Insulating glazing unit

An insulating unit (150) has a pair of glass sheets (12,14) about an edge assembly (152) to provide a compartment (18) between the sheets. The edge assembly has a U-shaped spacer (158) made of metal, metal coated plastic, gas and moisture impervious polymer, or gas and moisture impervious film coated polymer. The outer legs (156) of the spacer (158) and the glass (12,14) provide a long diffusion path to limit the diffusion of argon gas out of the compartment (18). The edge assembly (152) has materials selected and sized to provide edge assembly having an RES-value of at least 75. A spacer (158) for use in insulating units includes a plastic core having a gas impervious film e.g. a metal film or a halogenated polymer film. Also taught herein are techniques for making the unit (150) and spacer (158). The unit (150) has a long diffusion path to increase the time period in which insulating glass e.g. Argon gas may be retained in the compartment (18). The increased RES-value provides a unit (150) that has a low thermal conducting edge (152). In this manner heat loss through the marginal edge (152) of the unit (150) is reduced.