Rigid polyurethane foams (RPUFs) have been fabricated from crude MDI (CMDI) and polypropylene glycols (PPGs) of various functionalities (f) with HFC 365mfc as a blowing agent. Foam density increased, cell size and density distribution decreased with increasing f while the closed cell content was kept constant over 92%, The gel time, tack-free time, volume change, and the thermal conductive of the foam showed a minimum with f = 5, and the existence of minimum has been explained in terms of a large mixture viscosity and cell wall resistance. (C) 2008 Wiley Periodicals, Inc.