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.