The propagation of electrical trees is of a particular interest for the power engineering industry as it is one of the major causes for breakdown in high voltage equipments. Composites, materials with barriers and a surrounding polymer matrix are used to extend time to breakdown of the insulations. Since the material, structure or processing of the barriers are various and can determine time to breakdown significantly, the major influences of barriers on the propagation of electrical trees is described in this paper in experiments and numerical simulations. Although it was the motivation, to determine tree growth in the composite structure of mica-epoxy winding insulations, the results can also be used for other composite materials. The trees grew in needle-plane samples and their growth characteristic was optically analysed. The results show that tree propagation can be slowed down when introducing a barrier between the needle and the plane electrode and may cause significantly increased time to breakdown values. The increase depends on the barrier materials used, their thickness, the dielectric strength of the interface to the surrounding epoxy resin and the width of the barrier. When choosing multiple barriers, their arrangement influences time to breakdown significantly. Overlapped barriers show much higher time to breakdown values than impinged ones. From the results, it can be concluded that the selection of the barrier materials, their processing and their arrangement play a major role towards time to breakdown of composite insulation materials.