The mfr. uses a halogen-free organic liq. as a physical blowing agent for the foam. The organic liq. contains a positive azeotrope of min. b.pt. 15-80 deg.C at normal atmospheric pressure and comprises (a) a 5-7C cycloaliphatic hydrocarbon, and (b) a 4C aliphatic ether, a 2-5C aliphatic carboxylic acid ester, a 4C aliphatic ketone or a mixt. 1-10 pts. wt. of a nucleating agent per 100 pts. wt. of foaming ingredients are present during foaming pref. perfluoropentane. The wt. ratio of azeotrope components in the organic liq. is within +/- 20% (+/- 10%) of an ideal azeotrope compsn. The foam is pref. a rigid polyurethane, integral skin flexible polyurethane or urethane-modified polyisocyanurate foam. The azeotrope has a min. boiling temp. of pref. 15-45 (20-40) deg.C.; It comprises cyclopentane and (m)ethyl formate or methyl acetate, cyclohexane and methyl formate, cyclohexane or cyclohexene and 2-butanone, cyclopentane or cyclopentene and 2-butanone or cyclopentane and furan.