Common Window Terminology
Airspace
The space in the cavity between two panes of glass in an insulated glass unit.
Argon
An inert, nontoxic gas placed between glass panes in insulated windows in order to improve the insulating value of sealed glass units.
Condensation
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.
Conduction
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.
Convection
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.
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.
ENERGY STAR® label
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.
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.
Glaze/Glazing
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.")
Grids/Grilles
The decorative bars that divide the glass part of a window up and create a pattern.
Infiltration
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).
Lite
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 grids/grilles.
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.
NFRC
National Fenestration Rating Council - a nonprofit public/private collaboration that provides contractors and homeowners with a standardized energy-performance rating system for fenestration products.
Radiation
The transfer of heat in the form of electromagnetic waves from one surface to another.
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.
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.
Spacer
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.
Warm-edge
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.
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