Composite materials are increasingly used to contribute to structure lightening in electromagnetic shielding applications. The interactions between electromagnetic waves and composite materials are highly dependent on their microstructure. This gives rise to challenging modelling issues. Considering details of the microstructure would involve an excessive number of unknowns with standard numerical tools for structural analysis. Homogenisation methods—such as Maxwell-Garnett model—are a possibility to overcome this problem. The equivalent homogeneous medium obtained with such methods can be introduced into numerical tools to model full shielding enclosures. A homogenisation model has been recently proposed to obtain the equivalent homogeneous properties of composite materials subjected to electromagnetic waves. It relies on the introduction of a length parameter into classical non dimensional semi-analytical homogenisation methods—also known as mean field approaches. The model is applicable at microwave frequencies as long as the induced currents in the fibres (or inclusions) of the composite materials remain weak. This paper proposes an extension of the approach to include skin effect in the homogenisation method. This is done by considering Joule losses within the fibres of the composite. This extension significantly broadens the frequency range covered by the model. The results show that the optimization of composite shielding properties relies on a subtle compromise between internal reflections and Joule losses.