GLASS DEFINITIONS

U-Factor - The U-Factor (BTU/hr/ft2/F) measures the rate of heat loss through a window. The lower the U-Factor, the better the window is at reducing heat loss.

Solar Heat Gain Coefficient (SHGC) - The Solar Heat Gain Coefficient measures the amount of solar heat that passes through a window. The lower the SHGC, the better the window is at blocking heat from the sun.

Visible Transmittance (VT) - Visible Transmittance measures how much visible light passes through a window compared to the amount that would pass through an open hole of the same size. The higher the VT, the more light is transmitted. A low VT often increases the amount of artificial light needed which leads to an increase in energy costs. A VT of 60 percent or more often appears clear, while a VT below 50 percent begins to look dark and/or reflective.

Fading Transmission (Tdw) - Fading transmission measures the amount of fading-causing solar energy that enters through a window. The lower the Tdw value, the better the window is at protecting against fading. Insulating Glass - Insulating glass features two panes of glass separated by a spacer. The dead air space between the panes creates an insulating barrier and helps to reduce sound transmission.

Warm-edge Spacers - Warm-edge spacers are manufactured from low-conductivity material. They can improve the U-Factor of a window by 10 percent and increase the glass edge temperature by almost 5°F, helping reduce condensation.

Low-Emissivity (Low-E) - Coated - A metal or metallic oxide layer applied to the surface of the glass that lowers the window’s U-Factor. The coating blocks long wave infrared inside and short wave solar heat outside. This results in keeping heat inside during the winter and outside during the summer, reducing air conditioning or heating demand, thus lowering energy costs. It also reduces winter condensation because of higher indoor glass temperatures and blocks harmful ultraviolet rays, lowering the Tdw, without blocking as much VT as tints do.

Argon - Argon is a gas used to fill the airspace between panes of insulating low-E glass. It may increase insulating performance by slowing the transfer of energy through the window, therefore lowering the window’s U-Factor. Because the amount of argon in a unit may deplete over time, the insulating performance attributed solely to argon may diminish over time. Capillary/breather tubes are required on units with glass 12” and less in width or height and units shipped over high elevations. The capillary/breather tubes allow the unit to adjust to changes in pressure. Argon gas is not an option when capillary/breather tubes are used.

Tinted Glass - Tinted glass can lower the SHGC of a window significantly and can help control glare. Keep in mind that while blocking the sun’s rays, tinting also reduces VT.