Foam-forming compositions are disclosed which contain a mixture of cis-1,1,1,4,4,4-hexafluoro-2-butene and 1,1,1,3,3-pentafluoropropane. Also disclosed is a closed-cell polyurethane or polyisocyanurate polymer foam prepared from reaction of an effective amount of the foam-forming composition with a suitable polyisocyanate. Also disclosed is a process for producing a closed-cell polyurethane or polyisocyanurate polymer foam by reacting an effective amount of the foam-forming composition with a suitable polyisocyanate.