The diffusion of the blowing agents in the building insulation foams and the transfer of the air into the insulation foams cause the decrease of the thermal insulation performance. In order to evaluate the protection effectiveness of global warming for the long term, it is important to estimate the aging of the thermal conductivity in the building insulation foams. In this paper, the aging of thermal conductivity in building insulation foams using fluorocarbons or hydrocarbons as the blowing agents was discussed with measured data and numerical analysis. The equations to estimate the thermal conductivity in insulation foams were expressed and the measured data of the thermal conductivity in polyurethane foam and polystyrene foam were represented. The several numerical simulations using these equations were conducted. The calculated thermal conductivity was compared to the measured value and the calculated results showed the good agreement with the measured data. The decay of the partial pressure of the blowing agents was calculated with numerical simulation and the thermal conductivity in the insulation foams increased because the partial pressure within the insulation foams was decreased and the total pressure became the atmospheric pressure.