aspects of the ZERH program.

US DOE logo

US Department of Energy Logo from USGS.GOV

Also: see Zero Energy Ready Home


Depressurization describes a situation within a home when the air pressure inside is lower than the air pressure outside.

Depressurization can cause outside air to infiltrate the home. With conventional homes, even with windows and doors closed, depressurization will cause air to infiltrate the home through cracks and gaps in the home’s envelope. This brings in allergens and if the air flows through your home’s insulation can bring in particles of insulation.

Green homes are designed to minimize air infiltration even if there is depressurization in the home.

Kitchen and bath exhaust fans, dryers that exhaust air outside, as well as non-sealed fireplaces can all cause air to be pulled from the home causing depressurization.

Depressurization can also cause back drafting which can pull any radon gas present in the ground into the home. Green homes foundations are sealed to help prevent radon getting into the home. Radon gas is a health hazard.

Depressurization can also pull gas into the home from even small cracks in gas lines or gas connections that are not properly sealed. Gas in the home is a dangerous situation.

Design Temperature

A home’s design temperature is the expected minimum or maximum temperature for the home’s location. The design temperature, along with other metrics and factors, is used to properly determine the size and capacity for a home’s heating and cooling systems.

Direct-Gain System

A direct-gain system is one approach for passive solar heating systems. One type of direct-gain system uses south-facing windows through which the sun’s radiant energy, heats the home during the day. This is called heat-gain. This heat energy is also absorbed and stored in the home by building materials such as stone, brick, concrete, or other materials which can absorb and store heat. A wall, floor, fireplace, countertop, etc. made of such products acts as a thermal mass.

At night, when the sun has set and it is generally cooler, heat stored in the home’s thermal masses radiates into the home keeping it warm.

green home built by BPC Green Builders

Direct Gain Energy through south facing windows with overhangs that block most of the energy gain in the summer

For this type of system to work all year, roof overhangs, shades, filters, and other devices must be used to reduce or eliminate solar heat-gain during the daytime. This is possible because during the summer the sun is higher in the sky so often simply extending the roof further is all that is needed.

See Thermal Mass

Displacement Ventilation

Because warm air rises, displacement ventilation works by natural air convection without using additional energy to provide ventilation.

Simply having windows or air vents both high and low in a room or on different floors allows displacement ventilation to work.

displacement ventilation illustration

Displacement ventilation. Warm air rises and exits upper windows pulling in cooler air from windows and doors open below.

Also see Stack Effect

Diurnal Flux

Diurnal flux is the technical term for the difference between average  daytime and nighttime temperatures.


See Muntin

Drainage Plane (aka Water Plane)

A home’s drainage plane is the space or special material between the outer cladding (siding, brick, etc.) of your home and your home’s weather barrier which allows water that gets behind the cladding to drain down and out from the wall assembly thereby preventing moisture from damaging the cladding and inner sections of your wall assembly.

Let’s understand the structure of your home’s exterior walls. From the outside moving in you have cladding (siding, shingles, brick, etc.), then the drainage plane, then a weather barrier – sheathing that is affixed to the perimeter of your home’s framing – insulation, and then sheets of drywall which make up your home’s inner walls. Depending on your home’s climate you may have a vapor barrier in the wall assembly.

Drainage Plane illustration

Illustration of one type of drainage plane. Source BPC Green Builders, Inc.

A drainage plane can be created using thin strips of wood running vertically under the cladding to create a space for the water to flow downward. It can also be a specialized sheet or wrap applied outside the weather barrier that has channels for the water to flow down and out from the wall assembly. Some weather barriers have integrated drainage planes.

Double Pane Windows

Double Pane windows are windows where there are two layers of glass separated by a gap with a spacer. The glass and spacers are surrounded by an airtight frame that makes the entire glass assembly a unit. The gap is filled with air or a gas like argon or krypton for a glass unit. The gap between the glass panes windows reduces the transfer of heat from one side of the window glass unit to the other and increases the insulation value of the window. Argon and Krypton are more efficient thermal insulators than is normal air.

double pane window

Double Pane Glass. Source BPC Green Builders, Inc.

Double pane windows are more energy efficient than single pane windows but less energy efficient than triple pane windows that are filled with the same gas (air, argon, or Krypton). Most new homes today are built using Double Pane or Triple Pane windows.

Also: see Insulated Window and Triple Pane Windows

Double-Stud Wall

A Double Stud Wall is a type of wall design that has two layers of studs, one on the exterior side of an exterior wall, the other on the interior side of an exterior wall. This creates both a thicker wall with room for more insulation than in a single stud wall configuration. By staggering the studs or placing the inner and outer studs apart so they don’t touch, this wall design approach also eliminates thermal bridging.

example of double wall construction

Example of a double stud wall with the inner and outer studs spaced apart so they don’t touch, therefore eliminating the potential for thermal bridging. Source BPC Green Builders, Inc.

double wall with staggered studs

Double Wall Construction using staggered studs. Source BPC Green Builders, Inc.

See Thermal Bridging

Dual Flush Toilet

Dual Flush Toilets are designed to save water by allowing the user to use less water for flushing liquid waste and more water when solid waste is flushed. These toilets are usually High Efficiency Toilets, as are low flow and other WaterSense toilets can be.

Also: see WaterSense, High Efficiency Toilet

Ductless Mini-Split

A ductless mini-split refers to a type of small-capacity air-source heat pump (commonly called a heat pump) that is designed to be highly energy efficient and provide both heating and cooling or just cooling.

A ductless mini-split is called ductless because it’s outside unit (heat pump) is connected to one or more indoor wall-mounted units (ceiling and floor units are also available) by refrigerant lines. These lines move heat energy to or from the indoor units’ heat exchangers.

The indoor units suck in air from the room they are in, pass the air over the unit’s heat exchanger, and then back into the same room. The heat exchanger either heats or absorbs heat from (aka cools) the air passed through the heat exchanger.

Conventional central air heating, cooling and ventilation systems, heat, or cool air in the home in one location and circulate that air throughout the house using air ducts.

Ductless mini-splits are made that only work with a single indoor unit and some are made so that multiple indoor units can be connected to a single outdoor unit. With multiple indoor mini-split units you can have each indoor unit set to a different or the same temperature, thereby creating heating and cooling zones in the home.

Dutch Drain

A Dutch Drain is a type of drain system that carries storm-water runoff from the bottom of a house wall away from the house.

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EEM stands for Energy Efficient Mortgage.

See Energy Efficient Mortgage


See Energy Factor (EF)

Effective Leakage Area (ELA)

A home’s effective leakage area is the amount of air leakage a home’s envelope has with all its doors and windows closed, as compared to a 100% perfectly sealed home envelope with no air leakage.

Most older homes have an effective leakage area equivalent to the home have a hole in its envelope equal to one or square yards. This is like a home with many windows and doors open allowing cold air in or warm air out.

Even new, non-green homes built to current building codes often have effective leakage areas of several to many square feet.

effective leakage area illustration

Effective Leakage Area for Green Homes is very small, for older homes often very large. Source BPC Green Builders, Inc.

In an energy efficient, high-performance green home, the effective leakage area should be measured in square inches.

Embodied Carbon

Embodied carbon is the carbon (CO2 or CO2E of greenhouse gases) emitted into the atmosphere during the growth, mining, extraction, harvesting, transport, manufacturing, and distribution of a building material.

Embodied carbon does not include any carbon emissions the building material or product causes when in use after the home is finished such as would a coal, oil, or gas furnace. Those emissions are called Operational Carbon.

Also see CO2, CO2E, Operational Carbon, and Embodied Energy.

Embodied Energy

Embodied energy is the energy used in the growth, mining, extraction, harvesting, transport, manufacturing, and distribution of a building material. This is strongly related to Embodied Carbon.

Also see Embodied Carbon

Energy Assessment/Energy Audit

Energy assessment and energy audit are used interchangeably. They describe the evaluation of an existing home’s energy efficiency and usage. The purpose of this process is to identify where the home is wasting energy and develop recommendations for improving energy efficiency.

Energy audit using blower door and thermal imaging

Energy Audits typically include both blower door tests and thermal imaging

Typically, such an assessment or audit will recommend where to seal or caulk spaces around electrical outlets on exterior walls, gaps in walls or between windows and walls as well as between walls and floors where heat is leaking into or out of your home. The audit should also inspect and make recommendations if your current windows and doors should be replaced with more energy efficient windows and areas of your home that would benefit from added insulation. These audits also look to identify any air leaks from air duct systems, imbalances in air handling systems, and so on and also make recommendations on how to fix the problems.

A report should not only include recommendations but also expected costs per recommendation and its expected energy savings. Typically, you will find some remedies (like adding more insulation in your attic have low costs and high returns while others have high costs but lower returns.

When recommendations are followed a home becomes less drafty, more comfortable and more energy