Glossary

Apron : A piece of window trim attached to the wall underneath the interior stool (interior sill).

Argon Gas: A colorless, odorless, inert, and nontoxic gas (denser than air) used between windowpanes to enhance energy efficiency by reducing heat transfer.

Astragal: A component fastened to one sash/panel of a gliding window or patio door that becomes the closing shoulder of the operating sash/panel. Its function is similar to a mull post.

Awning Window: A window hinged at the top, opening outward from the bottom using a crank, push bar, or pressure hinges.

Balance System: A mechanical device (also called balancer) used on double-hung windows to counterbalance the weight of each sash during opening and closing.

Bay Window: Three or more window units attached together to project outward from a building wall. Bay angles vary; common ones are 30°, 45°, and 90°.

Bottom Rail: The bottom horizontal component of a window sash that supports the weight of the glass.

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

Brickmould: A trim piece that covers the gap between the window frame and the exterior wall, traditionally made from milled wood and used to attach the window to the wall.

Capstock: Various materials used to coat wood, vinyl, metal, or composite materials, providing protection against environmental exposure like ultraviolet light.

Casement Window: A window hinged at the top or bottom, may pivot inward or outward, and opens using a bar or manual crank.

Casing: The decorative molding or frame around a window or door that covers the gap between the wall and the window frame.

Check Rail: The horizontal middle rails that meet where the sashes come together and lock on a double-hung window.

Coil Stock: A sheet, typically of prefinished aluminum, used for external covering and trim, shaped using a brake.

Composite: A solid material made of two or more substances.

Condensation: Water vapor deposited on a surface with a temperature below the dew point, such as cold window glass.

Conduction: The transfer of heat through a solid material from a higher-temperature area to a lower-temperature area.

Convection: The transfer of heat through a fluid material, like air, due to differences in fluid density and gravity effects.

Divided Light: A window with several small glass panes held in place by muntin within the window sash.

Double Glazing: Two panes of glass separated by a spacer and often filled with insulating gas, like argon, to enhance energy efficiency.

Double-Hung Window: A window with two movable sashes that slide up and down independently.

Drip Cap: A type of flashing, usually sheet metal, used to divert water away from the head jamb of a window or door.

Egress Window: A window opening that provides a secondary means of escape or rescue in an emergency, regulated by local codes.

Energy Rating: A window’s energy performance based on the efficiency of framing, weatherstripping, and window glazing, usually certified by the National Fenestration Rating Council.

ENERGY STAR® Certification: A certification program by the EPA and DOE for windows meeting energy efficiency standards 30% above Model Energy Code requirements.

Extension Jamb: A wood trim accessory that fills the space between the window’s innermost edge and the interior wall face.

Extrusion: The process of forcing heated materials, like aluminum or vinyl, through a die to form specific shapes.

Fenestration: Refers to window and door products and accessories.

Fixed Light/Panel/Window: A window component that does not operate (i.e., a picture window).

Flashing: Material, such as sheet metal, installed to shed water away from window or door joints.

Frame: The structural component that surrounds and supports the glass in a window, made from materials like wood, aluminum, vinyl, or fiberglass.

Full-Frame Replacement: A method where the entire old window is removed and replaced with a new window.

Fusion Weld: A process for fusing window frame or sash components together for durability.

Gas Fill: Insulating gas, such as argon or krypton, used between glazing panels to reduce thermal conductivity.

Glazing: The glass component in a window. Single glazing refers to one pane of glass, while double or triple glazing refers to multiple panes for insulation.

Grilles (Muntins): Decorative elements that visually divide a window into smaller sections. They can be placed on or between glass panes.

Head Jamb: The topmost component of a window or door frame.

Infiltration: The passage of air or water through or around a window’s weatherstripping or joints.

Insulated Glass Unit (IGU): A sealed unit made of two or more glass panes separated by a spacer and typically filled with gas for insulation.

Jamb: The vertical components forming the sides of a window frame.

Low-E (Low-Emissivity) Glass: Glass coated with a special layer to reflect heat while allowing natural light through, improving energy efficiency.

Mullion: A vertical or horizontal bar between adjacent window units, providing structural support and separation between panes.

R-Value: A measure of a window’s resistance to heat flow. Higher values indicate better insulation.

Sash
The part of a window that holds the glass panes and may move (e.g., in double-hung windows, both sashes can slide).

Spacer: A component placed between glass panes to maintain even spacing and reduce thermal conductivity.

Tempered Glass: A type of safety glass that is heat-treated to increase strength and, upon impact, shatters into small, less dangerous pieces.

U-Value (U-Factor): A measure of how well a window insulates against heat transfer. Lower U-values indicate better insulation properties.

Vinyl Windows: Windows made from PVC, known for durability, low maintenance, and energy efficiency.

Weatherstripping: Material applied around a window or door to prevent drafts and improve insulation.