Window & Door Terms

Balance: a mechanical device used in hung windows as a means of counterbalancing the weight of the sash.

Basement window: any window type intended for ventilating or illuminating a basement or cellar.

Bay window: A composite of three windows, usually made up of a large center unit and two flanking units at 60- or 45-degree angles to the wall.

Bite: the dimension by which the inner or outer edge of the frame or glazing stop overlaps the edge of the glazing.

Bow window: A composite of four or more window units in a radial or bow formation.

Brickmold: A type of external casing which frames windows and doors.

British thermal unit (Btu): the heat required to increase the temperature of 1 lb. of water 1°F.

Building envelope: the assembly or assemblies of materials and components that enclose building spaces and are exposed to exterior space or separate conditioned interior space from unconditioned interior space.

Casement window: A window unit in which the single sash levers outward, to the right or left.

Casing: Molding of various widths, thickness and shapes applied to the framework of window and door units.

Cellulosic composite material — a composite material whose ingredients include cellulosic elements.
Note: These cellulosic elements appear in the form of, but are not limited to, distinct fibers, fiber bundles, particles, wafers, flakes, strands, and veneers.

Certification — a process that indicates a representative sample of a product line has been tested, that the product meets specified requirements, and that the product is subject to ongoing inspections by an outside certification agency.

Check rail — see Meeting rail.

Chemically bonded (when related to a welded corner) — a process where the two polymer profiles or pieces are heated and fused together with the aid of a chemical reaction. The reaction and bonding is similar to the original extrusion process.

Circlehead: A generic term referring to any of a variety of window units with one or more curved frame members, often used over another window or door opening.

Cladding: An aluminum material locked to the outside faces of many Pella® products to provide a durable, low-maintenance exterior surface.

Clerestory window: A venting or fixed window above other windows or doors on an upper outside wall of a room.

Closing force: see Operating force and Force to latch door.

Combination assembly: an assembly formed by a combination of two or more separate fenestration products whose frames are mulled together utilizing a combination mullion or reinforcing mullion.

Commercial entrance system: a system of products used for ingress, egress, and rescue generally in non-residential buildings.
Note: Commercial entrance systems typically utilize panic hardware, automatic closers, and relatively large amounts of glass. Commercial entrance systems are often site assembled. They are typically subject to high use and possibly abuse and are designed to withstand such use and abuse.

Composite unit: a fenestration product consisting of two or more sash, leaves, lites, or sliding door panels within a single frame utilizing an integral mullion.
Note: Composite units are not to be confused with products made from cellulosic composite materials.

Concentrated load: a force applied to a fixed point on a window, door, TDD, SSP, roof window, or unit skylight component.

Condensation: the deposition of moisture (liquid water or frost) on the surface of an object caused by warm, moist air coming into contact with a colder object.

Conditioned space: an area or room within a building that (a) is heated or cooled by any equipment or appliance; (b) contains uninsulated ducts; or (c) has a fixed opening directly into an adjacent area or room that is heated or cooled by any equipment or appliance or contains uninsulated ducts.

Corrosion: the deterioration of a material by chemical or electrochemical reaction resulting from exposure to weathering, moisture, chemicals, or other agents or media.

Curtain wall: a non-load-bearing exterior wall cladding that is hung to the exterior of the building, usually spanning from floor to floor.
Note: Curtain wall systems can be factory-glazed or designed to accommodate field fabrication and glazing, including optional structural glazing. Curtain wall employs deep rectilinear framing profiles (approximately 150 mm [6 in] or greater), which are often made available in “stock lengths”. Curtain wall vertical framing members run past the face of floor slabs, and provision for anchorage is typically made at vertical framing members only. In contrast to combination assemblies and composite units, non-residential curtain wall systems often need to meet additional performance requirements for interstory differential movement, seismic drift, dynamic water infiltration, etc. Operating vents and entrance doors are provided as separate inserts.

Deflection: displacement due to flexure of a member under an applied load.

Design pressure (DP): a rating that identifies the load, induced by wind and/or static snow, that a product is rated to withstand in its end-use application.
Note: Design pressure (DP) is not to be confused with Performance Grade (PG) or structural test pressure (STP). Loads induced by static snow are applicable only to TDDs, roof windows, and unit skylights.

Design wind load: the wind load pressure a product is required by the specifier to withstand in its end-use application.
Note: When other loads such as snow load are included, a “design load” results.

Divider: a member that divides glazing into separate vision areas. Dividers are either structural or decorative. Other common terms are muntin, true divided lite (TDL), simulated divided lite (SDL), grill, grid, or bar-in-glass.

Door: a means of access for the purpose of ingress and egress. See also Commercial entrance system, Dual-action side-hinged door, Folding door system, Interior door, Passive door, Revolving door, Secondary storm product, Side-hinged door system, Sliding door, Storm door, and Vehicular-access door.

Dormer: A space which protrudes from the roof of a house, usually including one or more windows.

Double glazing (STANDARD FEATURE): Use of two panes of glass in a window to increase energy efficiency and provide other performance benefits.

Double-hung window — a hung window with two sash in which both sash are operable.

Double Weatherstripping (STANDARD FEATURE): A material that has two or more levels of flexibility. An example is the weatherstripping used between the frame and sash of a casement window.

Drip cap: A molding placed on the top of the head brickmold or casing of a window frame.

Dual-action side-hinged door: a door system consisting of one or more leaves contained within an overall frame and designed such that one of the leaves is operable in a swing mode and can be tilted inward from the top for ventilation.

Dual-action window: a window consisting of a sash that tilts from the top and swings inward from the side for cleaning of the outside surface. Also referred to as a tilt-turn window.

Dual glazing: two layers of glazing material mounted in a common frame and/or sash, separated by a space, and sealed or non-sealed.

Dual mode: the primary and secondary window/door, or both primary windows/doors, are closed, the primary windows/doors are locked, and the insect screen (when offered or specified by the manufacturer) is in the stored position.

Extrusion: A form produced by forcing material through a die (if you think back to when you were a kid, this is similar to the spaghetti-like ‘extrusions‘ created by squeezing Play-doh through a mold). Pella window frames are clad with extruded aluminum.

Fenestration: openings in the building envelope, such as windows, doors, secondary storm products (SSPs) curtain walls, storefronts, roof windows, tubular daylighting devices (TDDs), sloped glazing, and skylights, designed to permit the passage of air, light, or people.

Fenestration cladding: the exterior components that cover the frame, sash, leaf, or sliding door panel members and constitute the weather-resistant surface.
Note: Some claddings function only as an aesthetic covering, while others contribute partially to the structural strength of the product. This use of cladding should not be confused with the definition of “Components and Cladding — Elements of the building envelope that do not qualify as part of the main wind-force resisting system” found in ASCE/SEI 7.

Fixed: Non-venting or non-operable.

Fixed door: one or more non-operable assembled leaves or sliding door panels within a door frame and threshold/sill.

Fixed window: a window that is designed to be non-operable and consists of a glazed frame or a non-operating sash within a frame.
Note: This category does not include non-operable unit skylights or TDDs, or products fabricated from curtain wall or storefront systems that are used in window openings.

Flashing: A thin strip of metal or synthetic material that diverts water away from a window or skylight.

Float glass: flat glass that has been formed on molten metal, commonly tin. The surface in contact with the tin is known as the tin surface or tin side. The top surface is known as the atmosphere side or air side.

Foam spacer: Foam material placed in the airspace of the insulating glass in windows to enhance the appearance and improve the performance of the window.

Folding door system: a door system that has, at a minimum, a hinge or pivot attachment of any type between two leaves and three vertical axes about which the leaves rotate. The leaves can be folded to the interior or exterior of the opening. These systems are either top hung or bottom supported by hardware that attaches to a single track system and include, at a minimum, two pivoting/folding leaves, a frame, and a track and roller assembly. The frame has vertical and horizontal members that are joined at the intersections that fully encompass the operating and inactive leaves in a closed position. A flush set track assembly can exist in place of a sill assembly. Additional hinged and pivoting/folding leaves and/or a single-side hinged leaf can be included in the door system.

Forced-entry resistance (FER): the ability of a window or door in the locked position to resist entry under a specified load and conditions.

Force to latch door: the force required to close a door and fully engage the latch.

Frame — the enclosing structure of a window, door, TDD, roof window, SSP, or unit skylight which fits into or attaches to the wall or roof opening and receives glazing, sash, panels, leaves, or vents.

French hinged door: Hinged door(s) which have wider panel members around the glass.

Fully tempered glass: glass that has been heat treated to a high surface and/or edge compression to meet the requirements of ASTM C1048 (kind FT) or CAN/CGSB 12.1.
Note: Fully tempered glass, if broken, will fracture into many small pieces (dice) which are more or less cubical. Fully tempered glass is approximately four times stronger than annealed glass of the same thickness when exposed to uniform static pressure loads.

Fusion welded: see Welded.

Gateway performance requirements: the requirements for minimum gateway test size, air leakage resistance, structural design load and overload testing, water penetration testing, forced-entry resistance, and auxiliary testing which are the conditions permitting a product entry into a Performance Class.

Gateway test size: the minimum test specimen size specified to enter a Performance Class.

Glass: a hard, brittle substance, usually transparent, made by fusing materials such as soda ash (NA2CO3), limestone (CaCO3), and sand under high temperatures.

Glazing: (n): an infill material such as glass or plastic.
(v): the process of installing an infill material into a prepared opening in windows, doors, TDDs, roof windows, SSPs, or unit skylights.

Glazing stop: The part of the sash or door panel which holds the glass in place.

Greenhouse window (garden window): a window consisting of a three-dimensional, five-sided structure, with provisions made for supporting plants in the enclosed space outside the plane of the wall. Operating sash are allowed but are not required.

Grille: A term referring to windowpane dividers or muntins, usually a type of assembly which may be detached for cleaning.

Handle: a component which enables the movement of a sash, leaf, or panel, or which activates a mechanism which locks or unlocks a sash, leaf, or panel.

Hardware: all the necessary equipment to retain, operate, and lock or unlock the sash, leaf, or panel within the frame.

Head: The main horizontal member forming the top of the window or door frame.

Header: A horizontal framing member placed over the rough opening of a window to prevent the weight of wall or roof from resting on the window frame.

Heat-strengthened glass: glass that has been heat treated to a specific surface and/or edge compression range to meet the requirements of ASTM C1048 (kind HS).
Note: Heat-strengthened glass is approximately two times as strong as annealed glass of the same thickness when exposed to uniform static pressure loads. Heat-strengthened glass is not considered safety glass and will not completely fracture into many small pieces (dice) as with fully tempered glass.

Heat treated: see Fully tempered glass and Heat-strengthened glass.

Hinged rescue window: any window that is mounted into a stationary perimeter frame and is permanently hinged at one jamb.

Hopper: A window unit in which the top of the sash swings inward.

Horizontally pivoted window: see Pivoted window.

Horizontal sliding window: a window that consists of one or more sash that slide or roll horizontally within a common frame and can also contain fixed lites/sash.
Note: Typically, operating sash are identified with an (X) and fixed lites or fixed sash are identified with an (O).

Hung window: a window consisting of vertically sliding sash which utilize counterbalancing devices to allow the sash to be opened to any variable position between its fully open and fully closed limits. See also Vertical sliding window.
Note: Common types are single hung, double hung, and triple hung.

Insulating glass (IG) (STANDARD FEATURE): Also referred to as a insulated glass unit, or IGU, a combination of two or more panes of glass with a hermetically sealed air space between the panes of glass. This space may or may not be filled with an inert gas, such as argon.

Inoperable: no longer opening, closing, locking, or unlocking as originally designed.

Integral ventilating system/device: an apparatus that is independent from but installed into a window, door, or unit skylight product for the purpose of controlling the transfer of air through the window, door, or unit skylight product.

Interior accessory window (IAW): a glazed frame and/or sash, attached inboard of existing prime windows, curtain wall, or storefront, in commercial buildings, to enhance control of thermal transmittance, solar heat gain, sound, air leakage, and/or daylight. IAWs are not intended for occupant operation or to be used with the exterior windows in the open position, nor are they intended to provide any specific resistance to air leakage or water penetration, or withstand structural load.
Note: The IAW frame is typically anchored to surrounding construction, to the existing window frame, or to the curtain wall or storefront interior frame. Unlike SSPs and multiple glazing panels, interior accessory windows are intended for use by trained custodial personnel only and are fitted with limited-access custodial locks to hinge or lift out for periodic cleaning of the non-hermetically sealed air space created. If IAWs are intended for regular occupant operation, or used with the exterior windows in the open position, the product should instead be rated as a prime window or SSP.

Interior door: a door system not intended for use in exterior applications.

Interior window: a window system not intended for use in exterior applications.

Jal-awning window: a window consisting of a multiplicity of top-hinged sash arranged in a vertical series within a common frame, each operated by its own control device which swings the bottom edges of the sash outward. See also Jalousie window and Tropical awning window.

Jalousie window: a window consisting of a series of overlapping horizontal frameless louvers which pivot simultaneously in a common frame and are actuated by one or more operating devices so that the bottom edge of each louver swings outward and the top edge swings inward during operation.

Jamb: The main vertical members forming the sides of a window or door frame.

Jamb liner: In a modern double-hung window, the track installed inside the jambs on which the window sashes slide.

Knocked down (KD): Unassembled window or door unit.

Laminated glass: two or more lites of glass permanently bonded together with one or more polymer interlayers.

Leaf: a part of a side-hinged door system, glazed or unglazed, surrounded by a frame. Leaves can be fixed in place (non-operable) or movable (operable).

Light (aka lite): A separately framed piece of glass in a window or door. A traditional double-hung window, for instance, often has several lights divided by muntins in each sash. Such windows are described as six-over-six, eight-over-one, twelve-over-twelve, etc., to indicate the number of lights in each sash. Sometimes spelled ‘lite.’

Limited water (LW) (as a designation): that the water penetration resistance performance is achieved by testing at a pressure less than the minimum test pressure required for the indicated Performance Class and Performance Grade (PG).

Low-emissivity (low-E) glass: A special type of glass having a transparent material fused into its surface which acts as a thermal mirror.

Manufacturer: a company which fabricates and/or assembles one or more parts, components, and/or accessories or supplies entire fenestration systems.

Masonry opening: The space in a masonry wall left open for windows or door.

Meeting rail or check rail: one of the two adjacent horizontal sash members that come together when in the closed position.

Meeting stile: one of the two adjacent vertical leaf, sash, or panel members that come together when in the closed position.

Minimum gateway test size: the test specimen size specified to enter a Performance Class at the lowest or minimum level.

Moisture content: the percentage of dry weight that is composed of water, such as in wood.

Mortise: A slot or rectangular cavity cut into a piece of wood to receive another part.

Mortise-and-tenon: A strong wood joint made by fitting together a mortise in one board and a matching projecting member (tenon) in the other. Often used as a corner joint in window sashes.

Mullion: A wood or metal part used to structurally join two window or door units.

Mullion assembly: a horizontal or vertical member formed by joining two or more individual fenestration units together.

Mullion stiffener: an additional reinforcing member used in a reinforcing mullion. Mullion stiffeners carry the entire load or share the load with adjacent framing members.

Multiple glazing panel (MGP): a glazed panel that can be installed in or on a sash, leaf, or panel on either the interior side or exterior side of the primary glazing. An MGP is tested only in conjunction with a specific primary window or door.

Muntin: Applies to any short or light bar, either vertical or horizontal, used to separate glass in a sash into multiple lights. Also called a windowpane divider or a grille.

Negative pressure: pressure acting in the outward direction.

Non-hung window: a window consisting of vertically sliding sash which utilize mechanical retainers or slide bolts to allow the sash to be opened to any one of the pre-selected positions between its fully open and fully closed limits. See also Vertical sliding window.

Non-operable: intended to not open or close.

Normal use: Regarding windows, doors, secondary storm products, operable unit skylights, and roof windows, normal use means intended for operation for reasons in addition to cleaning and maintenance of the window(s), door(s), secondary storm product(s), operable unit skylight(s), or roof window(s) in question.

Panel: Usually refers to the separate panel or panels in a door frame.

Parallel opening window: a window consisting of an operable sash that moves outward in a horizontal direction perpendicular to the plane of the frame for the purpose of ventilation. The sash remains parallel to the frame throughout its range of motion.

Passive door: one or more hinged leaves or sliding door panels that are normally held inactive by latching or locking hardware, but can become active on the release of the latching or locking hardware.

Performance Class: one of the five Performance Classes (R, LC, CW, AW, and SK) within the classification system that provides for several levels of performance.
Note: This allows the purchaser or specifier to select the appropriate level of performance depending on climatic conditions, height of installation, type of building, etc.

Performance Grade (Grade or PG): a numeric designator that defines the performance of a product in accordance with this Standard/Specification.
Note: Performance Grade (Grade or PG) is not to be confused with design pressure (DP) or structural test pressure (STP). Performance Grade (PG) is achieved only on successful completion of all applicable tests specified

Pivot: an axis or the hardware about which a window, sash, panel, or leaf rotates.

Pivoted window: a window consisting of a sash which pivots about an axis within the frame. The pivoting action of the window allows for easy access to clean the outside surfaces of the window. Two common types are the 180° compression seal pivoting window and the 360° pivoting window.

Plastic glazing: plastic infill materials (including, but not limited to, acrylic, co-polyester, fiberglass-reinforced plastic, and polycarbonate) that are glazed or set in a frame or sash.

Positive pressure: pressure acting in the inward direction.

Primary door: that door in a dual-door system so designated by the manufacturer, capable of protecting the building’s interior from climatic elements (as opposed to a secondary door used mainly for performance enhancement).

Primary window: that window in a dual-window unit so designated by the manufacturer, capable of protecting the building’s interior from climatic elements (as opposed to a secondary window used mainly for performance enhancement).

Rail: The top and bottom horizontal members of the framework of a window sash.

Reinforcement: the material added to individual sash, leaf, panel, or frame members to increase strength and/or stiffness.

Revolving door: an exterior door consisting of two or more leaves that pivot about a common vertical axis within a cylindrically shaped vestibule.

Roof window: a sloped application of a fenestration product that provides for in-reach operation.
Note: Roof windows used for emergency escape and rescue usually have a balanced sash.

Rough opening: The framed opening in a wall into which a window or door unit is to be installed.

R-Value: Resistance to thermal transfer or heat flow. Higher R-value numbers indicate greater insulating value.

Safety glass: a strengthened or reinforced glass that is less subject to breakage or splintering, such as glass for doors, unit skylights, and some windows. See also Fully tempered glass and Laminated glass.

Sash: A single assembly of stiles and rails made into a frame for holding glass.

Sash lift: A protruding handle screwed to the inside bottom rail of the lower sash on a double-hung window.

Sash weights: In older double-hung windows, the concealed cast-iron weights which are used to counterbalance the sash.

Screen: a product that is used with a window, door, secondary storm product, or unit skylight, consists of a mesh of wire or plastic material used to keep out insects, and is not for providing security or retention of objects or persons from the interior.

Sealant: a compound used to fill and seal a joint or opening.

Secondary door: that door in a dual-door system so designated by the manufacturer, used on the exterior of, or interior of, and in tandem with, a primary door designated by the manufacturer to be used for the purpose of performance enhancement, and not to be used by itself as a primary door.

Secondary storm product (SSP): a door, window, or skylight product intended to be used only in conjunction with a primary door, window, or skylight product for the purpose of enhancement of performance in a system with the primary product. A secondary storm product can be attached to the internal or external frame or sash of the primary product. A secondary storm product is also considered a secondary door or window.

Secondary window: that window in a dual-window unit so designated by the manufacturer, used on the exterior of, or interior of, and in tandem with, a primary window for the purpose of performance enhancement, and not to be used by itself as a primary window.

Serviceable: accessible without major reconstruction of the window, door, SSP, TDD, roof window, or unit skylight.

Setting block: a device or part that supports the weight of the glazing and is in direct contact with an edge of the glazing after final installation.

Side-hinged door system: a door system having, at a minimum, a hinge attachment of any type between a leaf and jamb, mullion, or edge of another leaf but having a single, fixed vertical axis about which the leaf rotates between open and closed positions. These systems include, at a minimum, a single operating leaf, surrounding frame, and components. The surrounding frame has vertical and horizontal members that are joined at the intersections and fully encompass the operating and/or fixed leaf/leaves.
Note: Additional operating, passive and/or fixed leaves, side lites, transoms, framing, and mullions are often included.

Side-hinged (inswinging) window: a window that consists of sash hinged at the jambs that swings inward using exposed butt hinges or concealed butt hinges, and in some cases friction hinges. It is used primarily for cleaning or emergency escape and rescue purposes, but not for ventilation other than in the case of emergency. The gateway test size is larger than for casement windows, but otherwise the same requirements are met. See also Hinged rescue window and Top-hinged window.

Sidelights (or sidelites): Narrow fixed units mulled or joined to door units to give a more open appearance.

Sill: The main horizontal member forming the bottom of the frame of a window or door.

Simulated divided light (SDL): A method of constructing windows in which muntins are affixed to the inside and outside of a panel of insulating glass to simulate the look of true divided light.

Single glazing: Use of single panes of glass in a window. Not as energy-efficient as double glazing.

Single-hung window: A double-hung type of window in which the top sash is fixed or inoperable.

Single mode: the primary window/door is closed and latched, the secondary window/door or outer primary window/door is opened fully, and the insect screen (when offered or specified by the manufacturer) is in the functional position.

Slider: see Horizontal sliding window.

Sliding door: a door that consists of manually operated door panels, one or more of which slide or roll horizontally within a common frame, and can also contain fixed lites/panels.
Note: Typically, operating panels are identified with an (X) and fixed lites or fixed panels are identified with an (O).

Sloped glazing (other than unit skylights): a glass and framing assembly that is sloped more than 15° from the vertical and which forms essentially the entire roof of the structure.
Note: Generally, this is a single slope construction.

Sound transmission class (STC): a single-number rating calculated in accordance with ASTM E413, using sound transmission loss values, that provides an estimate of the sound insulation performance of an interior partition in certain common sound insulation problems.
Note: The frequency range used is typical of indoor office noises.

Spacer: the linear material that separates and maintains the space between the glass surfaces of insulating glass units.

Span: the clear distance measured parallel to the length of a mullion or divider between support points.

Spandrel: the opaque areas of a building envelope which typically occur at locations of floor slabs, columns, and immediately below roof areas.

Specification: a written document often accompanying architectural drawings, giving such details as scope of work, materials to be used, installation method, required performance, and quality of work for work under contract.

Stile: The main vertical members of the framework of a sash.

Stool: An interior trim piece on a window which extends the sill and acts as a narrow shelf.

Stop: A molding used to hold, position or separate window parts.

Structural test pressure (STP): the pressure differential applied to a window, door system, TDD, roof window, SSP, or unit skylight.
Note: Structural test pressure (STP) is not to be confused with design pressure (DP) or Performance Grade (PG).

Sunroom: a multi-sided structure consisting of a high percentage of glazed area versus framing area.
Note: Usually a non-conditioned area attached to the exterior of an existing building.

System: the parts, components, hardware, and/or accessories that yield a complete, fully functional assembly.

Tempered glass (STANDARD): Glass manufactured to withstand greater than normal forces on its surface. When it breaks, it shatters into small pieces to reduce hazard.

Tenon: A rectangular projection cut out of a piece of wood for insertion into a mortise.

Thermal break: The addition of a thermal insulating material between two thermally conductive materials.

Thermoplastic: a polymer material that turns to liquid when heated and becomes solid when cooled and is able to repeat these processes.

Top-hinged window: a window consisting of sash hinged at the head which swings inward or outward using a continuous top hinge or individual hinges, primarily for cleaning or emergency escape and rescue purposes and not for ventilation.

Torsion: the twist induced in a product by the application of a static load to an extreme free corner of that product and normal to its plane.

Transom: A small window that fits over the top of a door or window, primarily for additional light and aesthetic value.

Tributary width — the width of wind-bearing area contributing to the load on a mullion or divider.

Tropical awning window: a window consisting of one or more top-hinged or pivoted sash that swing outward at the bottom edge and are operated by one control device that securely closes them at both jambs without the use of any additional manually controlled locking devices.

True divided light: A term which refers to windows in which multiple individual panes of glass or lights are assembled in the sash using muntins.

Tubular daylighting device (TDD): a non-operable fenestration unit primarily designed to transmit daylight from a roof surface to an interior space via a closed-end tubular conduit. The basic unit generally consists of an exterior glazed weathering surface, a light-transmitting tube with a reflective inner surface, and an interior closure glazing in a retainer frame. The interior closure glazing is generally sealed. A TDD product line can be tested and rated in either or both of the following configurations: (a) Closed ceiling (CC): the tubular conduit passes through unconditioned space. (b) Open ceiling (OC): the tubular conduit is suspended in conditioned space.

Turn-tilt window unit: see Dual-action window.

Unit skylight: a complete factory-assembled glass- or plastic-glazed fenestration unit consisting of not more than one panel of glass or plastic installed in a sloped or horizontal orientation primarily for natural daylighting. Unit skylights are either fixed (non-operable) or venting (operable).

Vehicular-access door: a door that is used for vehicular traffic at entrances of buildings such as garages, loading docks, parking lots, factories, and industrial plants, and is not generally used for pedestrian traffic.

Vent Unit: A window or door unit that opens or operates.

Vertical fenestration: fenestration products that are installed at an angle less than 15° from vertical.

Vertical sliding window: a hung or non-hung window consisting of at least one manually operated sash that slides vertically within a common frame.

Vinyl: A plastic material used by some window manufacturers.

Weatherstripping: A material or device used to seal the openings, gaps or cracks of venting window and door units to prevent water and air infiltration.

Weephole (weep): an opening that allows water to drain.

Welded: when materials are fused by heat to become one when cooled.

Windload: Force exerted on a surface by moving air.

Window: an operable or non-operable assembly that is installed in an opening within an exterior wall or roof intended to admit light or air to an enclosure, and is usually framed and glazed.
Note: Windows are typically designed to accommodate factory fabrication and glazing.

Window wall: a non-load-bearing fenestration system provided in combination assemblies and composite units, including transparent vision panels and/or opaque glass or metal panels, which span from the top of a floor slab to the underside of the next higher floor slab.
Note: Window walls are available with separate or integral slab edge covers and can be fabricated from windows or curtain wall or storefront systems. Primary provision for anchorage occurs at head and sill conditions. Receptor systems can be designed as a part of drainage and movement accommodation provisions.