The use of a cross-linked polysiloxane dispersion composition comprising (i) a crosslinked polysiloxane dispersion of: a reaction product of (a) a siloxane polymer having at least two -OH groups per molecule, or polymer mixture having at least two -OH groups per molecule, having a viscosity of between 5000 to 500,000 mPa.s at 25°C, as determined by the method disclosed in the specification, and (b) at least one self catalyzing crosslinker reactive with (a), and additionally comprising (c) a surfactant and (d) water; together with one or more of the following ingredients: (ii) one or more fillers selected from the group of colloidal silica, fumed silica, precipitated silica, diatomaceous earths, ground quartz, kaolin, calcined kaolin, wollastonite, hydroxyapatite, calcium carbonate, hydrated alumina, magnesium hydroxide, carbon black, titanium dioxide, aluminium oxide, vermiculite, zinc oxide, mica, talcum, iron oxide, barium sulphate and slaked lime or a mixture thereof; (iii) one or more stabilizers; and (iv) one or more rheology modifiers as a liquid applied, air and water barrier coating composition; characterised in that the cross-linked polysiloxane dispersion composition, once dried on a substrate, meets the requirements of ASTM E2178-11, Standard Test Method for Air Permeance of Building Materials, having an Air Permeance (L/s per m 2) of less than 0.006 at a differential pressure of 75 Pa at thicknesses of 10 mil (0.254mm) and 15 mil (0.381mm); and/or has a Water Vapour Transmission, once dried on a substrate of greater than 7 US Perm (400.5 ng.s -1 m -2 Pa -1), according to the Dry Cup Desiccant Method of ASTM E96/E96M-10 for both the 10 mil (0.254mm) and 15 mil (0.381mm) thicknesses; and, Standard Test Method for Water Vapour Transmission rate of Materials and in accordance with Water Vapour Transmission Wet Cup Water Method of ASTM E96/E96M-10, Standard Test Method for Water Vapour Transmission rate of Materials of greater than 30 US Perm (1716.41 ng.s -1 m -2 Pa -1) for coatings of 10 mil (0.254mm) thickness and greater than 24 US Perm (1373.12 ng.s -1 m -2 Pa -1) for coatings of 15 mil (0.381mm) thickness; and/or passes the Self Sealability (Head of Water) Test described in Section 8.9 of ASTM D1970 - 09. (i) a crosslinked polysiloxane dispersion of: a reaction product of (a) a siloxane polymer having at least two -OH groups per molecule, or polymer mixture having at least two -OH groups per molecule, having a viscosity of between 5000 to 500,000 mPa.s at 25°C, as determined by the method disclosed in the specification, and (b) at least one self catalyzing crosslinker reactive with (a), and additionally comprising (c) a surfactant and (d) water; together with one or more of the following ingredients: (ii) one or more fillers selected from the group of colloidal silica, fumed silica, precipitated silica, diatomaceous earths, ground quartz, kaolin, calcined kaolin, wollastonite, hydroxyapatite, calcium carbonate, hydrated alumina, magnesium hydroxide, carbon black, titanium dioxide, aluminium oxide, vermiculite, zinc oxide, mica, talcum, iron oxide, barium sulphate and slaked lime or a mixture thereof; (iii) one or more stabilizers; and (iv) one or more rheology modifiers as a liquid applied, air and water barrier coating composition; (ii) one or more fillers selected from the group of colloidal silica, fumed silica, precipitated silica, diatomaceous earths, ground quartz, kaolin, calcined kaolin, wollastonite, hydroxyapatite, calcium carbonate, hydrated alumina, magnesium hydroxide, carbon black, titanium dioxide, aluminium oxide, vermiculite, zinc oxide, mica, talcum, iron oxide, barium sulphate and slaked lime or a mixture thereof; (iii) one or more stabilizers; and (iv) one or more rheology modifiers as a liquid applied, air and water barrier coating composition; characterised in that the cross-linked polysiloxane dispersion composition, once dried on a substrate, meets the requirements of ASTM E2178-11, Standard Test Method for Air Permeance of Building Materials, having an Air Permeance (L/s per m 2) of less than 0.006 at a differential pressure of 75 Pa at thicknesses of 10 mil (0.254mm) and 15 mil (0.381mm); and/or has a Water Vapour Transmission, once dried on a substrate of greater than 7 US Perm (400.5 ng.s -1 m -2 Pa -1), according to the Dry Cup Desiccant Method of ASTM E96/E96M-10 for both the 10 mil (0.254mm) and 15 mil (0.381mm) thicknesses; and, Standard Test Method for Water Vapour Transmission rate of Materials and in accordance with Water Vapour Transmission Wet Cup Water Method of ASTM E96/E96M-10, Standard Test Method for Water Vapour Transmission rate of Materials of greater than 30 US Perm (1716.41 ng.s -1 m -2 Pa -1) for coatings of 10 mil (0.254mm) thickness and greater than 24 US Perm (1373.12 ng.s -1 m -2 Pa -1) for coatings of 15 mil (0.381mm) thickness; and/or passes the Self Sealability (Head of Water) Test described in Section 8.9 of ASTM D1970 - 09.