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Glossary of Terms

Air leakage (Al) – Air leakage rating indicate the amount of air leaking in and out of a building through closed windows, doors, or skylights in the presence of a specific pressure difference. These rating are expressed in units of cubic feet per minute per square foot of a frame are (cfm/sq ft). The lower a window’s air-leakage rating, the better its airtightness.

Gas fill – A gas other than air, usually argon or krypton, placed between window or skylight glazing panes to reduce the u-factor by suppressing conduction and convection.

Insulating glass – Two or more pieces of glass spaced apart and hermetically sealed to form a single glazed unit with one or more air spaces in between. Also called double and triple glazing.

Low emittance (low-E) coating – Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. Low-E coating are typically highly transparent to visible light but can reflect heat from infrared radiation.

National Fenestration Rating Council (NFRC) – A nonprofit, public/private organization created by the window, door, and skylight industry. It is composed of manufacturers, suppliers, builders, architects and designers, specifiers, code officials, utilities, and government agencies. The NFRC has developed a window energy rating system based on whole product performance.

R-Value –  R-value refers to the resistance of the window to heat conduction, and it is the inverse of the U-factor (that is, R-value = 1/U-factor). Better windows have high R-values and low U-factors

Reflective coatings – Coatings on window glass that reflect radiation striking the surface of the glass.

Solar heat gain coefficient (SHGC) – The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar hear gain coefficient has replaced the shading coeffient as the standard indicator of a window’s shading ability. It is expressed as a number between 0 and 1. The lower a window’s solar heat gain, 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.

U-factor (U-value) – A measure of the rate of non-solar heat loss or gain trough a materials or assembly. It is expressed in units of Btu/hr-sq ft- f (W/sq m- C). Values are normally given for NFRC/ASHRAE winter conditions of 0 f (18 c ) outdoor temperature, 70 f (21 C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the u-factor, the greater a window’s resistance to heat flow and the better its insulating value.

Visible transmittance (VT) – The percentage or fraction of the visible (380 to 720 nanometers) weighted by the sensitivity of the eye, that is transmitted through the glazing.

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