Standard pore size determination methods like mercury porosimetry, nitrogen sorption, microscopy or X-ray tomography are not suited to highly porous, low density and thus very fragile materials. For this kind of materials, a method based on thermal characterization has been developed in a previous study. This method has been used with air pressure varying from 10-1 to 10 5 Pa for materials having a thermal conductivity less than 0.05 W m K at atmospheric pressure. It enables the estimation of pore size distribution between 100 nm and 1 mm. In this paper, we present a new experimental device enabling thermal conductivity measurement under gas pressure up to 10 6 Pa, enabling the estimation of the volume fraction of pores having a 10 nm diameter. It is also demonstrated that the main thermal conductivity models (Parallel, Series, Maxwell, Bruggeman, Self-consistent) lead to the same estimation of the pore size distribution as the extended parallel model (EPM) presented in this paper and then used to process the experimental data. Three materials with thermal conductivities at atmospheric pressure ranging from 0.014 W m K to 0.04 W m K are studied. The thermal conductivity measurements results obtained with the three materials are presented and the corresponding pore size distributions between 10 nm and 1 mm are presented and discussed.