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What Window Glass Is Best?

Part 2

The window glass is an important part of a replacement window. When purchasing new or replacement windows it is important to look at the construction and manufacturing technologies of the window glass. The problem is that window glass, as a material, is an excellent conductor of heat and cold. A single pane window glass provides very little insulation value in itself.

diy window glass installation

It should be noted, that placing energy efficient glass windows in a poorly constructed or energy inefficient frame is a waste of money, as is having the best window assembly poorly installed.

It is the entire window, the window glass and the window frame;

that must be energy efficient in order to save money and have a home that is draft free.

In order to make the window glass portion of the window assembly thermally efficient, it is necessary to trap air or provide spacing between two pieces of glass. It is the air or other material, an inert gas, between the window glass panes that acts as the insulator. Air is a very poor conductor of heat and cold and other inert gasses, such as argon. are even better insulators.

The following window glass study was performed by the University of Illinois using controlled experimentation.

They set-up a room with a controlled temperature of minus seven degrees Fahrenheit and then measured the temperature on the inside of different styles of window glass panes, as shown in Figure 1.

Window glass thermal efficiency experiment
Figure 1 - Window glass thermal efficiency experiment

As the test is a measurement of the thermal efficiency of the number of panes of window glass with the addition of Low-E and Argon gas (between layers of window glass), it does not take into account any deficiencies or efficiencies that may exist in window frame materials or window frame construction techniques and technologies.

The outcome of the study is presented in Table 1. It is quite evident that the number of window glass panes and the addition of Low-E and Argon gas make a substantial difference to the thermal efficiency of the window glass and will save the homeowner substantial amounts of hard dollars on energy consumption.

Table 1 - Window Glass Efficiency

Glass System


Of Glass On
Inside Of Window

Single Pane Glass


10.4° F

Single Pane with Storm Window


17.0° F

Double Insulated Glass


40.4° F

Regular Triple Glass


51.0° F

Triple Glass with Low-E and Argon


63.7° F

*U-Value - The U-Value is a measurement of non-solar heat loss or gain as it relates to a specific material or unit. When purchasing windows it is important that the U-Value stated for the window is for the entire window assembly, not just the glass portion. The better the window the lower the U-Value.