Namesort descending Definition

Process whereby a porous material extracts one or more substances from an atmosphere, a mixture of gases, or a mixture of liquids (from Gatley, Understanding Psychrometrics).


abbr. Air Conditioning Contractors of America

ACCA is a group of over 4,000 air conditioning contractors who work together to improve the air conditioning industry, promote good practices, and keep homes and buildings safe, clean and comfortable.


abbr. air changes per hour

A common unit of measure of ventilation rate for a space, or air leakage rate for a building, defined as the volumetric flow rate divided by the volume of the space considered.


abbr. Advanced Customer Technology Test for Maximum Energy Efficiency

The ACT2 project was initiated in 1990 by the Pacific Gas and Electric Company. It was the first time that energy efficiency measures (EEMs)—such as advanced technologies for lighting, cooling, and insulation—were used as components of integrated systems. Since most of these EEMs had already been rigorously tested and proven individually, ACT2 focused on the cumulative, energy-related synergistic effects that EEMs can have when used in integrated packages.


Property that describes a material's ability to bond to a surface physio-chemically or chemically.


(1) A substance or compound used for bonding surfaces together, usually applied in the form of a liquid or paste. An adhesive and sealant or base coat may be the same material.
(2) Tending to adhere (as in “epoxy is adhesive”).

Adhesive Failure

Loss of adhesion of a material to the surface to which it has been applied. Usually refers to a sealant line, floor finish, or coating.


(1) Process in which fluid molecules are concentrated on a surface by chemical or physical forces or both; (2) surface adherence of a material in extracting one or more substances present in an atmosphere or mixture of gases and liquids, unaccompanied by physical or chemical change (from Gatley, Understanding Psychrometrics).

Aesthetic Joint

A groove cut into a cladding system (often EIFS, precast, or stucco) that serves the function of decoration and/or to provide a starting or stopping point for finish coat application.

Aesthetic Reveal

See Aesthetic Joint.


abbr. Annual Fuel Utilization Efficiency

A measure of annual efficiency of conversion of primary gas or oil consumption to the production of hot water or air. Widely used in boilers and furnaces.


abbr. air handling unit

A mechanical space conditioning device comprising an enclosure, and a fan to move air. In many cases, an AHU might also contain a filter, duct connection flanges, and/or heating or cooling coils (which, strictly, make it a fancoil unit).


abbr. American Institute of Architects

A professional membership association for licensed architects, emerging professionals, and allied partners.

Related resources:

Air Control Layer (formerly Air Barrier)

The complete air control layer system is comprised of materials and assemblies, each with their own performance requirements.

Air Control Layer Material (formerly Air Barrier Material)

A material that has sufficiently low air permeance and adequate strength that it can be part of an air control layer system. Recommended maximum air permeance for a material is 0.02 l/(s-m2)@ 75 Pa (0.004 cfm/ft2 @ 0.3” WC) when tested according to ASTM E 2178 or E 283.

Air Control Layer System (formerly Air Barrier System)

Air control layers are three-dimensional systems of materials designed, constructed, and/or acting to control air flow across a building enclosure, or between a conditioned space and an unconditioned space. In multi-unit/townhouse/apartment construction an air control layer system should also separate the conditioned air from any given unit and adjacent units. The pressure boundary of the enclosure should, by definition, be coincident with the plane of a functional air control layer system. In multi-unit/townhouse/apartment construction the air control layer system may also be the fire barrier and smoke barrier between units. In such assemblies the air control layer system must also meet the specific fire resistance rating requirement for the given separation.

Air control layer systems are assembled from materials incorporated in assemblies (or components such as windows) that are interconnected to create enclosures. Each of these three elements has measurable resistance to air flow. The maximum air permeances for the three components are listed as follows: 

•    Material        0.02 l/(s-m2)@75 Pa (0.004 cfm/ft2  @ 0.3” WC)
•    Assembly    0.20 l/(s-m2)@75 Pa (0.04 cfm/ft2  @ 0.3” WC)
•    Enclosure    2.00 l/(s-m2)@75 Pa (0.4 cfm/ft2  @ 0.3” WC)

Materials and assemblies that meet these performance requirements are said to be air control layer materials and air control layer assemblies. Air control layer materials incorporated in air control layer assemblies that in turn are interconnected to create enclosures are called air control layer systems.

Air Current Tester

See Smoke Pencil.

Air Infiltration

Uncontrolled inward leakage of air (that may contain entrained water vapor) through cracks and insterstices in any building element and around windows and doors of a building, caused by the pressure effects of wind or the effect of differences in the indoor and outdoor air density (from Gatley, Understanding Psychrometrics).

Air Leakage

Uncontrolled and/or unintended airflow through a building enclosure or between units of occupancy. Leakage from indoors to outdoors is known as exfiltration and leakage from outdoors to indoors is known as infiltration. Air leakage can cause indoor air quality problems, condensation, excess energy use, comfort complaints, and smoke transport.

Air Pressure Boundary

The air pressure boundary is the boundary (comprised of a series of planes to form a three dimensional boundary) that generates the largest pressure drop (usually much more than half the total) when the enclosure is subjected to a pressure difference.

Air Retarder

A material or assembly that does not meet the performance requirements of an air control layer material or assembly, but that is nonetheless designed and constructed to control air flow.

Air-Impermeable Material

An air impermeable material is an air control layer. An air-impermeable material has an air permeance equal to or less than 0.02 l/s-m2 at 75 Pa pressure differential when tested according to ASTM E 2178 or E 283.

Air-Permeable Material

An air-permeable material has an air permeance greater than 0.02 l/s-m2 at 75 Pa pressure differential when tested according to ASTM E 2178 or E 283.

Related resource: Info-500: Building Materials Property Table


abbr. American Lung Association

A non-profit organization that promotes lung health and improved air quality through education, advocacy and research.


abbr. American Society of Heating, Refrigerating and Air Conditioning Engineers

A building technology society with more than 50,000 members worldwide. ASHRAE’s activities include research, standards writing, publishing, and continuing education. Its areas of interest include building systems, energy efficiency, indoor air quality and sustainability within the industry.


abbr. ASTM International

A globally recognized leader in the development and delivery of international voluntary consensus standards. Formerly known as the American Society for Testing and Materials.

Back Wrapping

In EIFS, the carrying of the mesh reinforcement and base coat around the ends of the insulation boards, terminating between the insulation and substrate. Typically used at system interfaces and terminations to firmly attach the base coat to the substrate and protect the edges of the insulation board at these locations.

Backer Rod

A rod-shaped foam material (typically closed cell polyethylene) that is installed under compression in a joint. Sealant is applied over the backer rod. The backer rod provides a backing, controls sealant joint depth, acts as a bond breaker to prevent three-sided sealant adhesion, and provides an hour-glass contour for the finished sealant bead.

Barrier System or Perfect Barrier

The general term to describe a rain control approach that relies on the perfection of a single plane of material(s) to resist rain water penetration. Two sub-types, face-sealed and concealed barrier, are commonly used.

Base Coat

A compound used to embed and to cover the reinforcing fabric of an EIFS, depending on the system type. The base coat acts as the primary weatherproofing layer.


A line of sealant or adhesive. In glazing, an applied sealant in a joint such as a caulking bead, heel bead, glazing bead, etc. Also a molding or stop used to hold glass or panels in position.


A structural supporting member that resists loads primarily in bending. May be vertical or horizontal and made of any material or composite.


abbr. Building Energy Code Training

A training program for the building industry and code officials. Offers classroom, on-site, and webinar training on the California Building Standards Code, with a focus on the most comprehensive and cost effective ways to bring a home up to and above California Energy Code requirements.

Below Grade

The portion of a building that is below the line of the surrounding ground level.

Related resource: Info-511: Basement Insulation, BA-1015: Bulk Water Control for Foundations, Info-101: Groundwater Control


A computer program designed to find optimal building designs along the path to zero energy.


abbr. Building Energy Simulation Test

A benchmark for building energy simulation. Tests and diagnoses the simulation capabilities of the exterior enclosure portions of building energy simulation programs.


abbr. Building Industry Institute

A California-based organization that develops, implements and administers research and educational programs for homebuilders, developers and the general public.


abbr. brushless direct current

See Brushless Permanent Magnet (BPM).

Blower Door

A machine used to test the airtightness of buildings or of smaller spaces within buildings. A blower door is mounted in an opening such as a window or door and then uses a fan to pressurize or depressurize the measured space. The more airtight the space, the less air is needed from the blower door fan to create a change in air pressure.

Bond Breaker

A tape, sheet, wax or liquid applied treatment that prevents adhesion on a designated surface. Usually used with sealant to ensure a proper joint.