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Windows can be one of your home's most attractive features. They also provide several functions for your home: daylighting, solar heating, and ventilation. They serve as part of the building envelope, which provides a barrier between your house and the outdoors. Unfortunately, the energy lost through windows can account for as much as 10% to 25% of your heating bill.
If your home has single pane windows, consider replacing them. Look for the ENERGY STAR label. There are different glazings and frames available that can impact window performance.
In terms of the building envelope, the window's U-factor (a measure of its insulation ability) is critical. Look for National Fenestration Rating Council (NFRC) rated U-factors. NFRC ratings represent the whole window's performance (not just the glass area) are thus more accurate and representative of real conditions.
Glazings in both windows and glass doors can have dramatic effects on the energy performance of a house. A wide variety of coatings and configurations are now available to achieve a range of goals. In addition to a glazing's U-value, the other important factors are the daylight transmittance (the amount of visible light the window lets in) and the solar heat gain coefficient (the amount of heat the glazing lets in-glazings with low solar heat gain coefficients allow less heat in).
Glazings are now available with high daylight transmittances and low solar heat gain coefficients, allowing daylighting without heat gain. These windows are a good choice when solar heating is not the goal. For windows intended for solar heating, obviously a high solar heat gain coefficient is needed.
"Low-e" (low-emittance) coatings are a recent innovation for glazings. Low-e coatings reduce the heat transmitted through the pane, which significantly lowers the U-value (in other words, increases the insulating ability). There are two types of low-e glass, high solar transmitting (for climates where utility bills are highest in winter) and low-solar transmitting (for climates where utility bills are highest in summer). Choosing the right type of low-e coating for your home is critical. You may need different coatings for the various windows in your home depending on their location. Work with a reputable contractor or retailer to choose the appropriate glazings for your climate and requirements.
Credits: US Department of Energy (http://www.eere.energy.gov/consumerinfo/saveenergy/save_windows.html)