Glass Terms and Definitions

Solar Heat Gain Coefficient (SHGC)

Solar heat gain coefficient is a measurement of the amount of solar radiation through a piece of glass or door material. The lower the SHGC, the fewer UV rays that cause heat gain come through the glass.

Low-E Glass

This type of insulating glass significantly blocks harmful ultraviolet (UV) rays and the sun’s infrared heat, so homes stay cooler in the summer and interior furnishings fade less. In the winter, it helps keep homes warmer and reduces condensation. It also delivers greater visible light transmittance than tinted glass.

Tempered Glass

This glass is heat-treated during manufacturing for extra strength, enabling it to withstand abnormal force or pressure on its surface. And it will not break into sharp pieces. Local building codes often require tempered glass to be used in all windows that are close to the floor or near doors, bathtubs or showers.

Ultraviolet (UV)

UV refers to the type of rays outside of the visible spectrum (at its violet end). These rays are found in everyday sunlight and can cause fading or chalking of dark paint finishes. Extreme UV exposure can cause certain plastic materials to distort.

U-Factor/U-Value

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 0.35 or lower represents good insulating value.

R-Value

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

Double Glazing

Two panes of glass separated by an air space to form insulating glass; double glazing may also be accomplished by adding a storm panel.

Energy Star

Established to recognize the energy efficiency of various products, this independent U.S. government program is responsible for developing a standard set of guidelines (based on NFRC ratings for windows and doors). Energy Star guidelines are used in conjunction with a variety of building materials, including windows and patio doors.

Insulating Glass Unit

A sealed assembly of two or more panes of glass separated by a spacer. Manufacturing of insulating glass began in 1930.

Laminated glass

This glass is constructed by placing a plastic interlayer between two panels of glass. The interlayer helps protect the glass from impacts and prevents shattering.

Obscure glass

This is any type of decorative textured glass that fully or partially obscures the view. It allows for privacy while still letting light enter a home.

Radiation

Radiation is the transfer of energy by light rays. In winter, Low-E glass reduces the amount of radiant heat loss to the outdoors by reflecting the radiant heat back into the house. In summer, the energy is reflected back outside and prevented from entering the house.

Spacer

A component (usually metal) that separates the panes of glass to create the airspace in an insulating glass (IG) unit.

Sound Transmission Class (STC) Rating

The America Society of Testing Materials (ASTM) developed sound transmission class (STC) ratings to describe the sound isolation performance of various materials. A larger number indicates a greater sound dampening effect.

Thermal Break

This is an element of low conductance placed between elements of higher conductance to reduce the flow of heat or cold. It is often used in aluminum windows.

Tinted Glass

This glass has a subtle coloring (usually grey, bronze or green) that protects a home’s interior harsh direct sunlight.

Glazing

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 special low-emissivity coating for exceptional energy efficiency.

Argon

This is a colorless inert gas that is injected in the airspace of an insulating glass unit to improve energy efficiency.