An insulated glass unit comprising at least two spaced-apart sheets of glass in spaced relationship to each other, a low thermal conductivity gas therebetween and gas sealant element including a curable sealant composition comprised of a) diorganopolysiloxane exhibiting permeability to said gas; b) at least one polymer having a permeability to said gas that is less than the permeability of diorganopolysiloxane polymer, said polymer (b) being selected from the group consisting of low density polyethylene (LDPE), very low density polyethylene (VLDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene, polyisobutylene, polyvinyl acetate (PVAc), polyvinyl alcohol (PVOH), polystyrene, polycarbonate, polyester, such as, polyethylene terephthalate, polybutylene terephthalate, polyethylene napthalate, glycol-modified polyethylene terephthalate (PETG); polyvinylchloride, polyvinylidene chloride, polyvinylidene floride, thermoplastic polyurethane, acrylonitrile butadiene styrene, polymethylmethacrylate (PMMA), polyvinyl fluoride, Polyamides, polymethylpentene, polyimide, polyetherimide, polether ether ketone (PEEK), polysulfone, polyether sulfone, ethylene chlorotrifluoroethylene, polytetrafluoroethylene (PTFE), cellulose acetate, cellulose acetate butyrate, plasticized polyvinyl chloride, ionomers, polyphenylene sulfide, styrene-maleic anhydride, modified polyphenylene oxide, ethylene-propylene rubber (EPDM), polybutadiene, polychloroprene, polyisoprene, polyurethane, styrene-butadiene-styrene, styrene-ethylene-butadiene-styrene (SEEBS), and mixture thereof; c) cross-linker; d) catalyst for the cross-linker reaction; and e) a filler which is a clay, wherein the clay is modified with (i) an amine compound or ammonium ion having the structure R 3 R 4 R 5 N, wherein R 3, R 4, and R 5 are C 1 to C 30 alkyls or alkenes, and mixtures thereof, or (ii) an amine or ammonium ion having the structure R 6 R 7 R 8 N, wherein at least one R 6, R 7, and R 8 is C 1 to C 30 alkoxy silanes and the remaining are C 1 to C 30 alkyls or alkenes, on (iii) ammonium, primary alkylammonium, secondary alkylammohium, tertiary alkylammonium, quaternary alkylammonium, phosphonium derivatives of aliphatic, aromatic or arylaliphatic amines, phosphines or sulphides, or sulfonium derivatives of aliphatic, aromatic or arylaliphatic amines, phosphines or sulfides. a) diorganopolysiloxane exhibiting permeability to said gas; b) at least one polymer having a permeability to said gas that is less than the permeability of diorganopolysiloxane polymer, said polymer (b) being selected from the group consisting of low density polyethylene (LDPE), very low density polyethylene (VLDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene, polyisobutylene, polyvinyl acetate (PVAc), polyvinyl alcohol (PVOH), polystyrene, polycarbonate, polyester, such as, polyethylene terephthalate, polybutylene terephthalate, polyethylene napthalate, glycol-modified polyethylene terephthalate (PETG); polyvinylchloride, polyvinylidene chloride, polyvinylidene floride, thermoplastic polyurethane, acrylonitrile butadiene styrene, polymethylmethacrylate (PMMA), polyvinyl fluoride, Polyamides, polymethylpentene, polyimide, polyetherimide, polether ether ketone (PEEK), polysulfone, polyether sulfone, ethylene chlorotrifluoroethylene, polytetrafluoroethylene (PTFE), cellulose acetate, cellulose acetate butyrate, plasticized polyvinyl chloride, ionomers, polyphenylene sulfide, styrene-maleic anhydride, modified polyphenylene oxide, ethylene-propylene rubber (EPDM), polybutadiene, polychloroprene, polyisoprene, polyurethane, styrene-butadiene-styrene, styrene-ethylene-butadiene-styrene (SEEBS), and mixture thereof; c) cross-linker; d) catalyst for the cross-linker reaction; and e) a filler which is a clay, (i) an amine compound or ammonium ion having the structure R 3 R 4 R 5 N, wherein R 3, R 4, and R 5 are C 1 to C 30 alkyls or alkenes, and mixtures thereof, or (ii) an amine or ammonium ion having the structure R 6 R 7 R 8 N, wherein at least one R 6, R 7, and R 8 is C 1 to C 30 alkoxy silanes and the remaining are C 1 to C 30 alkyls or alkenes, on (iii) ammonium, primary alkylammonium, secondary alkylammohium, tertiary alkylammonium, quaternary alkylammonium, phosphonium derivatives of aliphatic, aromatic or arylaliphatic amines, phosphines or sulphides, or sulfonium derivatives of aliphatic, aromatic or arylaliphatic amines, phosphines or sulfides.