Green Building Terminology

Air Changes per Hour (ACH)

ACH is an acronym for Air Changes per Hour and is a measurement of air infiltration. It is the total volume of air in a home that is turned over in one hour. Tightly constructed homes may have an ACH of 0.35 to 0.25 or less. A typically built new home may have an ACH of around 1.75 ACH.  Older, poorly weatherstripped and air-sealed homes may have higher than 2.5 ACH.See also Blower-door below for how to test for ACH.

Air Flow (CFM)

CFM50 is defined as the air flow (in cubic feet per minute) needed to create a 50 Pascal pressure change in the building envelope.CFM50 is one of the most basic measurements of airtightness. Air flow measurements are sometimes referenced to different building pressures such as 25 or 50 Pascals, with 50 being most common.


Air-sealing is the practice of filling gaps, holes, or cracks in a builidng's envelope in an effort to tighten the home's pressure boundary. Air sealing is one of the most significant energy efficiency improvements you can make to your home.  Air sealing will not just reduce energy costs, it will also improve your home's comfort and durability. Some common air-sealing products include latex or silicone caulk, expanding foam products, weatherstripping, and even dense-packed cellullose in exterior walls.


A diagnostic tool designed to measure the airtightness of buildings and to help locate air leakage sites. A blower door consists of a calibrated fan for measuring an air flow rate and a pressure sensing device to measure the pressure created by the fan flow. The combination of pressure and fan flow measurements are used to determine the building airtightness. The airtightness of a building is useful knowledge when trying to increase energy conservation or decrease indoor air pollution or control building pressures.

The Minneapolis Blower Door has long been recognized as the best designed and supported building airtightness testing system in the world. Combined with specialized accessories and testing procedures developed by The Energy Conservatory, the Minneapolis Blower Door is the system of choice for utility DSM programs, energy raters, HVAC contractors and weatherization professionals.

Building Envelope

The separation between the interior and the exterior environments of a building. It serves as the outer shell to protect the indoor environment as well as to facilitate its climate control.  The red outline in the picture below follows the building envelope, also referred to as the pressure boundary.

For more detailed information on building envelopes and the materials that combine to make them, visit the Oakridge National Labratory's Buildilng Envelopes Program home page.

Building Performance

Building performance is a set of measurable building characteristics including:

  • StructuralDurability
  • Moisture Management
  • Energy Efficiency
  • Indoor Air Quality
  • Thermal Comfort

Building Science

Building science is the collection of scientific knowledge that focuses on the analysis and control of the physical phenomena affecting buildings. This includes the detailed analysis of building materials and building envelope systems. The practical purpose of building science is to provide predictive capability to optimize building performance and understand or prevent building failures.

Built-Up-Roof Systems (BURs)


A roof consisting of multiple plies of roof felts laminated together with bitumen. Built-up roof material can consist of bitumen-saturated felt, coated felt, polyester felt or other fabrics. A surfacing is generally applied and can be asphalt, aggregate (gravel or slag), emulsion or a granule-surfaced cap sheet.

This section was written simply to give people an idea of what a Built-Up Roof (B.U.R. for short) is and how it’s put together. The materials and information listed here are not comprehensive nor do they represent all types of built-up roof materials available. 

Built-Up Roofing is one of the oldest ways of installing a new roof. It was first known as composition roofing and started in the 1840's. B.U.R.’s come in two basic types, asphalt and coal tar, and three basic components – (1) the waterproofing component, (2) the reinforcing component, and (3) the surfacing component which is used to protect the other components from the elements.

Unfortunately, most BUR systems are not installed perfectly, and many leakage problems have been commonly found as a result. 

See our Services page and the section on moisture flat roof leak detection for how to locate these problems using infrared thermography.


In physics, buoyancy is the upward force on an object produced by the surrounding liquid or gas in which it is fully or partially immersed, due to the pressure difference of the fluid between the top and bottom of the object. 

Cellulose insulation

Cellulose insulation is a natural product made primarily of recycled newspaper, which is mixed with borates that provide it with excellent fire-retardant, mold & mildew retardant, and insect/rodent retardant properties.  Cellulose insulation has a high R-value per inch installed (3.6-3.8), and is one of the "greenest" insulation products available.

Cellulose can be blown into attics, referred to as "loose-fill", and sprayed into empty wall cavities in existing homes and new construction with different application methods.

For existing homes there are two methods for installing cellulose in exterior walls.  The older, traditional method is called the "low-density" or "two-hole" method.  This technique has been around for decades and is the most widely practiced application method, however there are serious drawbacks to this technique, mainly settling of the insulation due to its low-density.

For more on retro-fitting a home's walls with cellulose insulation, see

CFM 50

In order to compare the relative airtightness of buildings, it is useful to normalize the measurements for the size of the building. This allows easy comparison of various sized buildings to each other, or to program guidelines.

One of the most common ways to normalize building airtightness is to calculate the number of times per hour that the total volume of the enclosure is changed, when the enclosure is subjected to a 50 Pascal pressure difference. To calculate air changes per hour, the total volume of the enclosure is required in addition to the CFM50 measurement. It is also common to use the building enclosure surface area to normalize airtightness measurements.

A pressure of 50 Pa is equal to 0.2 inches of water.


The newer, more effective method for insulating an existing home that doesn't have any wall insulation is called "dense-packing" or the "tube-in method".  Dense-packing requires a slightly different procedure for filling the wall cavities than the low-density method and takes a little longer to complete, however the benefits are significant and long-lasting.

Instead of drilling two one-inch holes per cavity, the installer drills only one two-inch or larger hole about three feet from the bottom of the wall and inserts a smaller, flexible tube into the cavity, which runs all the way to the top of the wall.  The insulation is then installed with the tube inside the wall and is removed slowly once the material stops packing, completely filling the wall cavity without any gaps or voids. This higher-density method will never settled when done correctly and also provides a very effective air-barrier to outside air infiltration as well.

Digital pressure gauge

A pressure gauge, in terms of building performance diagnostics, is a testing device that reads air pressure in terms of Pacals (Pa) or inches of water collumn (w.c.), as well as air flow in cubic feet per minute (CFM).

The Energy Conservatory sells an excellent digital pressure gauge called the DG700.


The ability to maintain satisfactory aesthetic, economic, and functional performance for the useful life of the structure.  The more durable a structure is, the longer its functional life will be.

Energy Efficiency

Energy efficiency is using less energy to provide the same level of energy service. An example would be insulating a home to use less heating and cooling energy to achieve the same temperature. Another example would be installing fluorescent lights and/or skylights instead of incandescent lights to attain the same level of illumination. Efficient energy use is achieved primarily by means of a more efficient technology or process rather than by changes in individual behaviour.

Energy Star

ENERGY STAR is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy helping us all save money and protect the environment through energy efficient products and practices.


To become an Energy Star Home and earn the ENERGY STAR, a home must meet strict guidelines for energy efficiency set by the U.S. Environmental Protection Agency. These homes are at least 15% more energy efficient than homes built to the 2004 International Residential Code (IRC), and include additional energy-saving features that typically make them 20–30% more efficient than standard homes.

Fiberglass Insulation

Fiberglass insulation is what most people think of when they refer to insulation materials.  The "Pink Panther" brand comes to mind....Ownens Corning.  To this day most of the new homes and buildings are built using fiberglass "batts" or rolls.

Over the years, however, building-science R&D has shown how poor of an insulating material fiberglass can be, if not installed perfectly.  It is extrememely difficult to install perfectly, as bending, twisting, or compressing the material reduces the R-value significantly, and if the exterior membrane of the building envelope is not air-sealed well, then that air movement will virtually eliminate any potential insulating power.

Green Building

Green building is the practice of increasing the efficiency with which buildings use resources — energywater, and materials — while reducing building impacts on human health and the environment, through better siting, designconstruction, operation, maintenance, and removal — the complete building life cycle.

A similar concept is natural building, which is usually on a smaller scale and tends to focus on the use of natural materials that are available locally.[1] Other commonly used terms include sustainable design and green architecture.

The related concepts of sustainable development and sustainability are integral to green building. Effective green building can lead to 1) reduced operating costs by increasing productivity and using less energy and water, 2) improved public and occupant health due to improved indoor air quality, and 3) reduced environmental impacts by, for example, lessening storm water runoff and the heat island effect.

Practitioners of green building often seek to achieve not only ecological but aesthetic harmony between a structure and its surrounding natural and built environment, although the appearance and style of sustainable buildings is not necessarily distinguishable from their less sustainable counterparts.

Indoor Air Quality

(IAQ) deals with the content of interior air that could affect the health and comfort of building occupants. The IAQ may be compromised by microbial contaminants (moldbacteria), chemicals (such as carbon monoxideradon), allergens, or any mass or energy stressor that can induce health effects.

Recent findings have demonstrated that indoor air is often more polluted than outdoor air (albeit with different pollutants) although this has not changed the common understanding of air pollution. In fact, indoor air is often a greater health hazard than the corresponding outdoor setting. Using ventilation to dilute contaminants, filtration, and source control are the primary methods for improving indoor air quality in most buildings.


Infiltration is the unintentional or accidential introduction of outside air into a building, typically through cracks in the building envelope and through use of doors for passage. Infiltration is sometimes called air leakage.

Infiltration is caused by wind, building pressurization and by air buoyancy forces known commonly as the stack effect.

Infrared thermography

Infrared Thermography, thermal imaging, or thermal video, is a type of infrared imaging science. Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 900–14,000 nanometers or 0.9–14 µm) and produce images of that radiation.

Since infrared radiation is emitted by all objects based on their temperatures, according to the black body radiation law, thermography makes it possible to "see" one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature, therefore thermography allows one to see variations in temperature (hence the name).

When viewed by thermographic camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded animals become easily visible against the environment, day or night. As a result, thermography's extensive use can historically be ascribed to the military and security services.


Insulation in buildings is an important factor to achieving thermal comfort for its occupants. Insulation reduces unwanted heat loss or gain and can decrease the energy demands of heating and cooling systems. It does not necessarily deal with issues of adequate ventilation and may or may not affect the level of sound insulation.

In a narrow sense insulation can just refer to the insulation materials employed to slow heat loss, such as: cellulosefiberglassrock woolpolystyrene, urethane foamvermiculite But it can also involve a range of designs and techniques to address the main modes of heat transfer - conduction, radiation and convection materials.[1][2], and earth or soil.

The effectiveness of insulation is commonly evaluated by its R-value. However, an R-value does not take into account the quality of construction or local environmental factors for each building. Construction quality issues include inadequate vapour barriers, and problems with draft-proofing.

Leakage Area

Leakage area estimates are a useful way to visualize the cumulative size of all leaks or holes in the building enclosure. Estimated leakage areas can also be used in infiltration models to estimate natural infiltration rates (i.e. the air change rate under natural weather conditions).

In order to accurately estimate leakage areas, it is best to conduct the blower door test over a wide range of building pressures (e.g. 60 Pa to 15 Pa). There are a variety of standard calculation methods used to calculate leakage areas.

Leakage Area per Square Foot

Leakage area estimates can also be normalized for the size of the enclosure being tested. 

Low-Density cellulose

When a contractor retro-fits an existing house with cellulose using the low-density method, they typically drill two one-inch diameter holes per wall cavity; one about three feet from the bottom and another about one foot from the top.  The bottom hole is filled first and continues to fill until the material flow stops.  The top hole is then filled to finish the channel.

The inherant problem with this method is its low-density installation and the fact that air pockets form within the wall cavity when this method is used.  Once the air in these pockets dissapate from the wall cavity, the entire channel of insulation sags or settles, greatly reducing the effectiveness of the installation, as well as any air-sealing benefits the homeowner was seeking to achieve. 

Because of these problems, the dense-pack method was created.

Moisture Management

Moisture management refers to the active planning of how rainwater is collected and directed away from a building, so as to avoid any potential negative effects.

Regardless of project location or building type, the goal of a successful building design is to keep water out and provide thermal control within the interior spaces.

When moisture infiltrates the building envelope, several undesirable conditions can occur, including mold and mildew, structural steel corrosion, and rotting wood. These conditions can result in high energy costs, ongoing maintenance problems, compromised indoor air quality, and failure of one or more architectural and engineering building systems. If they are not adequately addressed, the problems caused by moisture infiltration can potentially increase risk and liability concerns for architects, design professionals, building owners, and building occupants.


Simply put, a pascal is a unit of pressure measurement used most often in building diagnostics when testing for duct leakage, pressure differentials, etc.

The Wikipedia definition is much more complicated and includes:

The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Young's modulus and tensile strength). It is a measure of perpendicular force per unit area i.e. equivalent to one newton per square meter or one joule per cubic meter. In everyday life, the pascal is perhaps best known from meteorological air-pressure reports, where it occurs in the form of kilopascal (1 kPa = 1000 Pa).

Pressure boundary

Often more commonly referred to as walls, ceilings, floors, and foundation walls.  Depending on how the HVAC system is designed and the construction style of the house, the pressure boundary generally encompasses all of the heated and cooled (conditioned) areas of the building, although it is often mis-placed in older homes, resulting in excessive heat loss and gain.

In order for the pressure boundary to function properly, it must be a continuous and uniform surface throughout, and be aligned exactly with the thermal boundary (insulation) in the building.

As you can see in the picture below, a mis-aligned or "open" pressure boundary can lead to outside air infiltration, degradation of insulation quality and R-value, and excessive heat loss, and high utility bills.

Pressure pan

The pressure pan is a duct leakage diagnostic tool which is used along with the Blower Door and digital pressure gauge to identify exterior air leakage in duct systems. This technique involves placing a gasketed pressure pan over each register or grille with the furnace air handler fan off and the Blower Door depressurizing the house to a standard reference pressure (-50Pa). 

The lower the pressure number found on the digital pressure gauge, the "tighter" the duct run is.  For example, a duct pressure reading of -0.5 is much tighter than -3.0.  Typically a pressure number less than -3.0Pa is considered "tight", while anything higher is in need of duct-sealing.

A measurement of the pressure between a duct run and the room where the duct register or grille is located provides a quick and reliable indication of whether significant exterior duct leaks exist in that section of the duct system.

Stack Effect

Stack effect is the movement of air into and out of buildings, and is driven by buoyancy.

Since buildings are not totally sealed (at the very minimum, there is always a ground level entrance), the stack effect will cause air infiltration. During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences.

Sustainable Building

Also referred to as "green building", is the practice of increasing the efficiency with which buildings use resources — energywater, and materials — while reducing building impacts on human health and the environment, through better siting, designconstruction, operation, maintenance, and removal — the complete building life cycle.

Thermal Comfort

Human thermal comfort is defined by ASHRAE as the state of mind that expresses satisfaction with the surrounding environment (ASHRAE Standard 55). Maintaining thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC design engineers.

Thermal comfort is affected by heat conductionconvectionradiation and evaporative heat loss. Thermal comfort is maintained when the heat generated by human metabolism is allowed to dissipate thus maintaining thermal equilibrium with the surroundings. Any heat gain or loss beyond this generates a sensation of discomfort.

It has been long recognized that the sensation of feeling hot or cold is not just dependent on air temperature alone.

US Weatherization Assistance Programs

DOE's Weatherization Assistance Program increases the energy efficiency of dwellings occupied by low-income Americans to reduce their energy costs while safeguarding their health and safety. DOE works directly with the states, the District of Columbia, and Native American Tribal Governments to carry out these goals. These agencies, in turn, contract with almost 1,000 local governmental or nonprofit agencies to deliver weatherization services to low-income clients in their areas.


Weatherization is the practice of protecting a building and its interior from the elements, particularly from sunlightprecipitation, and wind, and of modifying a building to reduce energy consumption and optimize energy efficiency.

Weatherization is distinct from building insulation, although building insulation requires weatherization for proper functioning. Many types of insulation can be thought of as weatherization, because they block drafts or protect from cold winds. Whereas insulation primarily reduces conductive heat flow, weatherization primarily reduces convective heat flow.

Background on Low-income Weatherization

Weatherization agencies operate under a grant that began with President Johnson.  These grants are intended to help low income residents pay for their basic utilities.  They will help pay off utility company debts, setup payment plans to turn back on utilities, and most importantly, pay for the complete weatherization of resident houses. 

This weatherization includes air-sealing and insulating the house, fixing any problems with the furnace, water heater, and/or oven, and making sure that there are no health concerns, such as risk for lead poisoning or carbon monoxide exposure.  Above all else, health and safety are the primary concern of the CAP agency and weatherization contractor.

A priority list is of services needed is created for each house, based on the best Savings to Investment Ratio (SIR) with the money available.  For example, if a house has at least 5" of insulation in the attic, but nothing in the walls, then the walls are dense-packed before anything is done to the attic.