Replacement Window Glossary

Bay window: a composite of three or more windows, usually made up of a large center unit and two flanking units at 30, 45 or 90 degree angles to the wall.

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

Brick mould: outside casing around window to cover jambs and through which nails are driven to install the window.

Casing: inside casing is a flat, decorative moulding that covers the inside edge of the jambs and the rough openings between the window unit and the wall. Outside casing (or brick mould) serves the same purpose, as well as being an installation device through which nails are driven to install the window unit to the wall.

Double glazing: two panes of glass in a window that increases energy efficiency.

Drip cap: moulding placed on the top of the head brick mould or casing of a window frame.

Fenestration: the arrangement of windows in a wall.

Flashing: 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 that has wider panel members around the glass, giving the appearance of a French hinged door.

Glazing: glass panes or lites in a sash of a window. 

Glazing bead: a vinyl or wood strip applied to the window sash around the perimeter of the glass.

Head: the main horizontal member forming the top of the window or doorframe.

Head expander: window accessory that slides over the window head frame and can be adjusted to eliminate any gap between the window head and the rough opening height.

Hopper: window that is hinged at the bottom and has a top sash that swings inward.

Keeper: piece of window hardware that engages with the sweep lock to draw the window sashes together to ensure security and weather seal.

Lift rail: handle or grip extruded on the bottom of the lower sash rail of a double-hung window to make it easier to raise or lower the sash.

Light/Lite: glazing framed by muntins and/or sash in a window or door.

Low-E glass: a common term used to refer to glass which has low emissivity due to a film or metallic coating on the glass or suspended between the two lites of glass to restrict the passage of radiant heat.

Masonry openings: openings in a masonry wall to accept a window or door unit, the same as a rough opening in a frame wall.

Mullion: vertical or horizontal divisions or joints between single windows in a multiple window unit.

Muntin: short bar used to separate glass in a sash into multiple lights. Also called a windowpane divider or grid.

Operator: metal arm and gear which allows for easy operation or closing of projecting casement windows.

Palladian window: large, arch-top window with flanking smaller windows.

Pivot: for ventilating windows, in which the sash pivots on a central axis.

Rails: horizontal members of a window sash or door panel.

Rough opening: opening left in a frame wall to receive a window or door unit.

R-value: measures thermal resistance in heat transfer. The higher the glass’ R-value, the higher the insulation value of the window.

Sash balance: system of coiled springs that help in raising double-hung sashes and keep the sashes in position by counterbalancing the sash weight.

Sash lock: a cam-action lock applied to the lock rails of a sliding window or at the open edges of a projecting window for security and weather-tightness.

Shims: wedges used to secure the window or door unit in a rough or masonry opening in a square, level, and plumb position during and after installation.

Side lights: tall, narrow, fixed, or operating sash on either or both sides of a door to light an entryway or vestibule.

Sill: horizontal member that forms the bottom of a window frame.

Sill (soldier course): 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: 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-hung: double-hung type of window in which the top sash is fixed or inoperable.

Solar heat gain: 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.

Stile: vertical side member of a window, sash, or door panel.

Transom: smaller window above a door or another window.

Triple glazing: sash glazed with three lites of glass, enclosing two separate air spaces.

U-factor: measure of heat transmission through a wall or window. The lower the U-factor, the better the insulating value.

Windload: force exerted on a surface by moving air.