This study investigates two ultra-lightweight earth-based composites made from rapeseed straw (RS) and hemp shive (HS) granulates and a binder of raw-earth. An experimental campaign was conducted to evaluate their mechanical strength, hygric behavior, and durability under extreme variations of relative humidity. The raw materials were characterized: two types of raw-earth were analyzed for particle size distribution and mineralogical composition, together with the particle size distribution of plant granulates. Water absorbency of the granulates was measured across time scales from 1 min to 14 days. Several series of test blocks (10 \(\times \) 10 \(\times \) 10 \(cm^{3}\) ) with varying densities (250–350 \(kg/m^{3}\) ) and different water content in the slurry were hand-casted and systematically evaluated. The drying phase, undertaken under free but monitored temperature and relative humidity (T-RH) conditions and measured by weighing, reveals a longer drying time for rapeseed than for hemp based composites. In the first phase, blocks were prepared with varying water content, highlighting that a high water content increases by up to 50% the mechanical strength under uniaxial load. In the second phase, hygric regulation potential was assessed by exposing blocks to extreme relative humidity (RH) (from 20% to 90%) showing that both composites have excellent moisture buffering, with uptake of 14wt.% after 6 days. Finally, durability under repeated RH variations was evaluated using an environmental chamber. Blocks underwent several weeks of cyclic RH changes (20%–95%) before testing for mechanical strength. Slight strength loss were revealed for rapeseed composites. Additionally, earth binders with very high clay content (60% clay) were more susceptible to strength degradation than those with lower clay mineral content (30% clay). The findings highlight the potential of both these sustainable insulation materials for the building sector.