Window Terms & Definitions

Air Chambers: Small spaces within the window sash and frame that help insulate (and strengthen) the window

Apron: A piece of decorative trim or casing installed against the wall, under the sill and stool of the window

Argon: A non-toxic, colorless, odorless noble gas that is often used in double-pane windows (between the two panes of glass) to help insulate (usually used in windows panes with a wider width)

Balance: A system that holds up sash units on single- and double-hung windows and is used to control the force required to lift and lower the sash

Casing: Decorative trim or molding around the interior frame of a window

Cladding: A material that is installed on the outside of a window to provide a durable, low-maintenance surface

Coated Glass: Glass with reflective coating that offers UV protection

Divided Lights: Division of light by the use of multiple muntins or grilles

Egress: The size of the opening a window creates for access

Fenestration: An architectural term that refers to the arrangement (design and placement) of windows in a wall

Fixed Window: A window that doesn’t open at all and is, therefore, very energy-efficient

Flanker: A “lateral” window used on either side of a main window; bay windows have two flanker windows

Frame: The enclosure where the sash of the window is mounted; structure that keeps the window from collapsing in

Glazing: The process of fitting a window with glass

Grilles/Muntins: The decorative dividers on the window pane that can usually be removed for cleaning

Head: The horizontal member that makes up the top of the window frame

Jamb: Vertical members of the frame that form the sides of the window

Krypton: Insert gas often used in double-pane windows between the panes of glass to help insulate the window (usually used in windows panes with a narrower width)

Low-E: A term used to describe a glass with low emissivity, which helps reflect heat from the sun and provides UV protection

Moisture Barrier: A material (usually polyethylene) that protects your windows and doors from water damage

Mulling: The process of attaching two or more windows together; product is called a mulled window

Mullion: The wood or metal piece that joins two or more window units

NFRC Rating: The energy performance rating for windows provided by the National Fenestration Rating Council (NFRC); independent ratings provide the basis for ENERGY STAR requirements

Orientation: Placement of windows in termsof the view, shading, etc.

PVC: A molded plastic material used for window framing (and sometimes cladding)

R-value: The measure of resistance to heat flow (the higher the R-value, the better the insulating properties)

Rails: The horizontal parts of the window sash

Reflective Glass: A type of glass that reflects heat from the sun and is, therefore, extremely energy-efficient

Rough Opening: A framed opening in a wall where the window is installed

Sash: The part of the window that holds the pane (made up of rails and stiles)

Sill: A horizontal piece that forms the bottom of a window and supports the frame

Simulated Divided-Light Window : A window with just one piece of glass (instead of multiple panes like true divided-light windows) and removable muntins, which are usually attached with a strong adhesive

Stiles: The vertical parts of the window sash

U-value: A measurement of heat entering or escaping through a window (the lower the U-value, the better the insulating properties)

Weatherstripping: A material that is used to create a seal between the sash and the frame of the window to reduce air and water infiltration



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