Glossary of Window and Glass Terms

A – C | E – I | L – R | S – T | U – W

Air leakage rating

A measure of the rate of infiltration around a window or a skylight in the presence of a strong wind, expressed in units of cubic feet per minute per foot.


The space in the cavity between two panes of glass in an insulated glass unit.


An inert, nontoxic gas placed between glass panes in insulated windows in order to improve the insulating value of sealed glass units.


An abbreviation for British thermal unit – a standard measure of the amount of energy required to raise one pound of water by one degree Fahrenheit.


The accumulation of water vapor or droplets as the result of warm, moist air coming in contact with a cold surface and cooling to its dew point temperature. Condensation may occur when a cold window glass or frame is exposed to humid indoor air. Low-conductivity, insulated glass and warm-edge spacers reduce condensation.


The transfer of heat through a solid material, such as glass or wood, through direct contact. Heat flows from a higher-temperature area to a lower-temperature one.


The flow of heat that occurs through a circulating gas or liquid (such as air) as warm air rises and cool air sinks. Convective heat transfer can take place in large areas (like rooms and buildings) and in small areas (like the cavity between two layers of insulated glass). Low-e insulated glass units lessen cold convection currents by maintaining warmer interior glass surface temperatures.


The relative ability of a surface to reflect or emit heat by radiation. Emissivity factors range from 0.00 to 1.00. The lower the emissivity, the less heat that is emitted through a window system. Emissivity is typically measured by U-factor (or its inverse, R-factor).

ENERGY STAR® windows program

A voluntary partnership between the U.S. Department of Energy (DOE) and the fenestration industry to promote sales of energy-efficient windows, doors and skylights. The program establishes three climate regions with one recommended product designation for each region. All ENERGY STAR® windows must be NFRC-rated, certified and labeled for both U-factor and solar heat gain coefficient.


A label that indicates a window meets ENERGY STAR® program energy-efficiency requirements for a specific region, and is at least 40% more efficient than products required under the most common national building codes.


A window or skylight and its associated interior or exterior elements (e.g., shades or blinds).

Gas fill

An insulating gas (such as argon or krypton) placed between window glazing panes to reduce the U-factor by suppressing conduction and convection.


Glass or plastic panes, as in a window or skylight. Note that the terms “double-glazed” and “double-paned” are interchangeable. (The term “glazed” should not be confused with “coated” or “tinted.”)


Also called “muntins.” The decorative bars that divide the glass part of a window up and create a pattern. Affordable Windows grids are located between the panes of glass.


The inadvertent flow of air into a building through breaks in the exterior surfaces of the building (e.g., through joints and cracks around window and skylight frames, sashes and glazings).

Laminated Glass

An insulated glass unit in which one internal piece of glass is covered (laminated) with an organic film. The film gives the glass an added degree of resistance to breakage.


A section of glass. This term can be confusing, as it can refer to an entire window (as in a 4-lite bow) or to one of the areas of glass divided up by muntins/grids/grilles. Sunrise uses the term “lite” only to refer to bow windows, which can come in 4-lite, 5-lite, or 6-lite sizes.

Low-emissivity (low-e) coating

Microscopically thin, virtually invisible metal or metallic oxide layers deposited on a window glazing surface and sealed in an insulating glass unit to reduce the U-factor by suppressing radiative heat-flow through the window.


Also called “grids” or “grilles.” The decorative bars that divide the glass part of a window up and create a pattern. Affordable Windows muntins are located between the panes of glass.


National Fenestration Rating Council – a nonprofit public/private collaboration that provides contractors and homeowners with a standardized energy-performance rating system for fenestration products.

Passive Solar Heat Gain

Solar heat that passes through a material and is captured naturally, not by mechanical means.


The transfer of heat in the form of electromagnetic waves from one surface to another.

Relative Heat Gain (RHG)

A calculated relationship of heat gain (through a window system) that accounts for center-of-glass U-value and center-of-glass shading coefficient based on a standard inside and outside temperature.

R-value (also R-factor)

A measure of a product’s ability to resist the transfer of thermal energy. The inverse of U-factor (R=1/U), R-value is expressed in units of hr-sq. ft -ºF/Btu. A high R-value window has greater resistance to heat-flow and a higher insulating value than one with a low R-value.


A substance, which adheres to the glass and the spacer, sealing an insulated unit. It must provide adhesiveness and a minimum of moisture transmission through itself. When one substance provides both these characteristics adequately, the unit is called a single-seal unit. Sometimes two materials are needed for the desired result, which makes a dual-seal unit.

Shading Coefficient (SC)

A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8-inch clear, double-strength single glass. This measurement is being phased out in favor of solar heat gain coefficient, and is approximately equal to the SHGC (see below for definition) multiplied by 1.15.

Solar Heat Gain Coefficient (SHGC)

The fraction of solar radiation transmitted through a window or skylight, expressed as a percentage. The lower a window’s SHGC, the less solar heat it transmits and the greater its shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly. Generally, a lower SHGC is desirable in warm climates, and a higher SHGC is desirable cold ones. SHGC has replaced shading coefficient (SC) as the standard indicator of a window’s shading ability.

Solar reflectance

The measured quantity of energy in the solar wavelength range that is reflected by a window, expressed as a percentage.

Solar transmittance

The measured quantity of energy in the solar wavelength range that passes through a window, expressed as a percentage.


A material placed between two or more panes of glass in an insulated glass unit to bond and seal the glazing unit.

Tempered Glass

Special heat-treated, high-strength safety glass which shatters into pebble-sized particles and not in slivers.

U-factor (U-value)

A measure of the rate of non-solar heat-flow through a material or assembly. It is expressed in units of BTU/hr.-sq. ft. -ºF (or W/sq. m-ºC), and may be expressed for the glass alone or for the entire window assembly, including frame and spacer materials. The lower the U-factor, the greater a window’s resistance to heat-flow and the better its insulating value.

Visible light transmittance (VLT or VT)

The percentage or fraction of the visible light spectrum that is transmitted through the glass of a window or skylight as reduced by the sash material and reflectance of the glass.

Visible reflectance

The measured amount of energy in the visible wavelength range that is reflected by a window system, expressed as a percentage.


Describes a window unit that uses a new spacer material (e.g., rubber or stainless steel or plastic vs. aluminum) and/or a thermal-break spacer design to reduce conductivity between interior and exterior glass panes.