Glossary of Window Terms

Argon Gas

Argon is about 25% more dense than air. Argon's higher density adds more insulation than air. Argon is also an inert gas, which means it does not react with many other elements.

Awning Window

Hinges from the top and opens outward.

Bay Window

Projects outward and can have double hung or casement windows on the ends with a picture window in the middle.

Bow Window

Projects outward with a curve and combines casement and picture windows.


CleanFree glass is specifically designed to eliminate frequent, strenuous exterior glass cleaning. CleanFree is an application added during manufacturing that repels water and prevents the glass surface from being exposed to harsh elements that lead to damage and corrosion.

Casement Window

Hinges from the side and opens outward. Can be used in combination with stationary lites. Lang Exterior sells casement windows in single casement, 2-lite, 3-lite, 4-lite, and 5-lite configurations.

Double-Hung Window

Contains two sashes that both move up and down. Lang Exterior double hung windows also tilt in top and bottom for easy cleaning.

Garden Window

A projected window with side trapezoids that can open for ventilation. Optional glass shelf available.

Glass Block Window

Made up of individual glass blocks mortared together. May include optional vent.

Grids or Muntins

Decorative bars between the glass panes that create the appearance of a divided window design.

Hopper Window

Hinges from the bottom and opens inward.

Krypton Gas

Krypton occurs only in trace amounts in the atmosphere. The name "krypton" comes from the Greek word meaning "the hidden one." Krypton is a denser gas than argon, making it a better insulator. Like argon, krypton is chemically non-reactive.


A segment of a window that admits light. For example, a 3-lite slider window consists of two operable sashes and one picture window in the center, and each of these parts is a lite.


A heat-reflective coating that can add to the energy efficiency of a window. For example, Low-E glass on the interior pane of a window reflects heat back into the room, which makes the inside glass temperature closer to room temperature and can also help reduce heating energy...

Mechanical Window

Screwed together frame with welded sashes.

Nailing Flange

Used to attach windows to new construction.

Oriel Window

A double-hung window where the upper or lower sash is taller than the other sash.

Picture Window

A non-operating, stationary window.


The resistance of a material to heat flow; the higher the number the better the insulating quality. This common measurement can be compared to U-Value by dividing 1 by the R-Value.

R5 Triple Pane Windows

R5 windows have a U-Value of .20 and come with 3 glass panes, krypton gas between the panes, and Low-E glass on both outside and inside panes.

Slider Window

Opens by sliding to the left and right.

Solar Heat Gain Coefficient (SHGC)

An energy performance rating that measures the fraction of incident solar radiation admitted through the total window unit. The lower the windows SHGC, the less solar heat transmits.

Storm Window

Mounts outside a normal window for protection and insulation in bad weather or winter.

Tempered Glass

lass designed for added safety. When tempered glass breaks it creates small cubes or chunks instead of large, sharp pieces of glass.

Ultra Violet Transmission (UV)

UV radiation is the main source of fading furnishings, drapes, and carpeting. The lower the UV transmission number the better fade protection.


The rate of heat flow through a window; the lower the value, the better the insulating quality. U-Value can be compared to R-Value by dividing 1 by the R-Value. For example, an R-Value of 2 equals a U-Value of 0.5. R-Value is most commonly used to measure the total window...

United Inches

United Inches, or UI, are an easy way of measuring windows where you just add the width and the height of the window to get the UI value. For example, a window that is 40" wide and 60" tall would be 100 UI. UI units are sometimes used for pricing.

V-Bay Window

A two-lite projected window consisting of two casements.