Fenestration Glossary - Exterior Windows & Doors

Browse Atlas Supply’s Fenestration Glossary below for definitions about the placement of exterior windows and doors. Visit Fenestration OEM afterwards for more information about high-performance glazing components.

A – F
G – L
M – R
S – Z

Active Solar Heat Gain: Solar heat that passes through a material and is captured by mechanical means.
Air Space: Refers to the space between the two lites of glass in a IG unit. Air space is usually 1/32″ greater than spacer width.
Adsorbent: A highly porous solid with the ability to concentrate and hold gasses and vapors in contact with the solid. This includes moisture, as well as many other organic and inorganic molecules.
Adsorption: The adhesion in an extremely thin layer of molecules (as of gases, solids, or liquids) to the surface of solid bodies or liquids with which they are in contact. Adsorption is the result of attractive forces that can be likened to tiny “magnetic” microelectric charges on the surface of the adsorbent and the adsorbed substance. Variables affecting capacity include polarity, temperature, molecular size, adsorbate concentration, and type of adsorbent.
Argon Gas: An inert gas used to fill IG units as a replacement for air. Argon filled IG units have better thermal and sound insulating qualities then air filled units.
Bed or Bedding: The bead of compound applied between a lite of glass or a panel and the permanent stop or sight bar of the sash or frame, and usually the first bead of compound to be applied when setting glass or panes.
Bead: A Sealant/compound after application to a surface of the spacer.
Beveled or Cap Bead: Bead of compound applied so as to have a slanted top surface so water will drain away from the glass or panel.
Bite: The dimension by which the edge of a glass product is engaged into the glazing channel.
Block: Rectangular, cured sections of neoprene or other approved material, used to position the glass product in the glazing channel.
Bow: Refers to any curve in the spacer. These could be up, down, or sideways curves.
Breather (Tube) Units: An insulating glass unit where a tube or a hole is factory placed into the unit’s spacer to accommodate elevation of pressure differences encountered in shipping. These tubes or holes are to be sealed on the jobsite prior to unit installation.

BTU: An abbreviation of British Thermal Unit (BTU) that defines the amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit (F).
Butt Glazing: The installation of glass products where the vertical glass edges are without structural supporting mullions.
Butyl: A sealant which is used to adhere the glass to the spacer in an IG unit.
Capillary Tube Units: An insulating glass unit where a very small metal tube of specific length and inside diameter is factory-placed into the unit’s spacer to accommodate both the pressure differences to the point of installation and also the pressure differences encountered daily after installation. Capillary tubes are not sealed after installation.
Cavitywall: A type of building wall construction consisting of an outer wall fastened to an inner wall separated by an air space.
Certified IG Unit: An insulating glass unit constructed like a unit test model which has successfully passed the ASTM E773 and ASTM E774 tests of insulating glass seal durability performance at specific levels.
Channel Glazing: The installation and sealing of glass products into U-shaped glazing channels employing removable stops. Usually associated with a sealant, it is a measure of the structural strength of the sealant material (tensile strength).
Cohesive Failure: The splitting and opening of a sealant/compound within its body, resulting in structural damage and/or water penetration.
Condensation: The visible formation of moisture on the inside (room side) surface of a IG unit.
Conduction: The transfer of heat through matter, whether solid, liquid, or gas.
Convection: A transfer of heat through a liquid or gas when that medium hits against a solid surface.
Corner Flare: (As applied to a folded INTERCEPT spacer) is an increase in spacer width at the corner bend point; caused by the corner punches on the Feeder Press being set incorrectly (the folded spacer should have no corner flare on any of the corners).

Corner Gap: (As applied to a folded INTERCEPT spacer) is the space between the two adjacent corner edges; it should be a minimum of .030″.
Dead Loads: Load force due to glass weight.
Desiccant: A substance that adsorbs and holds moisture and organic and inorganic molecules. It is used to adsorb air space moisture and prevent condensation between the lites.
Dew Point: The temperature above 32°F at which visible water vapor or other liquid vapor begins to deposit on the air-space glass surface of a sealed insulating glass unit in contact with the measuring surface of the dew-point apparatus.
Double Glazed Units: Units of two lites of glass and one air space.
Dual Sealed Unit: Sealed insulating glass unit fabricated with an inner primary seal and an outer secondary seal. Generally, each of the two seals has been selected for its special performance characteristics, i.e.: adhesion and moisture vapor transmission properties.
Desiccant Station (stand): The machine that applies the hot melt matrix to an Intercept spacer. It also controls the application time delays on the Extruder.
Emittance: Heat energy radiated by the surface of a body, usually measured per second, per unit area.
Equivalent Combined Glass load: Combination of the instant applied load of wind and the factored long term loading of glass weight and snow accumulation.
Exterior Glazed: Glass set from the exterior of the building.
Extruder: Machine which applies the sealant to a spacer.
Extrusion: An application of sealant or matrix.

Face Glazing: On a rabbetted sash without stops, the triangular bead of compound is applied with a glazing knife after the bedding setting and clipping the lite in place.
Failed IG Unit: An installed unit failure exhibits permanent material obstruction of vision through the unit due to accumulation of dust, moisture or film on the internal surface of the glass. Surface numbers 2 or 3 in dual-pane units; surface numbers 2,3,4 or 5 on triple-pane units.
Feed Hold: Puts the Intercept in a pause condition, there are two types, zfeed press feed hold” and “system feed hold”.
Feeder Press: The station on the Intercept Spacer Frame Machine that punches or stamps Intercept® spacer material to make corners, grid punches, tabs (lead corners), gas and rivet holes. It is also referred to as the punch station, punch press, or feed press.
Feeder Press Feed Hold: The only feed hold that deactivates the air cylinders on the feeder press. The button is located on the feeder press itself.
Float Glass: Transparent glass with flat, parallel surfaces formed on the surface of a pool of molten tin.
FRL: (Filter/regulator/lubricator), Regulates the pressure of the air supply, filters dirt and moisture from the air supply, and lubricates the air lines and cylinders.
Flying Cut-Off: (FCO) The station on the Intercept Spacer Frame Machine that parts and swedges the Intercept spacer.
Full Loop: Refers to the point at which the material strip is furthest from the loop control sensor.
Fully Tempered Glass: Transparent or patterned glass with a surface compression of not less than 10,000 psi or an edge compression of not less than 9,700 psi.
Gas-Filled Unit: Insulating glass unit with a gas other than air in the air space to decrease the unit’s thermal conductivity U-value and to increase the unit’s sound insulating value.
Gas Hole: A hole punched approximately 1″ from the beginning of the last side of the Intercept spacer. It is used to fill the unit with argon or krypton.

Gasket: Pre-formed shapes of rubber, or rubber-like composition, used as the weather seal. Also, a spacer for supplemental application of a sealant.
Glazing: The installation and weather-sealing of a glass product in a prepared sash opening.
Glazing Bead: A strip surrounding the edge of the glass in a window or door; applied to the sash on the outside, it holds the glass in place.
Heel Bead: Compound applied at the base of channel, after setting lite or panel, and before the removable stop is installed, its purpose being to prevent leakage past the stop.
Heat-Absorbing Glass: Glass (usually tinted) formulated to absorb an appreciable portion of solar energy.
Heat Strengthened Glass: Transparent or patterned glass with a surface compression of not less than 3,500 psi or greater than 10,000 psi, or an edge compression of not less than 5,500 psi.

Hot Melt: Refers to any sealant or matrix that must be heated to extrude. It is also used to describe the process of sealing units through a oven/roll press.
Insulating Glass: (IG, Insulated Glass) Two lites of glass sealed together with an air space between them.
Intercept: (Intercept Spacer Frame Machine) A machine which manufactures Intercept IG spacer frames.
Intercept: The registered trademark for an insulating glass unit designed by PPG. The design is based on a one-piece spacer frame that gives manufactures the ability to make a warm edge insulating glass unit.
Interior Glazing: Glass set from the interior of the building.
Laminated Glass: Two or more lites of glass bonded together with a plastic interlayer.
Leading Edge: (Leading corner; tab) The lead end of the Intercept spacer, it inserts inside the trailing end of same spacer.
Light Reducing Glass: Glass formulated to reduce the transmission of visible light.
Lip: The top portion of the Intercept spacer; measured from the side of the spacer in. The lip should measure from .070″ to .083″.
Lite: A piece of glass that is used in making a IG unit (two or more lites per IG unit).

Live Load: Load force due to weight of non-permanent attachments; people, glazing rigs, washing rigs.
Low Emissivity Glass: Glass with a transparent metallic or metallic oxide coating applied onto or into a glass surface which reflects long-wave intrared energy and thus improves the “U” value.
Matrix: Is a mixture of hot melt adhesive and powder desiccant. Called holt melt matrix, it’s the form of desiccant used in the Intercept spacer.
Metal Spacers: Roll-formed metal shapes used at the edges of an insulating glass unit to provide the desired spacing of the glass. Metal spacers allow areas for sealant applications and contain desiccants.
Multiple-Glazed Units: Units of three glass lites (triple glazed) or four glass lites (quadruple glazed) with two and three air spaces respectively.
Muntins: (Grids) Bars used to divide IG units into geometric lites, they are permanently mounted between two lites. Similar to mullions but smaller in dimensions and weight.
Mullion: A horizontal or vertical member that holds together two adjacent lites of glass or sashes, or curtain wall sections.
MVT: Moisture vapor transfer (MVT); is the rate at which moisture passes through the seal of an IG unit.
Needle Glazing: Application of a small bead of sealant/compound at the site line by a nozzle gun.
Passive Solar Heat Gain: Solar heat that passes through a material and is captured naturally, not by mechanical means.
Patterned Glass: Rolled glass having a distinct pattern on one or both surfaces.

Permeability: The time rate of water vapor or gas transmission through a unit area of the material unit thickness induced by unit vapor pressure difference between two specific surfaces under specific temperature and humidity conditions.
Permeance: The time rate of water vapor or gas transmission through a unit area of a body, normal to specific parallel surfaces, under specific temperature and humidity conditions.
PIB: Polyisobutylene (PIB) A substance normally used as the primary seal of a dual seal unit.
Pre-Shimming: A preformed tape continuing a built-in continuous elastomer rod to eliminate use of individual shims which can be inadvertently omitted.
Primary Sealant: A sealant applied to the inner shoulders of a spacer with its principle purpose to minimize moisture, gas and solvent migration into the unit’s air space.
Profile: Refers to the height of the spacer material; standard, low or ultra low are most common.
Purlins: Structural members, generally horizontal, in slope glazing frames.
R-value: The overall resistance to heat transfer. The R-value is the reciprocal of the U-value.
Rabbet: A two-sided L-shaped recess in a sash frame to receive glass products. Addition of a removable stop will convert it to a glazing channel.

Racking: Movement and distortion of sash or frame because of lack of rigidity, or can be caused by adjustment of ventilator sections. Puts excessive strain on the sealant and may result in joint failure.
Rectangle Spacer: (Box) Spacer material used in the linear process; utilizes folding corner keys.
Reflective Coated Glass: Glass with metallic or metallic oxide coating applied onto or into the glass surface to provide reduction of solar radiant energy, conductive heat energy and visible light transmission.
Rivet Hole: Holes located at the tab and the end of the Intercept spacer; they align when the tab is inserted in the opposite end.
Roll Former: The station on the Intercept Spacer Frame Machine that forms the U shape spacer out of the flat coil stock.
Sealant: Refers to material that joins the two lites of glass to a spacer forming a IG unit.
Sealant Spacer: A permanent adhesive sealant extrusion which may contain a structural metal insert and a precompounded desiccant.
Sealed Insulating Glass Units: Units constructed of two or more lites of glass separated and hermetically sealed to spacer frames at the glass edges with the enclosed air chamber(s) dehydrated at the plant’s atmospheric pressure.
Secondary Sealant: A sealant applied into the exterior glass-spacer cavity to provide elastic, structural bonding of the assembly. In single-sealed units, this sealant also has low gas and moisture vapor transmission property to achieve effective unit performance.
Setting Blocks: Rectangular, cured extrusions of neoprene rubber or other approved material on which the glass product bottom edge is placed on glazing to effectively support the glass weight.
Shear: Strain put on a compound between two surfaces when there is a slipping movement of the two surfaces, parallel to and in opposite directions along the length of the joint, such as occurs when an aluminum channel expands to a greater length than a glass panel when both are subjected to the same pronounced rise in temperature. This kind of strain tends to rub or knead the compound in opposite directions along the joint, as contrasted to other forms of strains which may try to pull the compound apart, by reason of the strain being at a right angle to the joint.

Shore A Hardness: Measurement of the hardness of a cured elastomeric material by means of a durometer hardness gauge. Range of 20-25 is about the firmness of an art gum eraser. Range of 40-50 is about the firmness of a rubbr heel.
Sight Line: Imaginary line around the perimeter of a glazed glass product defined by the top edge of stationary and removable stops, or the line where the glazing sealant or gasket contacts the glass.
Single-Sealed Unit: Sealed insulating glass unit where the structural bonding and moisture sealing is accomplished by a single substance (sealant).
Sloped Glazing: Any installation of glass that is at a slope of 15 degrees or more from vertical.
Spacer Corners: Specific methods used in joining the spacer lengths into spacer frames including interlocking keys, bending, soldering, or welding.
Spacer Frame: The component that separates the lites of glass in an IG unit.
Spacers (Shims): Small blocks of neoprene, or other approved material, placed on each side of the glass product to provide glass centering, maintain uniform width of sealant bead, and prevent excessive sealant distortion.
Spandrel: That portion of the exterior wall of a multi-story commercial building that covers the area below the sill of the vision glass installation and above the head of the glass installation below.
Splay: The difference in spacer width comparing the width at the top of the spacer to the width at the bottom of the spacer; it should not exceed .015″ total.
Stops: The stationary lip of the back of the glazing channel and removable molding (retainer) at the front of the glazing channel.
Structural Glazing Gaskets: Cured elastomeric channel-shaped extrusions used in place of a conventional sash to install glass products onto structurally supporting sub-frames with the pressure of sealing exerted by the insert of separate lock-strip wedging splines.
Structural silicone Glazing: A system in which the glass product is bonded to the framing members of a curtain wall utilizing a structural silicone adhesive/sealant without the presence of outdoor retainers or stops.

Suspended Film Insulating Glass: IG unit manufactured with a light and energy controlling film suspended within the air space.
Swedge: Preformed at the flying cutoff, it slightly reduces the tab width, which makes the tab small enough to fit into the end of the spacer without forcing the spacer to flare; it should measure .025″ to .030″ less than the finished spacer width.
System Feed Hold: Puts the Intercept machine into a pause condition, continuing when feed the hold button is pulled out. It can be initiated from all machine feed hold buttons except the feeder press feed hold button. It can also be activated from the flying cutoff guards and the semi-auto mode proximity switch. A system feed hold does not deactivate the air cylinders on the feeder press.
“T” Bar: Spacer type used in polysulfide and hand gunned hot melt applications.
Tight Loop: Refers to the point at which the material strip is closest to the loop control sensor.
Time Delay: Is the time between sensor activation and extrusion application. It is set with the time delay switches on the desiccant stand.
Tin Plate: Refers to the plating on the steel. Tin plated steel is used for manufacturing Intercept spacer frames.
Transducer: An electrical device used on loop controls to maintain the proper material distance from the floor. It works by transmitting and receiving sound waves, then emitting a signal to the uncoiler motor.
Twist: Deformation of the spacer. Both ends of the spacer turned in opposing directions.
U-Value: The amount of conductive heat energy (BTU’s) transferred through a one-square-foot area of a specific insulating glass unit for each degree Fahrenheit temperature difference between the indoor and outdoor air. It is the inverse of the R-value; U=1/R.
Warm Edge Technology: Is used to define thermal characteristics for the edge of glass region of the IG unit in relation to heat loss as well as to condensation potential.
Weep Holes: Slots or holes in the sill (bottom) member of the sash frame to provide outdoor release of infiltrated water.
Wet-Out: The area of seal created by the butyl to glass press after the butyl/glass is heated and pressed. This is also called seal width or MVT path.