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Builder's Glossary of Window and Glass Terms 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. 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. BTU 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. CFD-TCO Coating for Display glass Transparent Conductive Oxide CFR-TCO Coating for Refrigerator and freezer Transparent Conductive Oxide. 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. Dessicant A chemical which absorbs and holds moisture. Dessicant performs two functions. First, it absorbs any moisture trapped in the dead airspace during manufacturing. Second, it must remain active and absorb moisture that may violate the dead airspace during the life of the unit. One type of dessicant is tiny beads which are poured into tube-shaped spacers. Another type is actually suspended in the sealant. This compound is then applied to the spacer frame. Electrochromic A material that when a small voltage is applied, changes to a darkened state and returns to a lightened state when the voltage is reversed. Actively controlled electrochromic glazings are being developed for use in "smart window" applications allowing the window to be adjusted to maximize energy performance with varying outdoor conditions, ie. daytime, night, sunny, cloudy. For example, during the heat of the day a window could be darkened to reduce the solar heat gain and during the evening lightened to maintain visibility. Emissivity 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. 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. Fenestration 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. 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.") Haze The scattering of visible light resulting in a decrease in transparency of a window system and a cloudy appearance. 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). 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. 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. PV-TCO Photovoltaic Transparent Conductive Oxide. Passive Solar Heat Gain Solar heat that passes through a material and is captured naturally, not by mechanical means. Photovoltaic A device that produces electricity (voltage) directly from sunlight (photons). Radiation 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. RESidential FENestration is a computer-simulation application developed at Lawrence Berkeley Lab. 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. Sealant 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 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. Spacer A material placed between two or more panes of glass in an insulated glass unit to bond and seal the glazing unit. Spectrally selective glazing A specially engineered low-e coated or tinted glazing that blocks out a portion of the sun's heat while maintaining high visible light transmittance (VLT). Most spectrally selective glazings feature low solar heat gain coefficients (SHGC). Titanium An ultra-hard metal with low light absorption and high visible light transmittance (VLT) properties. Used in AFG's Comfort Ti line of low-e glass, titanium low-e coatings improve transmitted color and reduce emissivity. U-factor (U-value) A measure of the rate of nonsolar 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. 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.