In most areas of the United States, windows are by far the poorest insulating materialsused in buildings. As a result, approximately 3% of the nations energy use is used tosoffset heat lost through windows. Under cold conditions, conventional double glazingsscreate uncomfortable spaces and collect condensation. However, with the recent introductionsof low-emissivity (low-E) coatings and low/conductivity gas filling to respectivelysreduce radiative and conductive/convective heat transfer between glazing layers, somesmanufacturers are beginning to offer windows with R-values (resistance to heat transfer)sof 4 hr-ft2- F/Btu (0.70 m2-C/W). This paper presents designs for and analysis and testsresults of an insulated glass unit with a center-of-glass R-value of 8-10; approximatelystwice 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 twoslow-emissivity coatings, a thin glass middle glazing layer, and a Krypton orsKrypton/Argon gas fill. The units overall width is 1 (25 mm) or less, consistent withsmost manufacturers frame and sash design requirements. Using state-of-the-art low-emissivity coatings does not significantly degrade the solar heat gain potential or visiblestransmittance of the window. Work to date has substantiated this concept of a high-Rswindow 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.