An insulating glass unit including a first pane of glass, a second pane of glass, and a spacer system. The spacer system includes (i) a spacer positioned between an inner surface of the first pane of glass and an inner surface of the second pane of glass and (ii) a sealant system for adhering the inner surfaces of the glass panes to the spacer. The sealant system includes at least one sealant having (a) at least one thermoplastic hot-melt material having a melt temperature ranging from about 125° F. (51° C.) to about 250° F. (121° C.), and (b) at least one curable material. The sealant, when cured, forms a covalent bond between the spacer and the panes. The sealant has an initial hardness ranging from about 25 Shore A to about 450 Shore A and a post-cure hardness measured about 48 hours thereafter ranging from about 30 Shore A to about 50 Shore A.