This study focuses on the surface treatment of an extremely hydrophilic natural plant material, hemp shiv, using a functionalised silica based coating to provide hydrophobicity while retaining its moisture buffering ability. The chemical composition and physical structure of bio-based materials results in their extremely hydrophilic behaviour. In this work, a simple one step coating process was used to enhance the water-repellence of hemp shiv without compromising its ability to adsorb and release moisture. The coating modified the morphology and surface roughness of hemp shiv providing a hydrophobic surface having a water contact angle of 118° and reduced the bulk water absorption by 250% over 24 h. Mercury intrusion porosimetry (MIP) showed that the treatment refined the pore size distribution of hemp shiv, reducing the size of larger pores but not completely blocking the smaller pores thereby allowing hemp shiv to buffer moisture. Fourier-transform infrared spectroscopy (FTIR) revealed the chemical composition was modified by the coating, reducing the hydroxyl groups. Hemp shiv aggregates treated with functionalised silica based coating show potential for the development of robust lightweight building materials with enhanced hydrophobicity.