Glossary of Terms
A quick guide to some key window terms you may see while shopping for your next project.
AAMA (American Architectural Manufacturers Association): the leading trade association representing window, door, skylight, curtain wall and storefront manufacturers and their suppliers, serving as the source of performance standards, product certification and educational programs
Apron: Inside flat trim member, which is used under the sill at the bottom of a window
Argon gas: a colorless, odorless and non-reactive gas and a cost-efficient solution to reduce heat loss in sealed windows
Astragal: a framing device used as the central part of a double door, which is attached to the door panel
Balance System: a device used for vertically sliding windows to help hold the open window in place by counterbalancing the weight of the window through a spring or weight
Bay Window: a composite of three or more windows, usually made up of a large center unit and two flanking units at a 30°, 45° or 90° angles to the wall
Bow Window: a composite of four or more window units in a radical bow formation
Brick Mould: outside casing around a window to cover jambs and through which nails are driven to install the window
Butyl: a rubber material (with the lowest gas permeability of all rubbers), which seals the spacer to the glass between the panes of a window
Casing: inside casing is a flat decorative mold that covers the inside edge of the jambs and the rough openings between the window unit and the wall outside casing (brick mould) serves the same purpose, while it also is an installation device through which nails are driven to install the windows unit into the wall
Check Rail: the bottom rail of the upper sash where the lock is mounted on a double-hung window
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 (aka window capping): a material (typically aluminum) placed on the exterior of window frame used for weather protection and aesthetic purposes
Clerestory: a window near the top of an outside wall
Cottage Double-hung: a double-hung window in which the upper sash is shorter than the lower sash
Cripples: the short 2”x4” members used to frame in under the sill or above the header in a rough opening for a window in a frame wall
Curb: a watertight wall of frame used to raise slope glazing above the surface to the roof as a preventative measure against water leaking from melting snow or rain runoff
DOE (Department of Energy): a government organization promoting energy technology and advancement in the U.S.
Dormer: a space that protrudes from the roof, usually including one or more windows
Double Glazing: use of two panes of glass in a window to increase energy efficiency and provide other performance benefits
Double Rafter: the doubling (side-by-side) of the roof members to reinforce an opening in the roof for a slope glazing installation
Drip Cap: a molding placed on the top of the head brick mould or casing of a window frame
Egress Window: a window providing an alternative exit route in case of emergency
Energy Rating: the level of energy performance, in terms of the framing, weatherstrip and window glazing, which is determined by the National Fenestration Rating Council
ENERGY STAR® Certification Program: a certification program managed by the EPA and the DOE, determining windows that are 30% or more energy efficient than the Model Energy Code requires
EPA (Environmental Protection Agency): the organization responsible for writing regulations for federal environmental laws
Failed Unit: a window with a broken seal, allowing moisture to get caught between the panes of glass
Fenestration: an architectural term referring to the arrangement of windows in a wall
Finger-Jointing: a means of joining individual pieces of wood together to form longer lengths, by forming a set of interlocking fingers, coating with an adhesive, and meshing them together under pressure
Fixed: Non-venting or Non-operable
Flashing: a metal or plastic strip attached to the outside of the head or side jambs to provide a weather barrier, preventing leakage between the frame and the wall
French Sliding Door: a sliding door with wider panel members around the glass, giving the appearance of a French hinged door
Gas Fill: a gas (usually argon) that is placed between two panes of glass, reducing the U-Factor
Gasket: a pliable, flexible, continuous strip of material used to create a watertight seal between the sash and frame of roof windows, much like the seal around a refrigerator
Glazing (n): the glass panes or lights in the sash of a window
Glazing (v): the act of installing a window sash
Glazing Compound: a pliable substance applied between the window sash and the lites of the glass to seal against the elements and sometime to adhere the glass to the sash
Glazing Stop: the part of the sash or door panel that holds the glass in place
Head: the main horizontal member forming the top of the window or door frame
Head Board: a flat board cut to fit the contour of a bow or bay window and installed between the head jambs and the flat wall surface to finish off that area, which would normally be the ceiling
Header: a heavy beam extended across the top of the rough opening to prevent the weight of the wall or roof from resting on the window frame
Hopper: a window with a top sash that swings inward
Insulating Glass (IG): a grouping of two or more panes of glass with sealed air between the panes, which may or may not be filled with argon
Insulating Spacers: a piece separating and sealing two panes of glass, which helps insulate a double-pane window
Jack Stud: framing members, generally 2”x4”s, which form the inside of the window or door rough opening, running from the sole plate to the header
Jamb: the piece of a window or door that stops the sash from continuing to close
Jamb Liner: a metal or plastic covering the inside surface and head jambs or sliding windows
Keeper: the protruding, hook-shaped part of a casement window lock, which is mounted to the inside surface of the sash stile
Lift: a handle or grip installed on the bottom rail of the lower sash of double-hung to make it easier to raise and lower the sash
Lite: the glazing framed by muntins and/or sash in a window or door
Light Shaft: an insulated shaft built to direct the light from a roof window or skylite through the attic to the room below
Low-E Glass: a term short for low emissivity glass, which has a coating allowing the sun’s visible spectrum into the interior of a home and reflects most heat energy back outside, working like sunblock to protect your home from UV rays
Masonry Openings: the opening in a masonry wall to accept a window or door unit, aka rough opening
Mortise: a slot or rectangular cavity cut into a piece of wood to receive another part
Mortise & Tenon: a strong wood joint made by fitting together a mortise in one board and a matching projecting member (tenon) in the other
Mullion: the vertical or horizontal divisions or joints between single windows in a multiple window unit
Mullion Casing: an interior or exterior casing member to cover the mullion joint between single units
Muntin: a short bar used to separate glass in a sash into multiple lites, aka window pane divider, T-bar or Grille
NFRC (National Fenestration Rating Council): the only federally recognized organization for determining the energy performance of windows, doors and skylights
Operator: a metal arm and gear that allows for easy operation or closing or projecting windows
Outer Frame Member: the exterior protruding portion of a window frame that has no exterior casing
Palladain Window: a large, arch top window flanked by smaller windows on each side
Panel: the separate panel or panels in a door frame
Passive Solar Collector: any glazed area in the walls or roof of a building pointed to the south to take maximum advantage of the sun’s heat without mechanical (or active) method of storage or distribution of the heat
Picture Frame Casing: the use of casing on all four sides of the interior of a window, replacing the stool and the apron at the sill, aka bound casing
Pitch: the degree of the inclination upward from horizontal or flat, expressed in degrees or as the ratio of the number of inches it rises in each 12 inches of horizontal span (ex: 4/12 means the roof rises four inches in every foot of horizontal span)
Pivot: a mode of operation for ventilating windows, which generally means the sash pivots on a central axis and turns 90 or more degrees
Rafter: the structural members of a roof that support the roof load and run from the ridge to the eaves
Rails: the horizontal members of a window sash or door panel
Rough Opening: the framed opening in a wall where a window or door unit is to be installed
Rough Sill: the horizontal rough framing member, usually two inches by four inches, which forms the bottom of the rough opening, toe-nailed into the jack studs and supported by cripples
Sash: a structure built into a frame for holding one piece of glass
Sash Balance: a system of weights, cords and/or coiled springs that assist in raising double-hung sash and tend to keep the sash in a any placed position by counterbalancing the weight of the sash
Sash Cord: the rope of chain that attaches the sash to the counter balance in double-hung windows
Sash Lock: a cam-action type lock applied to the check rails of a sliding window or at the open edges of a projecting window to pull the check rails tightly together or to seal the sash tightly to the frame, both for security and weather tightness
Sash Weights: the conceiled cast-iron weights that are used to counterbalance the sash in older double-hung windows
Seat Board: a flat board cut to fit the contour of a bow or bay window and installed between the sills and the flat wall surface, providing a seat or shelf space
SHGC (Solar Heat Gain Coefficient): a measurement to determine how well a window blocks heat from sunlight. The lower the SHGC, the lower the heat gain through a window, ranging from 0 to 1
Shims: Wood wedges (often wood shingles) used to secure the window or door unit in the rough or masonry opening in a square, level and plumb position during and after installation
Side Lites: Tall, narrow, fixed or operating sash on either or both sides of a door to light an entryway or vestibule
Sill: a horizontal member that forms the bottom of a window frame
Sill Course (aka Soldier Course): the row of brick, cement blocks or stones laid across the bottom of a masonry opening, which lie under the outside edge of the window sill
Simulated Divided Lite (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: the use of single panes of glass in a window, which is not as energy-efficient as double glazing
Single-hung: a double-hung type window in which the top sash is fixed or inoperable
Slope Glazing: any glazed opening in a sloped roof or wall, such as a stationary skylight or fully operable roof window
Solar Gain: the process of providing a net heat gain within a structure, over and above the normal heat loss, by passive collection of the sun’s heat through windows and other glazed areas
Sole Plate: the bottom horizontal member in a frame wall, nailed to the deck or rough floor with studs nailed into it
Stile: the vertical side member of a window sash or door panel
Stool: the inside horizontal trim member of a window sash or door panel
Stop: a wood trim member nailed to the window frame to hold, position or separate window parts, often molded into the jamb liners on sliding windows
Storm Window: a second window, placed temporarily on the inside or outside of an existing window to provide added insulation
Stud: the vertical wood framing members that form a frame wall, normally 8-ft long 2”x4”s
Tenon: a rectangular projection cut out of a piece of wood for insertion into a mortise
Transom: a smaller window above a door or other window
Transom Joint: the horizontal joining area between two window units that are stacked on top of each other
Triple Glazing: a sash glazed with three lights of glass, enclosing two separate air spaces
True Divided Light (TDL, aka T-Bar): windows in which multiple individual panes of glass or lights are assembled in the sash using muntins.
U-Factor (aka U-Value): a measurement to determine how well a window, door or skylight prevents heat from escaping. The lower the number, the more efficient the window, typically ranging from 0.20 to 1.20
Unison Lock: a casement locking system that secures the window at two locking points by operation of one handle
Vapor Barrier: a watertight material used to prevent the passage of moisture into or through floors, walls and ceilings
Venting Unit: a window or door unit that opens or operates
Weatherstrip: the sealant for any openings, cracks or gaps in windows and doors to block precipitation and air
Wind Load: the force exerted on a surface by moving air
Window Replacement (vs. glass replacement): the replacement of the glass AND the frame of the window
Windowpane Divider (aka muntin, grille or T-bar): a short bar used to separate glass in a sash into multiple lights
