Hempcrete is low carbon footprint building material, which consists in a mix of a cement and/or lime binder and vegetal particles (shiv). Its high porosity and low density give it good insulation, hygrothermal and acoustic properties. On the other hand, the mechanical strength of hempcrete is very poor, which limited its used like a filling material in a load bearing structure. Its limits mechanical properties are due, among other things, to an Interfacial Transition Zone (ITZ) around the shiv, which is not hydrated like the rest of the matrix. The identification of the size and the characteristics of this ITZ is a key parameter to improve the mechanical properties of these kind of bio-based material. A new experimental test protocol, based on image analysis, was developed to achieve repetitive and robust visual observations of the formation of the ITZ. The high-water absorption and the leaching of shiv are the main parameters, which drive the shape and strength of the ITZ. A microstructural characterization was also conducted to understand the nature and origin of this less hydrated zone around the hemp. This experimental protocol will allow to determine the mix parameters that impact ITZ development and select the most appropriate binder/vegetal fiber couple to use.