Types of Window Glass

When selecting windows for your home, it's important to consider what type of glazing or glass you should use to improve your home's energy efficiency. Based on various window design factors—such as window orientation, your climate, your building design, etc.—you may even want different types of glazing for different windows throughout your home.

There are many types of glazing available for windows, especially since many glazing technologies can be combined. These window glazing technologies include the following:

Gas fills
Heat-absorbing tints
Insulated (double-glazed, triple-glazed)
Low-emissivity (Low-E) coatings
Reflective coatings
Specially selective coatings

Window Gas Fills
To improve the thermal performance of windows with insulated glazing, some manufacturers fill the space between the glass panes with gas.

For these gas fills, window manufacturers use inert gases—ones that do not react readily with other substances. Because these gases have a higher resistance to heat flow than air, they (rather than air) are sealed between the window panes to decrease a window's U-factor.

The most common types of gas used by window manufacturers include argon and krypton. Argon is inexpensive, nontoxic, nonreactive, clear, and odorless. Krypton is more expensive but has a better thermal performance.

Heat-Absorbing, Tinted Window Glazing or Glass
Heat-absorbing window glazing contains special tints that change the color of the glass. Tinted glass absorbs a large fraction of the incoming solar radiation through a window. This reduces the solar heat gain coefficient, visible transmittance, and glare.

Some heat, however, continues to pass through tinted windows by conduction and re-radiation. Therefore, the tint doesn't lower a window's U-factor. However, inner layers of clear glass or specially selective coatings can be applied on insulated glazing to help reduce these types of heat transfer.

Gray- and bronze-tinted windows—the most common—reduce the penetration of both light and heat into buildings in equal amounts (i.e., not specially selective). Blue- and green-tinted windows offer greater penetration of visible light and slightly reduced heat transfer compared with other colors of tinted glass. In hot climates, black-tinted glass should be avoided because it absorbs more light than heat.

Tinted, heat-absorbing glass reflects only a small percentage of light, so it does not have the mirror-like appearance of reflective glass.

Note: when windows transmit less than 70% of visible light, indoor plants can die or grow more slowly.

Insulated Window Glazing or Glass
Insulated window glazing refers to windows with two or more panes of glass. They are also called double-glazed, triple-glazed, and—sometimes more generally—storm windows.

To insulate the window, the glass panes are spaced apart and hermetically sealed to form a single-glazed unit with an air space between each pane of glass. The glass layers and the air spaces resist heat flow. As a result, insulated window glazing primarily lowers the U-factor, but it also lowers the solar heat gain coefficient.

Some window manufacturers use spacers—which separate two panes of glass—that conduct heat less readily than others. These spacers can further lower a window's U-factor.

Low-Emissivity Window Glazing or Glass
Low-emissivity (Low-E) coatings on glazing or glass control heat transfer through windows with insulated glazing. Windows manufactured with Low-E coatings typically cost about 10%–15% more than regular windows, but they reduce energy loss by as much as 30%–50%.

A Low-E coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass. The Low-E coating reduces the infrared radiation from a warm pane of glass to a cooler pane, thereby lowering the U-factor of the window. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. A Low-E coating can also reduce a window's visible transmittance unless you use one that's specially selective.

Reflective Window Glazing or Glass
Reflective coatings on window glazing or glass reduce the transmission of solar radiation, blocking more light than heat. Therefore, they greatly reduce a window's visible transmittance (VT) and glare, but they also reduce a window's solar heat gain coefficient (SHGC).

Reflective coatings usually consist of thin, metallic layers. They come in a variety of metallic colors, including silver, gold, and bronze.

Reflective window glazing is commonly used in hot climates where solar heat gain control is critical. However, the reduced cooling energy demands they achieve can be offset by the resulting need for additional electrical lighting, so reflective glass is mostly used just for special applications.

Spectrally Selective Window Glazing or Glass
A special type of low-emissivity coating is specially selective. Specially selective coatings filter out 40%–70% of the heat normally transmitted through insulated window glass or glazing, while allowing the full amount of light to be transmitted.

Specially selective coatings are optically designed to reflect particular wavelengths but remain transparent to others. Such coatings are commonly used to reflect the infrared (heat) portion of the solar spectrum while admitting a higher portion of visible light. They help create a window with a low U-factor and solar heat gain coefficient but a high visible transmittance.

Specially selective coatings can be applied on various types of tinted glass to produce "customized" glazing systems capable of either increasing or decreasing solar gains according to the aesthetic and climatic effects desired.