The insulation efficiency of polyurethane foam is mainly (60 to 65%) determined by the thermal conductivity of the cell gas, which is a mixture of carbon dioxide (CO2) and a blowing agent. Until recently CFC-11 (trichlorofluoromethane) was the most widely used blowing agent. To find adequate substitutes for CFC-11, the vapor-phase thermal conductivity of several potential blowing agents has been measured. The experimental procedure is based on the transient hot-wire method. The apparatus was designed in cooperation with the University of Stuttgart (Prof. K. Stephan). The measuring cell (stainless steel, 1.4571) with a length of 270 mm and a diameter of 48 mm consists of two parallel chambers with platinum wires of different lengths. In this paper experimental results for the vapor-phase thermal conductivity of carbon dioxide(CO2), 1,1,1,2,2-pentafluoropropane (HFC-245cb), 1,1,1,3,3-pentafluoropropane (HFC-245fa), and 1,1,2,2,3-pentafluoropropane (HFC245ca) are given. Experiments have been performed at pressures between 0.2 h MPa and 1.5 MPa and temperatures between 298 K and 419.91 K.