The design and fabrication of a highly insulating window glazing is being studied at SERI. Computer aided design analyses indicate that an all glass, edge sealed vacuum window with spherical glass interpane spacers and a low emittance, coating,on one internal surface could exhibit a thermal conductance of less than 0.6 W/m2K (thermal resistance, R > 10°F ft h/Btu). Cost effective means of mass-producing such a glazing have been explored. A CO2 laser has been used to produce a continuous, leak tight welded glass perimeter at 580°C, and this process appears to be a promising approach. However, at this temperature in vacuum, few low-emissivity coatings retain their desirable properties. Systemmatic measurements were made on tin oxide (fluorine doped) and indium-tin oxide low-e coatings. The indium-tin oxide was shown to be improved by vacuum heating. The ratio of solar weighted transmittance to emittance (313 K black-body weighted), a measure of performance in this application, is shown to have a sharply defined maximum at a coating sheet resistance of approximately 5 ohms per square in both of these oxide semiconductor coatings.