What Is the Purpose of Air Spaces in Window Glass?

Heat Flow

• Air spaces between panes interrupt heat flow, which increases a window's energy efficiency. A single pane conducts heat, but an air space left between two or more panes cuts the heat flow through a window. The California Energy Commission compares the results with a thermos bottle, which reduces heat losses and heat gains to keep its contents hot or cold. The commission indicates that a double-pane window with a 1/2-to-3/4-inch air space between the panes has an effect on the window's insulating properties that's similar to that of a thermos bottle.

Window Ratings

• The "U" factor is a rating that indicates how well a window resists heat flow, and the lower the rating, the better a window's energy efficiency. The air space between panes isn't the only thing that affects a window's "U" factor. For example, a low emissivity (low-E) coating on a windowpane also lowers the "U" factor. The thin metal coating controls heat flow along with air spaces, because it draws heat into a home in winter and deflects heat in summer.

Gas-Filled Windows

• Some manufacturers fill the air spaces between panes with argon, krypton or another gas to increase a window's energy efficiency. Argon is inexpensive, and it prevents heat transfer more than air alone, according to the Efficient Windows Collaborative. The EWC indicates that krypton controls heat flow more effectively than argon, but it costs more to produce. Some manufacturers mix the two gases to create windows with better thermal performance while keeping costs down.

Costs

• The additional panes required to create air spaces increase the price of windows. The cost to install a standard single-pane window ranges from $200 to $400, according to CostOwl.com. The Service Magic contractors’ website indicates that a mid-range, double-pane window would cost about $600, as of time of publication. In any case, a salesperson's commission at a home improvement store could drive up your overall cost for new windows an additional 30 percent, according to CostOwl.com data.