Irradiating porous silicon is expected to reduce thermal conductivity without altering the porous structure and can be studied by optical techniques provided optical properties can be established reliably. Toward this end, meso-porous silicon (PSi), with a porosity of 56%, was prepared from p + Si wafer (0.01-0.02 .cm-1 resistivity) and was partially amorphized by irradiation in the electronic regime with 129 Xe ions at two different energies (29 MeV and 91 MeV) and five fluences ranging from 10 12 cm-2 to 3.10 13 cm-2. The PSi structure is monitored by scanning electron microscopy. High-resolution transmission electron microscopy shows that the amorphous phase is homogeneous in volume and that there is no formation of amorphous-crystalline core-shell structures. An agreement is found between the thermal conductivity results obtained with micro-Raman thermometry, which is an optical contactless technique heating the sample in the depth, and scanning thermal microscopy, which is an electrical technique heating the sample by contact at the sample surface. A linear relation is established between the effective thermal conductivity and the amorphous fraction, predicting the thermal conductivity of fully-amorphous porous Si below 1 W.m-1 .K-1. The obtained values are comparable to that of SiO2, reduced by a factor 6 in comparison to non-irradiated porous samples (~6.5 W.m-1 .K-1) and smaller than bulk silicon by more than two orders of magnitude.