Below is a list of commonly used terms in the building envelope industry.

For more information click You will be directed to the ASTM (American Society for Testing and Materials) web site.

AIR BARRIER:  A material which is applied in conjunction with a building component (such as a wall, ceiling or sill plate) to prevent the movement of air through that component.

AIR BARRIER SYSTEM: The assembly of components used in building construction to create a plane of air tightness throughout the building envelope and to control air leakage.

AIR PASSAGES: Openings through or within walls, through floors and ceilings, and around chimney flues and plumbing chases, that permit air to move out of the conditioned spaces of the building.

BUILDING ENVELOPE: Elements of the building, including all external building materials, windows, and walls, that enclose the internal space.

BUILDING-RELATED ILLNESS (BRI): Diagnosable illness whose symptoms can be identified and whose cause can be directly attributed to airborne building pollutants (e.g., Legionnaire's disease, hypersensitivity pneumonitis). Also: A discrete, identifiable disease or illness that can be traced to a specific pollutant or source within a building. (Contrast with "Sick building syndrome").

CAPILLARY ACTION, CAPILLARITY: The movement of liquid within a material against gravity as a result of surface tension.

CFC (Chloroflourocarbon): Any of various halocarbon compounds consisting of carbon, hydrogen, chlorine, and fluorine, once used widely as aerosol propellants and refrigerants. Chlorofluorocarbons are believed to cause depletion of the atmospheric ozone layer.

CFM: Cubic feet per minute. The amount of air, in cubic feet, that flows through a given space in one minute. 1 CFM equals approximately 2 liters per second (l/s).

COMMISSIONING: Start-up of a building that includes testing and adjusting HVAC, electrical, plumbing, and other systems to assure proper functioning and adherence to design criteria. Commissioning also includes the instruction of building representatives in the use of the building systems.

CONDITIONED AIR: Air that has been heated, cooled, humidified, or dehumidified to maintain an interior space within the "comfort zone." (Sometimes referred to as "tempered" air.)

DEW POINT: The temperature at which a vapor begins to condense.

DIFFUSION: The movement of water vapor from regions of high relative humidity (RH) toward regions of lower RH driven by a higher to lower temperature differential.

EXFILTRATION: Uncontrolled leakage of conditioned air from inside the home to the outside.

FLAME RETARDED: The property of a material to which flame-retardant has been added.

FLAME SPREAD: Standard test for determining relative combustibility. The flame spread of a tested material is rated relative to red oak (flame spread = 100).

GREEN BUILDINGS: The building industry is increasingly focused on making its buildings greener, which includes using healthier, less polluting and more resource-efficient practices. Indoor environmental quality (IEQ) refers to the quality of the air and environment inside buildings, based on pollutant concentrations and conditions that can affect the health, comfort and performance of occupants -- including temperature, relative humidity, light, sound and other factors. Good IEQ is an essential component of any building, especially a green building.

HCFC (Hydrochlorofluorocarbons): Compounds containing carbon, hydrogen, chlorine and fluorine. They have shorter atmospheric lifetimes than CFCs and deliver less reactive chlorine to the stratosphere where the "ozone layer" is found.

HYDROPHOBIC: Having no affinity for water; not compatible with water. "Water fearing"

HVAC: Heating, ventilation, and air-conditioning system.

IAQ: Indoor air quality.

INDOOR AIR POLLUTANT: Particles and dust, fibers, mists, bioaerosols, and gases or vapors.

INFILTRATION: Uncontrolled leakage of air into a building through cracks around doors, windows, electrical outlets and at structural joints.

INSULATION: Materials with low thermal conductivity characteristics that are used to slow the transfer of heat.

LEED: The Leadership in Energy and Environmental Design Green Building Rating System, developed by the U.S. Green Building Council (USGBC), provides a suite of standards for environmentally sustainable construction.

MOLD: Fungal growths often resulting in deterioration of organic materials, especially under damp conditions.

NEGATIVE PRESSURE: Condition that exists when less air is supplied to a space than is exhausted from the space, so the air pressure within that space is less than that in surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurized space.

OPEN JOINT CLADDING SYSTEM: A rain screen cladding system where the joints between the cladding elements are intentionally left open to promote drainage and ventilation of moisture.

OUTDOOR AIR SUPPLY: Air brought into a building from the outdoors (often through the ventilation system) that has not been previously circulated through the system. Also known as "Make-Up Air."

PERM: A unit of water vapor transmission defined as 1 grain of water vapor per square foot per hour per inch of mercury pressure difference (1 inch mercury = 0.49 psi). Metric unit of measure is ng/m2 s Pa. 1 perm = 55 ng/m2 s Pa

PERMEABILITY: The time rate of water vapor transmission through unit area of a material of unit thickness induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions.

POLLUTANT PATHWAYS: Avenues for distribution of pollutants in a building. HVAC systems are the primary pathways in most buildings; however all building components interact to affect how air movement distributes pollutants. Also - a term used in the IAQ Tools for Schools: IAQ Coordinator's Guide.

POSITIVE PRESSURE: Condition that exists when more air is supplied to a space than is exhausted, so the air pressure within that space is greater than that in surrounding areas. Under this condition, if an opening exists, air will flow from the positively pressurized space into surrounding areas.

RADIANT HEAT TRANSFER: Radiant heat transfer occurs when there is a large difference between the temperatures of two surfaces that are exposed to each other, but are not touching.

RAIN SCREEN DESIGN: An exterior wall detail where the wall cladding stands off from the moisture-resistant surface of an air barrier applied to the sheathing to create a capillary break and to allow drainage and evaporation away from the building.

RELATIVE HUMIDITY: The ratio expressed as a percentage of the amount of moisture air actually contains to the maximum amount it could contain at that temperature.

SICK BUILDING SYNDROME (SBS): Term that refers to a set of symptoms that affect some number of building occupants during the time they spend in the building and diminish or go away during periods when they leave the building. Cannot be traced to specific pollutants or sources within the building.

STACK EFFECT: The overall upward movement of air inside a building that results from heated air rising and escaping through openings in the building super structure, thus causing an indoor pressure level lower than that in the soil gas beneath or surrounding the building foundation.

THERMAL BRIDGE: A thermally conductive material which penetrates or bypasses an insulation system; such as a metal fastener or stud.

VAPOR RETARDER/BARRIER: A layer of moisture resistant material usually which controls moisture diffusion (defined as less than 1 perm) to prevent moisture build up in the walls.

VENTILATION AIR: Defined as the total air, which is a combination of the air brought inside from outdoors and the air that is being re-circulated within the building. Sometimes, however, used in reference only to the air brought into the system from the outdoors; this document defines this air as "outdoor air ventilation."