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Recent measurements have shown a record-breaking low thermal conductivity lambda_total of smaller than 0.25*10-3 W/(mK) at temperatures of 120K for an evacuated sample consisting of polyimide fibres with trilobal fibre cross section. Existing models for the heat transport in fibre insulations cannot sufficiently describe fibre-insulations consisting of fibres with non-cylindrical cross sections. In this article, a modification for the model for cylindrical fibres will be presented. The modifications for the trilobal cross section of the fibre will be explained and compared to the original cylindrical model. The results of the theoretical calculations will be discussed in comparison to experimental results of measurements performed with a guarded hot plate apparatus at temperatures in the range from 120K to 420K.