Builder’s Glossary of Window and Glass Terms
Argon
An inert gas used in insulating windows to improve the insulating value of sealed glass units.
Btu
An abbreviation for British Thermal Unit – the heat required to increase the temperature of one pound of water one degree Fahrenheit.
Condensation
The accumulation of water vapor for the air’s humidity on any cold surface where the temperature is below dew point, such as a cold window glass or frame that is exposed to humid indoor air. Low conductivity or warm edge spacers reduce condensation.
Emissivity
The relative ability of a surface to reflect heat, with emissivity factors ranging from 0.00 to 1.0. Emissivity, U-Factor, and R-Factor are different ways to evaluate insulating values.
Low-Emmittance (Low-E) Coating
Microscopically then, visually invisible, metal or metallic oxide layers deposited on a pane of glass and sealed in an insulating glass unit to reduce U-Facture by suppressing radiant heat flow.
NFRC
National Federation Rating Council
Passive Solar Heat Gain
Solar heat that passes through a material and is captured naturally, not by mechanical means.
R-Value
A measure of the resistance of a glazing material to heat flow. It is the inverse of the U-Factor, (R = 1/U) and is expressed in units of Fx Sq. Ft x Hr/Btu. A high R-Value window had a greater resistance to heat flow and a higher insulating value than one with a low R-Value.
Shading Coefficient (SC)
A glass measurement comparing solar heat transmission, related to 1/3-inch clear glass. It is being phased out in fewer of the 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. The SHGC has replaced the shading coefficient as the standard indicator of a window’s shading ability. It is expressed as a percentage. The lower a window solar heat gain coefficient, 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. SHGC should vary based on climate and geography with a lower SHGC in the south and higher SHGC in the north.
U-Factor (R-Value)
A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of 8*** x Sq. Ft x Hr. Values are normally given for: NFRC/ASHRAE winter conditions of 0 F (18 C) outdoor temperature 70 F (21 C) indoor temperature. 15mph wind and no solar load. 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 spectrum (380 to 720 nanometers) weighted by the sensitivity of the eye that is transmitted through the glazing.