An insulated glass unit comprising at least two spaced-apart sheets of glass in spaced relationship to each other, a low thermal conductivity insulating gas or mixture of gases therebetween and a gas sealant element including a cured sealant composition resulting from the curing of a curable sealant composition comprising: a) at least one silanol-terminated diorganopolysiloxane; b) at least one crosslinker for the silanol-terminated diorganopolysiloxane(s); c) at least one catalyst for the crosslinking reaction; d) a gas barrier enhancing amount of at least one inorganic-organic nanocomposite, the inorganic-organic nanocomposite comprising at least one inorganic component which is a layered inorganic nanoparticulate and at least one organic component which is a quaternary ammonium organopolysiloxane having the formula: M a D b D' c wherein "a" is 2, and "b" is equal to or greater than 1 and "c" is zero or positive; M is [R 3 z NR 4] 3-x-y R 1 x R 2 y SiO 1/2 wherein "x" is 0, 1 or 2 and "y" is either 0 or 1, subject to the limitation that x + y is less than or equal to 2, "z" is 2, R 1 and R 2 each independently is a monovalent hydrocarbon group up to 60 carbons; R 3 is selected from the group consisting of H and a monovalent hydrocarbon group up to 60 carbons; R 4 is a monovalent hydrocarbon group up to 60 carbons; D is R 5 R 6 SiO 1/2 where R 5 and R 6 each independently is a monovalent hydrocarbon group up to 60 carbon atoms; and D' is R 7 R 8 SiO 2/2 where R 7 and R 8 each independently is a monovalent hydrocarbon group containing amine with the general formula: [R 9 a NR 10] wherein "a" is 2, R 9 is selected from the group consisting of H and a monovalent hydrocarbon group up to 60 carbons; R 10 is a monovalent hydrocarbon group up to 60 carbons; and, optionally, e) at least one solid polymer having a permeability to gas that is less than the permeability of the crosslinked diorganopolysiloxane(s). a) at least one silanol-terminated diorganopolysiloxane; b) at least one crosslinker for the silanol-terminated diorganopolysiloxane(s); c) at least one catalyst for the crosslinking reaction; d) a gas barrier enhancing amount of at least one inorganic-organic nanocomposite, the inorganic-organic nanocomposite comprising at least one inorganic component which is a layered inorganic nanoparticulate and at least one organic component which is a quaternary ammonium organopolysiloxane having the formula: M a D b D' c wherein "a" is 2, and "b" is equal to or greater than 1 and "c" is zero or positive; M is [R 3 z NR 4] 3-x-y R 1 x R 2 y SiO 1/2 wherein "x" is 0, 1 or 2 and "y" is either 0 or 1, subject to the limitation that x + y is less than or equal to 2, "z" is 2, R 1 and R 2 each independently is a monovalent hydrocarbon group up to 60 carbons; R 3 is selected from the group consisting of H and a monovalent hydrocarbon group up to 60 carbons; R 4 is a monovalent hydrocarbon group up to 60 carbons; D is R 5 R 6 SiO 1/2 where R 5 and R 6 each independently is a monovalent hydrocarbon group up to 60 carbon atoms; and D' is R 7 R 8 SiO 2/2 where R 7 and R 8 each independently is a monovalent hydrocarbon group containing amine with the general formula: [R 9 a NR 10] wherein "a" is 2, R 9 is selected from the group consisting of H and a monovalent hydrocarbon group up to 60 carbons; R 10 is a monovalent hydrocarbon group up to 60 carbons; and, optionally, e) at least one solid polymer having a permeability to gas that is less than the permeability of the crosslinked diorganopolysiloxane(s).