Brittle fracture initially occurs at the nanometric and/or interatomic scale without prior plastic deformation under elastic loading. Fibres of different dimensional scales in hybrid composites provide dense packaging and act as reinforcements in the corresponding scales and dimensions. This study evaluated the effect of the inclusion of cellulose pulp (CP) (5wt%, 4.5wt%, 4wt% and 3.5wt%) combined with cellulose nanofibrils (CNF) (0.5wt%, 1wt% and 1.5wt%) from recycled cardboard on the water absorption, density, dimensional stability, thermal insulation, flexural strength and durability of earth-based materials. The total cellulose content (CP and CNF) is set at 5wt%. The results show that the partial replacement of CP by CNF contributes to an increase in the moisture content of the composite from 18% to 21%, while there is a reduction in their dry shrinkage from 2.29% to 1.29% and in their thermal conductivity from 0.42 to 0.39 W/m.K. This is attributed to the different dimensional scales of the fibres in hybrid composites, which provide a dense assembly with greater dimensional stability. The 3.5% CP content combined with 1.5% CNF showed the highest flexural strength (2.76 MPa) and modulus of elasticity (1.93 GPa) of the hybrid materials under four-point bending loading, due to the high anchorage between the cellulose fibres and the matrix. CP and CNF as reinforcing agents in an earth-based matrix is a suitable solution for improving the flexural strength, density, dimensional stability and durability of building bricks. However, bricks reinforced only with CP are recommended where flexural strength and toughness are essential requirements.