Double Glazed Windows
Double pane windows offer homeowners an economical energy efficient replacement window option. The additional glass pane of a double glazed window serves as an additional barrier to prevent noise, air, dust, weather penetration and provides enhanced thermal performance. Air which is trapped between the glass layers provides a measure of insulating value.
There are several fundamental approaches to improving the energy performance of glass windows. These include:
- Altering the glass itself by changing its physical characteristics or chemical composition, as is the case with tinted glazing.
- Applying a coating to the glass surface, such as a reflective film or a spectrally selective coating. These coatings aid in reducing glare and heat gain. More recent low emittance (Low-E) coatings, also help heating and cooling performance.
- Assembling various layers of glazing, including controlling the properties in the spaces between the layers. This option called double glazing, often includes a low-conductance gas fill which is pumped between the layers. Thermally improved edge spacers are often installed with double glazed windows.
The first two approaches may be combined with double glazing, to offer a window with a high level of thermal resistance.
 Types of Double Glazed Windows
Double pane replacement windows are available as double pane clear glass windows, double pane tinted glass windows, double pane windows with argon or krypton gas fills and double pane windows with a Low-E coating. Double glazed windows are manufactured using a spacer which separates the two panes of glass. Their glass panes are held in place within the window sash by a secondary frame -- a mullion.
 Clear Glass
A double glazed window with clear glass has two clear glass lites which are both clear. They are separated by an air gap which cuts heat loss in half due to the insulating space which is present between the glass layers. This gap reduces the heat flow and allows the transmission of solar heat gain and visible light.
 Tinted Glass
A double-glazed window with green, blue, gray or bronze tinted glass is constructed with two panes of glass of which the outer glass pane is tinted while the inner layer is clear. An air gap separates the two layers which cuts heat transfer through the glass panes. These tinted windows reduce light transmission due to the windows' tint. Tinted windows are useful for controlling glare but the solar heat gain may be lessened in comparison to other glazing options. These windows can be beneficial during the summer months as opposed to the winter months.
 Argon / Krypton Gas
Double-glazed replacement windows with argon or krypton gas fills are designed to admit solar gain while reducing heat loss. Argon or krypton gases slow heat transfer and help to maintain the warm temperatures of a warm house and the cool temperatures of a cool house. Argon and krypton gases are inert gases and serve as effective thermal insulators by reducing the movement of heat through a solid material --the conductance -- of air space that exists between the glass layers of a double-glazed window.
Many window manufacturers include double glazing with argon or krypton gas fills as a standard in a window order. Krypton gas is a more expensive gas fill but is used for thinner gaps. Argon and krypton gas combinations are also used in some window applications.
Gas-filled double-glazed replacement windows may be used in conjunction with tinted or patterned glass and together with Low-E coatings.
 Low E Coatings
A "Low-Emittance" -- Low-E coating is a microscopically thin, virtually invisible, metallic or metal layer of oxide which is deposited on a window surface. It reduces the U-factor of the window by suppressing radiative heat flow and helps a window's heating and cooling performance. Low-E coatings are applied to the outer layer of a double pane window.
 Window Styles
Double glazing is available for all replacement window styles including fixed windows, windows with special architectural shapes and conventional operating hinged and sliding windows. Many window manufacturers include double panes as part of the standard order for a replacement window.
 Glazing Options
Double glazing is available for all types of replacement windows including clear glass, tinted or patterned privacy glass, laminated glass or tempered glass.
Double glazing is the minimum standard required by many governmental bodies which oversee the standards for the replacement window market. Most window manufacturers offer the option to purchase of their double pane replacement windows.