A blowing agent for use in preparing a polyisocyanate-based foam which comprises a major portion of HCFC-123, a minor portion of HCFC-141b, a branched or unbranched aliphatic alkane having from 6 to 10 carbon atoms or mixtures thereof, and a polyether component which reduces cell size in the resultant polyisocyanate-based foam. The polyether component includes a mixture of polymerized, oxidized perfluorinated propene polyethers having a molecular weight distribution of from about 400 to about 1800 units. Alternatively, the polyether component can be added to the foam precursor materials separate from the blowing agent. Blowing agents and polyether additives for polyurethane-based foams are also provided.