When it comes to replacement windows; your options include the frame material, type of glass and how many layers of glass to have in the frame. Since 80% of a window is glass, most energy savings come from improved glass performance, not a high R-value of the frame.
The biggest choice in glass is the number of panes of glass: one, two or three. The decision often comes down to a choice of weighing your initial investment against expected long-term benefits.
A single pane window is the least expensive alternative. They are also the easiest to repair and weigh the lightest too. But a single pane window won’t do much to lower heating bills as they permit heat to escape easily.
Most homeowners now opt for insulated glass when replacing existing windows, with either double or triple window panes. A double pane window with inert gas between the panes is a big improvement over a single pane window. But a quality window with three panes, inert gas fill, and low-e glass will always have the lowest U-factor, an important measure of insulation value.
Dual pane windows are made up of two pieces of glass separated by an airspace. The panes are physically sealed together, and cannot be separated. Dual panes can be made up of countless combinations of glass and spacers to make the overall thickness anywhere from 7/16" all the way up to 1 and 1/4".
A triple pane glass package consists of three panes of glass separated by two sealed chambers of inert gas, usually argon or krypton. This creates a glass “sandwich” that provides substantially more insulating value than a single pane of glass, or even a double-glazed window. Insert gases like argon and krypton are used because their molecular composition make them particularly effective at blocking energy transfer.
In general the more airspace that there is in the unit the better the insulation the window provides. There are other factors that can improve the efficiency of of your windows. Among the best options you can have on your windows is a low-e coating. This will allow your windows to reflect much of the heat and UV radiation away from the inside of your home.
Low-e glass is short for low-emissivity glass. It has a very thin coat of material on the glass to make it more efficient. It helps reflect standing heat away from the surface of the glass, keeping unwanted heat out in the summer and desired heat inside in the winter. Low-e glass is the most cost effective way to increase the energy efficiency of the windows.
Low-e coatings can also help reduce furniture and carpet fading by reducing the amount of ultraviolet (UV) radiation that enters the home. Harmful ultraviolet radiation can alter the chemical structure of dyes and other colorants in carpets and furniture causing fading.
In order to provide proper insulation the layers of glass in a window must be appropriately separated by spacers. In the 1960s, window manufactures began using aluminum spacers. They soon realized because aluminum is an excellent heat conductor when used as a spacer for most edge systems there was a major heat loss at the edge of the insulated glass unite, reducing the energy efficiency benefits. This heat loss also made the windows susceptible to condensation.
Modern windows use spacers made of thermoplastic or other advanced materials that will not conduct heat and cold like aluminum, helping to further reduce heat convection and energy transfer through the window.
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