Water-blown rigid polyurethane foams manufactured in eco-friendly manners attract great attentions for applications in various industrial products. Especially, the polyurethane foams are widely applicable as thermal insulation materials for LNG carrier, electronic appliances, pipes, and building. Heat transfer mechanisms in foamed materials have strong relationships with gaseous molecules in cells, solid parts, and cellular morphologies. In this study, thermal conductivity of the water-blown rigid polyurethane foams was investigated by controlling the cellular morphologies using different types of surfactant molecules and gelling catalysts. The cell sizes were controlled from 551μm to 153μm by varying surfactants and gelling catalysts. The small cell sizes showed low radiative thermal conductivity due to the high number of thick cell walls and struts to obstruct the photon transport process. More importantly, there was a clear cell threshold size (230?μm) in variations of the overall thermal conductivity value (about 24.3mW/mK) of polyurethane foams, and the delayed slowly weaken thermal insulation property was also noted in the cell sizes less than 230μm.