Glossary of Terms

Glossary of Terms2020-02-19T03:52:49-07:00
[ivory-search id="2067" title="Design Guide Search Form"]


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

Anchored Veneer

Veneer secured to and supported by approved mechanical fasteners attached to backing.

Adhesive Failure

Loss of adhesion of a material to the surface to which it is applied. See also Adhesion.

Air Control Layer (or Air Barrier System

Air control layers are three-dimensional systems of materials designed and constructed and/ or acting to control air flow across a building enclosure or between a conditioned space and an unconditioned space. The pressure boundary of the enclosure should, by definition, be coincident with the plane of a functional air control layer system. 

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. Common minimum recommended air permeances for the three components are: 

Material: 0.004 cfm/sf @ 0.3” WC
Assembly/Component: 0.04 cfm/sf @ 0.3” WC
Enclosure: 0.4 cfm/sf @ 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 that are incorporated into air control layer assemblies that in turn are interconnected to create enclosures are called air control layer systems.

Air Leakage

Uncontrolled and/or unintended air flow 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 insulation quality problems, condensation, excess energy use, comfort complaints, and smoke transport.

Backer Rod

A resilient foam material (typically closed-cell polyethylene) of circular cross-section installed under compression in a joint to provide a backing, to control sealant joint depth, to act as a bond breaker to prevent three-sided sealant adhesion, and to provide an hourglass contour of the finished sealant bead.

Bed Joint

The horizontal layer of mortar on which a masonry unit is laid.

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. See also Backer Rod.


A solid masonry unit of clay or shale, usually formed into a rectangular prism while plastic and burned or fired in a kiln.

Building Enclosure

The elements of a building that act as the environmental separator between the interior environment and the exterior environment. Walls, windows, roofs, slabs, basements, and joints are all part of the building enclosure.

Building Envelope

An outdated term for the building enclosure.


A material or assembly that forms the exterior face of a wall such as brick/stone veneer

Concrete Masonry Unit (CMU)

Precast hollow block or solid brick of concrete conforming to ASTM C-90.


The change of state from vapor to liquid. A common factor in moisture damage. Occurs on surfaces when they are cooler than the air containing vapor next to it.

Control Joint

Formed, sawed, or tooled in a masonry structure to regulate the location and amount of cracking and separation resulting from the dimensional change of different parts of the structure, thereby avoiding the development of high stresses.

Control Layers

Notional concepts used to describe which materials and/or assemblies provide the control functions in a building enclosure and as an aid to ensure continuity of the functions in design and construction. They comprise one or several materials and are formed into planes to create a three-dimensional boundary. 

See: Thermal Control Layer, Vapor Control Layer, Vapor Barrier, Air Control Layer (or Air Barrier System), Water Control Layer


To develop the ultimate properties of a wet state material by a chemical process. Different than drying, which is not a chemical process— although drying is often a necessary part of a chemical process.

Drainage Plane

A water-repellent layer designed and constructed to allow the flow of water by gravity without allowing penetration of the layer. The materials that form the drainage plane often overlap each other shingle-fashion or are sealed so that water flow is downward and outward. They are part of the water control layer of drained enclosure systems and require interconnection (sealed or lapped) with flashings, with window and door openings, and with other penetrations of the building enclosure. See also Water-Resistive Barrier.

Perfect Barrier

One of the three available rain penetration control approaches (the other two are drained and mass/storage). This approach relies on the exterior face of the enclosure to act as the rain control layer and as a perfect barrier to rain penetration; neither drainage nor drying are required for successful performance.


A wood product made of three or more layers of veneer joined with glue and usually laid with the grain of adjoining plies at right angles to one another.

Pressure Boundary

The primary air enclosure boundary separating conditioned air and unconditioned air or conditioned air and semi-conditioned air. The air control layer/air barrier system is by design intended to define the extent of the pressure boundary


A cladding system and/or rain control strategy that accepts that some water will penetrate the outer surface (the cladding, which “screens” rain) and removes this water back to the exterior by gravity drainage over a drainage plane and through a drainage gap and then exiting via flashing and weep holes. In this guide, this term applies to drained systems (see also Drained) and may include systems that are ventilated.

Relative Humidity

The ratio (expressed as a percentage) of the amount of moisture within the air to the maximum amount of moisture that the air could possibly contain at a specific temperature.


Replacing mortar in masonry joints. Also known as tuckpointing.


Quantitative measure of resistance to heat flow, the reciprocal of the U-factor. The units for R-value are ft2 °F hr/Btu (English).

Rigid Insulation

Rigid board material that provides thermal resistance. Foam plastic such as EPS, XPS, and polyisocyanurate are commonly used.

Rough Opening

The opening in a wall into which a door, window, or other enclosure component is to be installed.


The transition of small horizontal surfaces, such as the top of a balcony guardrail or parapet wall, with a vertical surface, such as a wall.


A flexible, polymer-based elastomeric material installed wet and used in the assembly of the building enclosure to seal gaps, seams, or joints and to provide a clean finish, or to waterproof or airtighten the joint.

Semi-Rigid Insulation

Formed board material that provides thermal resistance and comprises mineral fibers. Mineral fiber insulation is normally used for its noncombustible properties and is typically composed of glass or rock wool.


A board material used to provide stiffness to the wall and/or to provide sufficient strong and stiff backing for attaching cladding, membrane control layers. Typical materials are oriented strand board (OSB), plywood, timber, fiberboard, various forms of gypsum board, and some new high-density polymer hybrids.


A fragment of material, such as concrete or masonry, detached from a larger mass by a physical blow, freeze-thaw, high levels of compression, or subfluorescence.

Split Face

A rough concrete masonry face formed by splitting slabs in a split-face machine.

Stack Effect

Air movement driven by buoyancy—that is, the density difference between two columns of air at different temperatures. Often described as warmer air rising or cold air falling. Stack effect generates small but steady pressures over a height in direct relation to the temperature difference and the height of the column of air. The resulting pressure differences can lead to air leakage and can generate unplanned air flows within buildings, which can result in indoor air quality problems or which may be used to ensure a chimney evacuates smoke or drives natural ventilation.


One of a series of wood or light-steel vertical structural members placed as supporting elements in walls and partitions.

Termination Bar

Also called retention bar. A bar of rigid material (often metal) used to end a roofing, flashing or air control membrane in a secure and durable manner. Mechanical clamping is the primary function, but a gum lip is also usually provided to allow sealant as well. Site-built versions often use simple 1×2 wood strips, small galvanized angles, or flat steel.

Thermal Bridge

A material with higher thermal conductivity transferring heat through an assembly with substantially lower thermal conductivity. For example, a steel stud in a wall will transfer more heat than the surrounding insulation, reducing the overall thermal control of the system.

Thermal Boundary

The layer in a building enclosure that controls the transfer of energy (heat) between the interior and the exterior. It is a component of the building enclosure and it may, but does not have to align with the pressure boundary

Thermal Control Layer

The layer in a building enclosure (comprising one or several materials and formed into planes to create a three-dimensional boundary) that is designed, installed, and/or acts to form the primary control of heat flow in an enclosure assembly. It is often partially penetrated by thermal conductive elements, which, if large, are termed thermal bridges.

Through-Wall Flashing

Flashing that extends completely through a wall system and is designed and applied in combination with counterflashings to prevent any water that enters the wall above from proceeding downward.


Quantitative measure of heat flow or conductivity; the reciprocal of R-value. While building scientists will use R-values for measures of the resistance to heat flow for individual building materials, U-factor is usually used as a summary metric for the ease of heat transfer through building assemblies.

Vapor Barrier

A layer (often comprising a single material) that has a water vapor permeance of 0.1 perm or less, and is thus a Class I vapor control layer. A vapor barrier is a material that is essentially vapor-impermeable. The test procedure for classifying vapor barriers is typically ASTM E-96 Test Method A—the desiccant or dry cup method when the vapor barrier is located on the interior of the enclosure assembly. Examples include metal, glass, polyethylene, asphalt membranes, etc.

Vapor Control Layer

The element (or elements) that is (or are) designed and installed in an assembly to control the movement of water by vapor diffusion.

Vapor Permeance

A layer property that describes the ease with which water vapor molecules diffuse through it. It is defined as the quantity of vapor flow across a unit area that will flow through a unit thickness under a unit vapor pressure difference. The unit of measurement is typically the “perm” (gr/h·ft2 ·in. Hg).

Vapor Retarder

A vapor retarder is a material that has a permeance of 1.0 perm or less and greater than 0.1 perm. A vapor retarder is a material that is vapor–semi-impermeable. A vapor retarder is a Class II vapor control layer. The test procedure for classifying vapor retarders is ASTM E-96 Test Method A—the desiccant or dry cup method.

Water-Resistive Barrier (WRB)

A water control layer product or system. See drainage plane.


Nonstructural facing of brick, concrete, stone, tile, or other similar material attached to a backing for the purpose of ornamentation or protection.

Water Control Layer

The continuous layer (comprising one of several materials and formed into planes to form a three-dimensional boundary) in an enclosure assembly that is designed and installed to act as the innermost boundary for rainwater. Penetration of any substantial amount of rainwater further into the enclosure is deemed or results in a performance failure.

Water-Shedding Surface

The outermost surface or material of a building enclosure exposed to rain. By definition it occurs in all building enclosure rain control strategies.

Weep Hole (i.e., weeps)

An opening placed in a wall or window assembly to permit the escape of liquid water from within the assembly. See also Weep vent.

Weep Vent

An opening placed in a wall to permit the escape of liquid water from within the assembly and to permit the flow of air.


Thank you to these industry leaders for underwriting the development of this Guide.

You deserve a great building!
Get started by downloading your free copy of the Masonry Systems Design Guide.