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Shopping for windows and doors can be a confusing experience if you're unfamiliar with window technology. Whether you're retrofitting an existing home or building a new one, selecting windows and doors is crucial in improving your home's energy efficiency. Other benefits of high performance windows and doors include increased comfort, reduced maintenance and repair bills and lower energy costs. Windows and doors have traditionally been a weak link in the home insulation system, but rapid improvements in insulating quality are being achieved. Selecting Windows and Doors We considered these key elements:
For the Millennium Star Home, we wanted windows with an overall U-Value of .35 or less. We choose a Platinum II, fiberglass frame window with three panes of glass manufactured by Accurate Dorwin in Winnipeg, Manitoba. There are two argon filled air spaces with two low E coatings and triple weatherstripping. The spacer between the glass is non metallic and made from extruded silicon, reducing thermal bridging at the glass edge. R- and U-Values are measurements used in rating how well a material resists heat conduction. The higher the R-Value, the better the insulating quality. In the past, window performance was measured by center of glass R-Value, which did not accurately reflect the total window performance. The more effective method is to compare the overall unit U-Value, which includes the performance of all window components such as frame, glass, and spacer. The lower the U-Value, the better the insulating quality. 
If your windows are 40-50 years old and have aluminum frames, double glass and one-half inch air space, they probably have a U-Value of about .62. If you're installing new windows, you'll want to select windows with a U-Value of .35 or less as we did for the Millennium Star Home. 
Lower U-Value windows not only help lower energy bills, they also maintain higher relative humidity levels in your home without a condensation problem. Comfortable humidity levels are about 40 - 60 percent. Windows with a low U-Value have a glass sur face temperature of about 58° -- 20° warmer than standard older windows. The cooler the window surface the greater the chance for condensation. Be sure to find out if the U-Value rating for a window applies to the entire unit (including frame) o r to the center of the glass only. Here are some other energy efficient options we considered in selecting the windows: Multiple glazing: Increasing the number of glass panes (or glazings) improves the insulating quality of the window. Gas-filled windows: The air space between the glass panes adds to the windows insulating quality. The R-Value of the window increases even more if the space between the panes is filled with argon or another gas with insulating properties super ior to ordinary air. Low-E Glass: Materials such as standard window glass that readily radiate heat to a colder object are said to have high emissivity. Ultra thin layers of special metallic compounds,deposited on window glass, reduce radiant heat transfer and the refore save energy. They are commonly called low-emissivity coatings. Sash and frames: The sash and frame comprise only 10 to 20 percent of the total window area. Steel and aluminum frames are durable, but they have little resistance to heat movement. If you choose a window with a metal frame, it must have a the rmal break to combat heat loss. (A thermal break is a section of material - usually polyurethane or wood - that is sandwiched between the frame parts.) Wood is the traditional frame material and has higher R-Value than steel. It's sometimes clad with metal or vinyl to protect the wood surface and reduce maintenance. Fiberglass frames, with a core of fiberglass or foam insulation, are making their appearance on the market and offer high insulating qualities as well as low susceptibility to expansion. Selecting Window Design 
For the Millennium Star Home, we are using the awning type and the casement type of window. Casement windows (the type that crank out) and awning windows (they hinge at the top and open at the bottom) are less vulnerable to air infiltration than other styles of windows because they have fewer seams. However, there is no quarantee they will be superior in airtightness. The horizontal glider type of window and the double hung type of window have more frame area per opening making them less energy efficient per square foot of glass area.
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