The invention relates to a prepolymer mixture for producing impervious and isolating foams, said prepolymer mixture containing a prepolymer compound, a pore-forming compound and traditional additives required for foam formation. The prepolymer compound is a silane-terminated polyurethane prepolymer with at least two groups Si(OR)x(R)3-x in its molecule, R representing respectively an alkyl residue with 1 to 6 carbon atoms, and x is an integer between 1 and 3.