Glossary of Common Window Industry Terminology

Air Leakage

Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.

American Architectural Manufacturers Association (AAMA)

The American Architectural Manufacturers Association or AAMA sets all performance standards, product certification and educational programs for the window industry. When a product passes AAMA tests, you can expect long and reliable service as long as the window is installed and maintained correctly. Visit the AAMA Website at


A homogenous mixture or solid solution of two or more substances. AMSCO® Renaissance® Series windows are an alloy of resins.


A malleable metallic element that has good electrical and thermal conductivity, high reflectivity, and resistance to oxidation.

Aluminum-Clad Windows

Windows with an aluminum material locked to the frame to provide a durable, low-maintenance exterior surface.

Casement Window

A window that opens from the side like a door. Historically, casements were the first working windows. They were strategically placed throughout a house to capture breezes and direct them through the rooms. Screens are hung internally to prevent dirt and insects from entering the house.


The trim around door and window openings. Interior casings are shaped and decorative pieces of moulding cover the inside edges of the jambs and the rough opening between the window unit and the wall. Exterior casing is an alternative to brick moulding.


A blend of thermoplastic alloys that is heated, pressurized and melted through a main extruder creating a solid core extrusion.


Condensation occurs when excess humidity in warmer air is released in the form of water droplets onto a colder surface such as a pane of glass.

Double Glazing

Two panes of glass separated by an air-space to form insulating glass.

Double-Hung Window

A window with two sashes, upper and lower, that slide vertically past each other.


ENERGY STAR is an independent U.S. government program establishing a standard set of guidelines to recognize the energy efficiency of various products. ENERGY STAR guidelines are used in conjunction with a variety of building materials, including windows and patio doors. Over the past 10 years, ENERGY STAR guidelines have helped double the efficiency of windows they endorse. Visit the ENERGY STAR website at

Exterior Casing

Trim around the exterior of a window or door frame that serves as the boundary moulding for the siding material.


A form produced by forcing material through a die.


The placement (or arrangement) and design of the windows and exterior doors of a building. In Greek architecture, windows began as simple openings in temples. These openings began to contain glass in the 13th century, when clear glass was available for buildings such as Westminster Abbey. Another important shift in fenestration occurred in the 20th century when large windows became important components in commercial buildings.


The assembly of structural members (head, sill, jambs) used to fasten a window sash or a door panel to a structure.


The process of mounting glass into windows and doors. Glazing also refers to the lowest quality of plate glass. The purpose of glazing is to retain the glass adequately under the design load, provide an effective weather seal, prevent loads or pressure points on the glass resulting from building movement, prevent glass-to-metal contact, and minimize glass breakage from mechanical or thermal stress. An insulating glass (IG) unit is two glass panes separated by a spacer and sealed. IG glass is offered in clear (no special coating) and high performance, which has a tinted, low-emissivity coating for exceptional energy efficiency.


The K-value of any material describes the amount of heat that moves through a material over time. The higher the K-value the more heat is transferred. Therefore a material with a lower K-value is a better insulator.

Low-Emmisivity (Low-E) Glass

Low-E glass is manufactured by depositing a microscopically thin, transparent metal or metallic oxide layer on the glass. Low-E coatings reduce radiant heat loss, and can reduce the passage of UV rays. Use of heat-resistant (or absorbing) glass began in the 1950s, as did the use of reflective (or mirror) glass.


The individual pieces of a decorative grid that help divide a window opening into smaller sections.

National Fenestration Rating Council (NFRC)

The NFRC is an independent, third-party certification organization with industry-accepted standards for evaluating and certifying energy performance. The NFRC Certificate contains U-factor, SHGC (Solar Heat Gain Coefficient) and VT (Visible Transmittance) values. These values form the basis for the ENERGY STAR® Door and Window Program. Visit the NFRC Website at


A single section of glass.

Picture Window

A fixed window – typically of a large size in relation to adjacent windows.


Any of various synthetic substances similar to natural resins normally used in plastics.


Refers to a window’s resistance to thermal transfer or heat flow. The higher the value, the better the insulation.

Safety Glass

See “tempered glass.”


An assembly of stiles and rails that forms a frame for holding the glass in a window.

Single-Hung Window

A window with a fixed upper sash and movable lower sash that slides vertically.

Slider Window

A window with a sash or sashes that move horizontally.

Solar Heat Gain

Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed, then subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits.


An acrylic-based capstock used to form a protective durable layer on AMSCO composite windows and doors. This capstock helps create the strong construction and prevents chipping, peeling and fading.

Tempered Glass

Glass that is treated with heat during the manufacturing process. Safety glass can withstand abnormal force or pressure on its surface and doesn’t break into sharp pieces. Code requires tempered glass in all doors (including patio doors) and in windows that are located near doors, bathtubs or showers. Also called safety glass.

Tilt Window

A double-hung window designed in such a way that the sashes tilt inward for easy cleaning of both sides.


U-factor or U-value is a number that represents the rate of heat loss through a window or door. The lower the number, the greater a window resists the transfer of heat. A U-factor of 0.4 or lower represents good insulating value.


Any of a various number of tough, flexible, shiny plastics.

Visible Transmittance

Visible Transmittance (VT) measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.


Performance in various climates.

Wood Veneer

A thin layer of finely grained wood adhered to the frame of the window.