Window Glossary and Glass Terminologies

Width of the overall glass unit is 1 3/8" top rating in energy efficiency.

It is essential to be familiar with the basic terms used for window and glass installation, as it helps in establishing your requirements and negotiating with your contractor.

Fundamental window and glass terminologies:

Conduction – Heat transmission through a substance. Heat flows from a higher temperature area to a lower temperature area.

Emissivity – The relative ability of a surface to reflect or release heat through radiation. Emissivity factors vary from 0.00 to 1.00 and are generally calculated in a U-Factor (or its opposite, R-Factor). The lower the emissivity, the lesser heat that is produced through a window system.

Relative Heat Gain – Measured in watts per square meter, relative heat gain is a calculated relationship of heat transfer (through a window system) that accounts for center-of-glass U-Value and center-of-glass shading coefficient (solar gain) based on a standard inside and outside temperature.

R-Value – Also known as "R-Factor", R-Value is a measurement of a material's power to resist the transfer of thermal energy. A window with a high R-Value has better resistance to heat flow and a higher insulating value versus one with a low R-Value.

Passive Solar Heat Gain – Solar heat that passes through a substance and is captured by natural means, and not mechanically.

Shading Coefficient – A glass calculation comparing solar heat conduction, related to 1/8-inch (3 mm) clear glass.

U-Factor – Used to compare the performance of windows, U-Factor measures the rate of non-solar heat flow through a substance or assembly. The lower the U- Factor, the greater a window's resistance to heat flow and the superior its insulating factor.

Visible Light Transmittance – The proportion of the visible light spectrum that passes through the glazing material of a window or skylight as reduced by the sash material and reluctance of the glass. The greater the Visible Light Transmittance (VT), the more light is transmitted; a high VT is advantageous if you want to maximize daylight and view.