Abstract: A glazing method and system for upgrading an existing single glazed window installation includes providing an insulating glass unit comprising a first glazing lite and a second glazing lite. Providing a spacer located between the existing lite and the insulating glass unit. The spacer is formed from a material which is capable of forming a seal between the insulating glass unit and the existing lite and forms a cavity between the insulating glass unit assembly and the existing glazing installation. Providing a supplemental holder for holding the insulating glass unit in place against the existing glazing installation.