Term Description
A-component (A-side)

One component of a two-component polymer system; for polyurethane foam and coatings, the isocyanate component. See also "isocyanate" and "methylene diphenyl diisocynate (MDI)."

Abrasion resistance

The ability to resist being worn away by contact with another moving, abrasive surface, such as foot traffic, mechanical equipment, wind-blown particles, tree limbs, etc.


The ability of a material to accept within its body quantities of gases or liquid, such as moisture.

Accelerated weathering

The exposure of a specimen to a specified test environment for a specified time with the intent of producing in a shorter time period effects similar to actual weathering.

Acrylic coating

A liquid coating system based on acrylic binders dispersed in water (occasionally in solvents) with pigments and several proprietary additives that cures by coalescence and air drying.

Acrylic resin

Polymers of acrylic or methacrylic monomers, often used as a latex base for coating systems.

Active metal (anodic)

A metal or material that readily gives up electrons to a cathodic (noble) material. (See "anodic.") An active metal will corrode in the presence of moisture when in contact with a cathodic metal.


To cause two surfaces to be held together by the combined strength of the molecular forces and the mechanical interlocking achieved between adhesive and the bonded surface. See "adhesion," "adhesive" and "bond."


(1) The degree of attachment between two surfaces held together by interfacial forces—mechanical, chemical or both; (2) the degree of attachment or bonding between application of the same substance; (3) the combined ultimate strength of the molecular forces and the mechanical interlocking achieved between the adhesive and the surface bonded. Adhesion is measured in shear and peel modes.


A cementing substance that produces a steady and firm attachment or adhesion between two surfaces.

Adhesive bond break

A material to facilitate independent movement between two units that would otherwise bond together.


Associated General Contractors of America.

Aged R-value

Thermal resistance value established by using artificial conditioning procedures for a prescribed time period. See "Long-term thermal resistance (LTTR).‚"


(1) Crushed stone, crushed slag or water-worn gravel used for surfacing a built-up roof system; (2) any granular mineral material.

Aggregate, lightweight (LWA)

Aggregate of low density—examples include coal bottom ash, pumice, scoria, volcanic cinders, tuff and diatomite, expanded or sintered clay, shale, slate, diatomaceous shale, perlite, vermiculite or slag and bonded or sintered coal combustion products (CCPs) used to produce lightweight concrete or component products.


(1) The effect on materials of exposure to an environment for an interval of time; (2) the process of exposing materials to an environment for an interval of time.


American Institute of Architects.

Air barrier

See "air retarder."

Air barrier accessory

See ‚"air retarder accessory.‚"

Air barrier system

See ‚"air retarder system.‚"

Air infiltration

Air leakage into the building driven by positive pressure.

Air leakage

The movement/flow of air through the building envelope, which is driven by either positive (infiltration) or negative (exfiltration) pressure difference, or both.

Air leakage rate

The quantitative measure of air passage through a set surface area of an assembly within a given time period under a pressure differential between the two sides of the assembly.

Air retarder

A material or system in building construction that is designed and installed to reduce air leakage either into or through the opaque wall.

Air retarder accessory

A transitional component of the air retarder that provides continuity.