This paper aims to qualify and quantify the various scales of porosities and densities in plant particles packings (hemp shiv) used for building applications. These characteristics are key parameters for the understanding of the mechanical, thermal and acoustical behaviors of particles used as loose aggregates or in bio-based concretes. A study was built around this topic, and enabled to evaluate skeletal, particle and packing densities and associated level of porosities. Many experimental methods have been used to characterize these parameters: manual and automated packing density evaluation, fluid and powder pycnometry, mercury intrusion and X-ray computed tomography. These measurements were performed as a function of particle size and level of aging by immersion in water. It was eventually shown that: - smaller particles have higher packing and particle densities, - aged particles present a strong evolution of their microstructure, visible through lower packing and particle densities and higher skeletal density, - powder pycnometry is the most representative method to measure particle density while X-ray computed tomography enables to characterize individual particles and to validate the range of data in terms of particle density. - the results of mercury intrusion porosimetry should be taken with caution due to the ?ink-bottle? shape of the pores, but enable to qualify the pore size distribution. This communication is linked with a second contribution (part 2) dealing with modelling applications.