Resin Transfer Moulding (RTM) is a process used in some composite parts manufacturing. A fibrous 3D interlock preform is firstly compacted to achieve the desired Fibre Volume Fraction (FVF), and is then impregnated with a polymer resin. Its flow within and between the homogeneous equivalent porous yarns has to be precisely understood to predict impregnation defects. The process setting is therefore monitored accordingly to the 3D interlock fabric permeability, defined from Darcy’s law. A double-scale description is used to represent the fabric unit cells, taking into account both the mesoscopic yarn morphology and the intra-yarn FVF field. This field is then converted into a permeability tensor field that characterises the homogeneous equivalent intra-yarn properties. The unit cell has a multi-scale nature that leads consequently to multi-scale effects on the fluid flow. Darcy’s equation is used as a model of the intra-tow flow and Stokes’ equation is used between yarns. Therefore double-scale concerns on the effective fabric unit cell permeability tensor and the potentially resulting dry spot map. However, an accurate description of the resin flow within the homogeneous equivalent porous yarns is required to achieve this. A locally oriented intra-yarn permeability tensor field varying along yarns and a capillary stress tensor within them at the resin-air interface are added to model stationary and transient doublescale flow in 3D interlock fabrics.