Terms you need to know relating to glass energy efficiency

You’ve probably heard of several energy efficiency terms. But you may not know exactly what they mean. We put together this quick list for your reference. By understanding these terms, you can be knowledgeable and informed in your energy efficiency journey.

Common energy efficiency terms

Clear Insulating Glass

An airspace separates two clear panes of glass, providing insulating properties that benefit your home. This option offers maximum visible light transmittance and basic energy efficiency in most climates.


ENERGY STAR® is a voluntary program created by the U.S. Environmental Protection Agency that helps businesses and individuals reduce energy consumption to save money and protect the environment. You will see an ENERGY STAR® label on a window or door if it meets the energy saving specifications for a designated climate zone.

Gas fills

Manufacturers often fill the space between glass panes with gas, such as argon or krypton, to improve thermal performance. Both argon and krypton are naturally occurring, inert, nontoxic, nonreactive, clear and odorless. Krypton has better thermal performance and is more expensive to produce.

Glass options

Glass options may vary by region. Double-pane insulating glass is the most common. Other options might include triple glazed insulating glass, which is often better suited for more northern areas for best thermal performance.


Glazing refers to the act of furnishing or fitting a window with glass. The term glazing can be a noun or a verb. As a noun, it means the infill material (glass). As a verb, it is the process of installing the infill material (glass) into the window sash or frame.


Low-E, or low emissivity, is an invisible metallic coating on the glass designed to reflect infrared light. This keeps heat inside in the winter and outside in the summer. It also reflects part of ultraviolet light to protect interior furnishings from fading prematurely. Low-E is often the best choice for energy efficiency and frequently standard on many products.


SHGC (Solar Heat Gain Coefficient) indicates a window or door’s ability to block heat generated by sunlight. The lower the SHGC number, the greater the blockage will be.


U-Factor is the amount of heat flow through a product. The lower the number, the more energy efficient the product is.


VLT (Visible Light Transmission) is the percentage of sunlight that penetrates a window or door. Higher percentages mean more light will enter through the glass.

For more information related to energy efficiency, read our blog with tips on how to enhance the energy efficiency of your home.

The information contained on this page is provided solely for general informational and/or educational purposes, and may not be applicable to all products, applications, climates, and other factors. Please consult with your licensed contractor, architect, dealer or local building code official for information relevant to your geographic region and project. JELD-WEN makes no representation or warranty that this information is applicable for your circumstances. JELD-WEN disclaims all liability associated with the use or transfer of this information.

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