Thermal Performance of Insulating Window Systems

Energy efficient windows coupled with window management strategies can alter the role of windows from that of an energy drain to a net supplier of energy to the building. This will require effective utilization of winter solar gain and daylight, coupled with reductions in thermal losses. Thermal losses of conventional double glazing are less than those of single glass but fall far short of the lower loss rate of other building elements. This paper reviews several improvements in window design, which show promise of reducing the window U value to as low as .5 W/m² K (.1Btu/hr ft² °F). These include the use of: convection suppression in double glazed windows using low conductivity fill gases, with vertical and horizontal partitions; partial evacuation of the air space; transparent heat mirror coatings on glass and on plastic interlayers; movable insulating devices; and air flow windows. Thermal comfort, cost effectiveness and other non-energy related performance issues are discussed briefly relative to several of the proposed designs.