General window terminology

Here are a few window industry terms that might help during your purchasing process.
Window Styles

Awning – Awning windows are compatible with a variety of architectural styles. They are hinged at the top and open out from the bottom in an upward swing.

Bay – A bay window consists of three individual units. The center unit is parallel to the exterior wall. The left- and right-side (flanking) units are aligned at 30- or 45-degree angles.

Bow – Bow windows extend outward to create the illusion of a rounded (arched) wall. A bow window consists of four or more (commonly five) adjoining windows that are installed on a radius from the wall of the building.

Casement – A casement window is hinged on the left or right side and opens outward in a swinging motion. Casements deliver the most unobstructed viewing areas and ventilation of any operating window.

Fixed – Fixed (picture) windows are available in a wide variety of shapes, with either curved or straight frames. They can be used as freestanding units or in combination with other styles.

Single-Hung – A single-hung window features a stationary top sash and a bottom  sash that slides upward vertically. Single-hung windows are available with a tilt or side-load removable sash for easy cleaning.

Sliding – A (horizontal) sliding window has one stationary sash and one sash that moves to the right or left in grooves or on tracks. It may also be referred to as a roller or glider window.

Window Terms Glossary

Air Chambers -Small honeycomb spaces within the sash and frame which help to insulate and strengthen the window

Air Infiltration - The amount of air that passes between a window sash and frame. In windows it is measured in terms of cubic feet or air per minute, per square foot of area. The lower the number, the less air the window lets pass through.

Angled Exterior - A sloped extension from the frame that adds an aesthetically-pleasing dimension to the exterior of the window.

Argon gas - Argon is a safe, odorless, colorless, non-toxic, non-flammable inert gas that is commonly used in place of air between the glass panes of an insulated Low-E glass unit to reduce temperature transfer.

Awning window - A window unit in which the bottom of the sash swings outward for ventilation.

Balance System - Device for holding vertically sliding sash in any desired position through the use of a spring or weight to counterbalance the weight of the sash.

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

Beveled Exterior - An angled extension from the frame that adds an aesthetically-pleasing dimension to the exterior of the window.

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.

Butyl - A rubber material that seals the glass to the spacer, creating an airtight and watertight IG unit. Butyl has the lowest gas permeability of all rubbers.

Cam Lock and Keeper - The mechanisms which pull the sash together when placed in the locked position.

Casement Window - A window unit in which the single sash cranks outward, to the right or left.

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

Center of Glass U- and R-values - The U- and R-values measured from the center of the glass to 2-1/2" from the frame.

Circletop - 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 - Any material locked to the outside faces of doors and windows to provide a durable, low-maintenance exterior surface.

Conduction - Energy transfer from one material to another by direct contact.

Convection - Heat transfer by currents that flow from a warm surface to a colder one.

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

Double-hung window - A window unit that has two operable sashes which move vertically in the frame.

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

Double or dual pane glazing - Use of two panes of glass in a window to increase energy efficiency and provide other performance benefits.

Fixed window (Picture window) - Non-venting or non-operable window. Also known as picture window.

Foam Spacer - Foam material placed in the airspace of the insulating glass in a window to enhance the appearance and improve the performance of the window.

Frame - The enclosure in which window sash or door panels are mounted.

Glazing - Glass in a window or door; the act or process of fitting with glass.

Glazing bead - A plastic or wood strip applied to the window sash around the perimeter of the glass.

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

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

Insulating glass (IG) - 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.

Light or 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 lights of glass to restrict the passage of radiant heat.

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

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.

Muntin Bar - Any small bar that divides a windows glass. Also called a grille or windowpane divider.

Nail-fin - Windows with nail-on frames are for new construction.

National Fenestration Rating Council (NFRC) - This organization conducts tests for energy efficiency based on the entire product and is required to be posted by label on every window. 

Picture - Non-venting or non-operable window. Also know as a fixed window.

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

Pane - A framed sheet of glass within a window.

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

Sash balance - A system of weights, cords and/or coiled springs which assist in raising double-hung sash and tend to keep the sash in any placed position by counterbalancing the weight of the sash.

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

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.

Security Latch - Latch mechanism on the interior face of the sash that retains the window in a partially open position for ventilation.

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

Sloped Sill Adapter - Used to cover the gap between the old sloped sill window and the new block frame window. It adapts a new window to the existing sloping sill.

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.

Solar Heat Gain Coefficient (SHGC) - The ability to block heat generated by sunlight.  The greater the blockage the lower the SHGC number.

Tempered glass - Glass manufactured to withstand greater than normal forces on its surface. When it breaks, it shatters into small pieces to reduce hazard. Standard on all doors and large fixed windows.

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

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

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.

U-value or U-factor - Rate of heat flow-value through the complete heat barrier, from room air to outside air. The lower the U-value, the better the insulating value.

Unison lock - A casement locking system which 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.

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

Vinyl - A plastic material used for cladding or entire window units.

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.

Windload - Force exerted on a surface by moving air.