Secondary Glazing Systems, or SGS, are high-insulating, low-emissivity (low-E) panels that are retrofitted to existing commercial windows. SGS products come in a variety of configurations, such as single-, double-, and triple-pane (resulting in anywhere from a double-pane to quadruple-pane window when added to an existing single-pane window), and they are available for a variety of glazing types, such as curtain walls, storefront glazing, double-hung, and sliders. Products are also available with various low-E coating options to optimize for different climates and building orientations.
SGS was proposed as a new measure to the RTF in February of 2017, at which point the RTF decided to commit resources toward its development. In July of 2018, the RTF directed a subcommittee to assist in reviewing and vetting a set of simulation models being used to estimate SGS energy savings.