District heating pipes are usually polyurethane-(PUR)-insulated and the outside is protected by a polyethylene (HDPE) casing. The permeability of the cell gases (oxygen, nitrogen, carbon dioxide and cyclopentane) in the PUR foam and in the HDPE casing was determined for a temperature range of 20-60 C (PUR) and 5-40 C (HDPE).From the results, it was concluded that the casing acts as the main diffusion barrier for carbon dioxide, oxygen and nitrogen, whereas the main diffusion barrier for diffusion of cyclopentane is due to the PUR foam. This conclusion is valid for district heating pipes of small to medium dimensions.

This study focuses on the mass transfer properties of 1,1,1,3,3-pentafluorobutane (HFC-365mfc) in the insulating system used in most district heating pipes produced today, namely rigid polyurethane (PUR) foam with a protective layer of polyethylene (HDPE). The solubility, permeability and diffusion coefficients for HFC-365mfc in PUR foam and HDPE have been determined. The coefficients for HFC-365mfc in PUR foam are very similar to those of cyclopentane, currently the most common blowing agent in PUR foams used for district heating pipes in Europe. The polyethylene casing is a better diffusion barrier for HFC-365mfc than it is for cyclopentane. However, the main mass transfer resistance of HFC-365mfc in a district heating pipe is found in the PUR foam.

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Polyethylene terephthalate (PET) foam is a possible replacement option for polyurethane (PUR) foam as insulation material in district heating pipes. In this study, the diffusion coefficients and activation energies of cell gases in carbon dioxide blown PET foam (densities 148–157 kg·m−3) were determined at temperatures between 23 °C and 90 °C. The foam thermal ageing due to the mass transport of air into and carbon dioxide out of the foam was about ten times slower in PET foam than in PUR foam. The thermal conductivities of the PET foam boards were determined in a heat flow meter apparatus. The contribution to the foam thermal conductivity due to conduction in the solid polymer and radiation within the cell voids was determined to 17 mW·m−1·K−1 at 20 °C. This is higher than the value estimated for PUR foam in district heating pipes, 12 mW·m−1·K−1. This contribution can probably be reduced by developing low density PET foam and reducing the cell size.