This paper concerns the packing of plant particles (hemp shiv) used for building applications and aims to quantify skeletal, particle and packing densities and associated porosities. These parameters were measured on hemp particles as a case study and were used to assess the physical behaviour of this material in terms of water adsorption and absorption, mechanical compression and thermal and acoustic behaviours. A number of experimental methods have been used to characterise these parameters including fluid and powder pycnometry, mercury intrusion and X-ray computed tomography, leading to robust and complementary results as a function of particle size and level of aging by immersion in water. It was concluded that smaller particles are characterised by higher packing and particle densities, while aged particles present a strong evolution of their microstructure, which is visible through lower packing and particle densities and higher skeletal density. By comparing these experimental results with physical property characterisations, correlations were found between water absorption and open porosity, compression behaviour and the three density scales, and between acoustic dissipation and intra- and inter-particle porosities.