This chapter deals with a novel nanocomposite specifically developed for chemical treatment of walls, floors, ceilings, roofs, etc. to prevent or limit exposure to dangerous gases seeping into buildings through constructive materials. The developed silica-based organic nanocomposite contains the mixture of polyethylhydrosiloxane and the solution of alkyltriethoxysilane in isopropanol with a tin-based catalyst. The nanocomposite fills micropores inside the concrete or another constructive material against hazardous gases, including volatile organic compounds, organic and inorganic gases, and radioactive gases emitting alpha particles. The method allows for reducing the coefficient of gas (air, Ar, 222Rn, CO, 85Kr, 133,135Xe, etc.) permeability 200-400 times. The consumption of chemicals depends on the porosity of the materials; it is equal to 0.2L/m2 for gypsum and 0.3-0.4L/m2 for concrete and cement, respectively. © 2018 Elsevier Inc. All rights reserved.