Knowing the basic terms used for window and glass installation can help when determining your needs, and talking to your contractor.
Conduction - Heat transfer through a material. Heat flows from a higher temperature area to a lower temperature area.
Emissivity - The relative ability of a surface to reflect or emit heat by radiation. Emissivity factors range from 0.00 to 1.00 and are typically measured in a U-Factor (or its inverse, R-Factor). The lower the emissivity, the less heat that is emitted through a window system.
Relative Heat Gain - 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.
R-Value (Also R-Factor) - A measure of a product's ability to resist the transfer of thermal energy. A window with a high R-Value has greater resistance to heat flow and a higher insulating value than one with a low R-Value.
Passive Solar Heat Gain - Solar heat that passes through a material and is captured naturally, not by mechanical means.
Shading Coefficient - A glass measurement comparing solar heat transmission, related to 1/8-inch clear glass.
U-Factor - A measure of the rate of nonsolar heat flow through a material or assembly. The lower the U- Factor, the greater a window's resistance to heat flow and the better its insulating value.
Visible Light Transmittance - 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 reluctance of the glass.
