Abstract: The specification describes and claims an insulating glass unit and a process for making. The unit comprises two glass panes spaced apart by a spacer of thermoplastics material adherent to the panes, an inert or heavy gas trapped within the unit and a layer of silicone elastomer located at the periphery of the unit between edge portions of the glass panes and in contact with external surfaces of the spacer. The thermoplastics material has a water vapor permeability of not more than about 0.2 l/m2/day (measured at 20° C. for 4 mm thickness) and a shear strength of more than 0.2 MPa as determined at a sealant thickness of 0.5 mm at 23° C. and a shear speed of 100 mm/min.