What Are Air Spaces in Window Glass?

How It Works

• Glass is an excellent heat conductor. Heat travels through a window whenever the inside and outside temperatures differ. During the summer, heat moves from the outside in, and during the winter, it travels from the inside out. A double-glazed or triple-glazed window traps some of the heat and significantly reduces this heat flow. The design works similarly to a thermos bottle, explains the California Energy Commission.

U-Value

• The efficiency of a window at preventing heat transfer is designated with a U-factor, also called a U-value. The lower the U-value, the better. Residential single-pane windows typically have a U-value of about 1, according to the Kansas State University Engineering Extension. The U-value of a double-glazed window with air spaces is about 0.5 and of a triple-glazed window, is about 0.31.

Gases

• You also can buy windows that have the air spaces filled with a gas, such as argon, krypton or carbon dioxide. This improves the window's energy efficiency further, because the gases are more viscous, more dense and less heat conductive than air. A window with argon in the air space has U-values ranging from 0.40 to 0.31, according to the KSU Engineering Extension. Argon may eventually begin leaking out, but the U-value does not change significantly unless the seal between the glass and edge spacer, which separates the glass panes, fails. Condensation between the panes is a sign of seal failure.

Options

• The air spaces in window glass generally are 1/2-inch to 3/4-inch thick, notes the California Energy Commission. If the air space is too thin, it does not insulate well, and if the space is too thick, it is inefficient. To further improve efficiency, consider windows that have an invisible heat-reflective coating. This product is called low-emissivity, or low-e, glass. Window tints also are available to block some heat from the sun, but you gain the energy efficiency by losing a certain level of visibility.