The invention relates to a foamable mixture which contains at least one alkoxysilane-terminated prepolymer and which can be produced of a) at least one alkoxysilane that was functionalized with isocyanate groups and that can be produced of at least one hydroxyfunctional and/or aminofunctional alkoxysilane and at least one diisocyanate or polyisocyanate, and b) at least one compound that contains at least one OH group, and at least one blowing agent, characterized in that the foamable mixture has a dynamic viscosity of 100 to 25,000 mPas, preferably 500 to 10,000 mPas, at 23° C. The invention further relates to a method for producing said foamable mixture. The foamable mixtures according to the invention are suitable for sealing, insulating and mounting joints, roof surfaces, windows and doors or for filling hollow spaces.