Polyurethane foam is a material with good insulating capacity that is used in many applications such as building, appliance and pipe insulation. The thermal conductivity of the polyurethane foam is generally low, but it changes over time as the foam ages. Cyclopentane is the dominating blowing agent of foam in Europe. It fills and expands the cells in the polyurethane matrix at foam production, together with carbon dioxide. The two insulating gases diffuse out of the foam and air diffuses into the foam over time. As the diffusion rates of the gases are temperature dependent, the ageing is a coupled heat and mass transfer problem. The change of the gas composition in the cells of the foam has a pronounced effect on the insulating capacity of the material. A model has been developed that describes how the insulating capacity of the foam changes over time. The model accounts for diffusion of cyclopentane, carbon dioxide, nitrogen and oxygen under a strong temperature gradient. Unlike models that have been used earlier, this model takes into account the coupled heat conduction and diffusion, and in particular all phases of cyclopentane. The cyclopentane occurs as gas in the cells, dissolved in the polymer and condensed liquid. The non-gaseous cyclopentane acts as a buffer to the cyclopentane in the gas phase. The gas phase concentration should be high in order to maintain a low thermal conductivity of the foam. The model has been used to determine the long-term insulation capacity of district heating pipes. This is an important task since heat generation to compensate heat losses from the networks constitutes a considerable part of the environmental impact and cost connected to the networks. The calculated time-dependent concentrations provide good insight into the ageing process of the pipes. Calculation with the model shows that the insulating capacity of an average sized forward pipe decreases with about 20% over a period of 30 years, the nominal lifetime of the pipes. Calculations show that it is important how much cyclopentane the foam contains initially. Different pipe manufacturers use different amount of cyclopentane at foaming. The insulating capacity of pipes may differ 3% between manufacturers, due to different initial gas compositions and cyclopentane content.