Abstract: A method of converting single pane glass to multiple pane hermetically sealed insulating glass without removing the existing glass sash and frame by using a preformed elastomeric (neoprene) glazing gasket as a mechanical seal. The preformed neoprene rubber seal glazing gasket is supported on a suitable frame of metal, aluminum, or other construction material. The glazing gasket support frame is attached to the existing sash or frame with suitable fastner. The preformed elastomeric glazing gasket is mechanically compressed against the existing lite of glass by the force of the fastner when the support frame is tightened against the existing sash or frame. This mechanical compression of the preformed elastomeric glazing gasket against the existing lite of glass forms a seal that is impervious to air and water vapor. The preformed elastomeric glazing gasket is grooved for support of the second lite of glass that is required to form the multiple pane hermetically sealed insulating glass unit.