Cross-linkable foamable composition with low content of monomeric isocyanates, containing a pre-polymer of polyester diols and/or polyether dials reacted i) with asymmetric aromatic diisocyanates to form a reaction product with OH and NCO groups, ii) subsequent addition of and reaction with symmetric aromatic diisocyanate in a quantity at least equivalent to the available OH-groups, 0.1 to 30 wt % additives, at least one propellant, wherein the polyols have a molar mass (MN) of less than 5000 g/mol, and the reaction product has a content of less than 0.1 wt % monomeric asymmetric isocyanate and less than 2 wt % monomeric symmetric isocyanate.