The thermal conductivity of the insulating polyurethane foam in district heating pipes is an important factor when improving the competitiveness of district heating, especially in areas of low heat density, where flexible district heating pipes are widely used. The standardised methods to measure the thermal conductivity of straight pipes are not applicable due to the small pipe dimensions and the fact that they are coiled. The paper presents a new transient method to determine the thermal conductivity, including the temperature dependence, for such district heating pipes. A coil of the flexible pipe is immersed into cold water (10°C). The temperature decline of hot water inside the service pipe (starting at 80ºC) is measured. The method is based on the fact that the temperature decline depends on the thermal conductivity of the insulation. The experimentally measured declining water temperatures give with rather simple theoretical expressions the thermal conductivity as function of temperature. In this paper, the method is analysed. The results of several experimental measurements on two different pipe types are used in the analysis. The final results, the thermal conductivities of both pipe types, are in agreement with values estimated by cell gas analysis of the polyurethane foam together with values taken from literature. The study focuses on the temperature dependence of the thermal conductivity of the polyurethane foam. Linear and quadratic relations, as well as a general “free” relation, for the temperature dependence are analysed.