Embodiments include a method for replacing air with an interpane gas during manufacture of a sealed insulating glass unit (IGU). The method includes forming an unsealed IGU assembly defining an IGU passage for fluid communication between an interpane space and an ambient environment; positioning the unsealed IGU assembly within an enclosure and sealing the enclosure around the unsealed IGU assembly; evacuating air from the enclosure; introducing a first gas into the interpane space through the IGU passage; introducing a second gas into the enclosure, wherein the second gas has a different composition than the first gas; and closing the IGU passage to seal the interpane space. Other embodiments are also included herein.