Blowing agents are mainly responsible for the insulation properties of polyurethane rigid foams. Detailed knowledge about the vapor conductivity of blowing agents and their mixtures is essential for their basic understanding. In addition to their insulation properties, blowing agents to a large extent determine the mechanical properties of the foam. Today, blowing agent mixtures are often used in an attempt to combine both worlds, low thermal conductivity and high vapor pressure at low temperatures. Our investigations aim at a better understanding of the relevant practical properties of blowing agents and their mixtures and to focus on their meaning for actual and potential foam systems of interest to the rigid foam insulation market. A newly developed transient hot wire method was used to determine the vapor phase thermal conductivity of various blowing agents used in rigid polyurethane foam. Data for CFC-11, HCFC-141b, HFC-365mfc, HFC-356mffm, HFC-245fa, HFC-245ca, cyclopentane, isopentane, n-pentane, isobutane, n-butane, and carbon dioxide have been generated. In addition to measurements of the pure compounds, current market mixtures of cyclopentane with low boiling hydrocarbons have been investigated. Experiments have been performed at pressures between 0.2 MPa and 1.5 MPa and temperatures between 298 K and 421 K.