LBNL Report Number
In most areas of the United States, windows are by far the poorest insulating material used in buildings. As a result, approximately 3 percent of the nation’s energy use is used to offset heat lost through windows. Under cold conditions, conventional double glazings create uncomfortable spaces and collect condensation. However, with the recent introduction of low-emissivity (low-E) coatings and low/conductivity gas filling to respectively reduce radiative and conductive/convective heat transfer between glazing layers, some manufacturers are beginning to offer windows with R-values (resistance to heat transfer) of 4 hr-ft2 -F/Btu (0.70 m2 -C/W). This papers presents designs for and analysis and test results of an insulated glass unit with a center-of-glass R-value of 8-10; approximately twice as good as gas-filled low-E units, and four times that of conventional double glazing. This high-R design starts with a conventional insulated-glass unit and adds two low-emissivity coatings, a thin glass middle glazing layer, and a Krypton or Krypton/Argon gas fill. The unit’s overall width is 1 in. (25 mm) or less, consistent with most manufacturers’ frame and sash design requirements. Using state-of-the-art low-emissivity coatings does not significantly degrade the solar heat gain potential or visible transmittance of the window. Work to date has substantiated this concept of a high-R window although specific components require further research and engineering development. Demonstration projects, in conjunction with utilities and several major window manufacturers, are planned. This high-R window design is the subject of a DOE patent application.