Abstract: The present invention relates to an improved essentially closed cell polyurethane foam comprising the reaction product of at least one polyol with at least one isocyanate, in the presence of water to provide a carbon dioxide blowing agent, the improvement comprising said reaction being effected in the presence of a catalyst system consisting essentially of N,N,N',N", N"-pentamethyl-diethylenetriamine and triethylene diamine in order to provide a polyurethane foam characterized by enhanced shrinkage resistance. Also claimed is a process for producing this foam.