Windows serve several functions in a house: They provide views to the outdoors, allow sunlight to enter, they provide fresh air, and block the flow of heat. The relative importance of these functions varies by climate and by location of the windows in the house. For example, Minnesotans will most likely be interested in passive solar heat gain and will thus want windows that transmit a lot of solar heat, especially on the south side of the house. Florida residents, though, will probably be more concerned with blocking solar heat gain. Next to the amount of insulation in the house envelope and the home’s airtightness, selection of windows and doors has the greatest impact on energy use for heating and air conditioning. Fortunately, this is where some of the most significant advances have been made in building technology over the past several decades.
The best windows today have up to four times the insulation value of the best windows from the early 1970s. These dramatic improvements in window energy performance have been made possible by several major technical developments, the most important of which was transparent, low-emissivity (low-e) coatings on window glass or on plastic films suspended between the layers of glass. These extremely thin layers of metal transmit sunlight and visible light fairly well, but they block the escape of longer-wavelength heat radiation. In this way, they significantly slow down the loss of heat through a window.
Another important development was the introduction of special low-conductivity gasses into the space between panes of glass in sealed, insulated units. The most common gas fill, argon, has an insulating value 40% higher than that of air. Krypton, used in the highest-performance windows, has an insulating value 1.4 times that of air. By combining low-conductivity gas fill in windows with two or more low-e coatings and accounting for solar heat gain, it is now possible to buy windows that, in effect, insulate as well as a fiberglass-insulated 2×6 wall. It can make a great deal of sense to “tune” glazing to the window’s orientation.
This means using one type of glazing on one wall of a house, and other types of glazing on other walls. For example, with a passive solar house in a cool northern climate, it can make sense to install south-wall windows that transmit a lot of solar energy, but on the east and west orientations — where solar heat gain in the summer is more of a problem — install windows that block more of the solar energy. On the north, maximizing the insulation performance of windows is most important, since there is little direct solar gain to worry about. Some, but not all, window manufacturers offer different glazing options for their windows, although there may be an additional charge to order a non-standard glazing.
As you already realize that selecting windows are tremendously important for controlling energy flow into and out of buildings, pick the best window glazing which effectively block heat loss, while appearing looks fabulous. NFRC (National Fenestration Rating Council) provides you information from its Certified Window Products Directory, which lists the energy performance of several thousand windows on the market.
Reference: Your Green Home by Alex Wilson, 2006