A wall structure for insulating the exterior surface of a high voltage coil having Roebeled windings is provided that comprises an inner insulating layer formed from a hardenable epoxy material for surrounding the coil and filling void spaces on the surface of the coil created by the Roebeled windings, a ground wall layer surrounding the inner layer for grounding the wall structure, and a semiconductive layer formed from a carbon filled epoxy material integrally molded around the outer surface of the inner insulating layer for reducing the electric stress across the insulating layer and in particular across any voids remaining after the insulative layer has been applied over the Roebeled windings on the exterior of the coil. The improved wall structure advantageously increases the lifespan of high voltage coils by reducing the electric stress across the inner moulding layer, thereby reducing the probability that damaging electric arcing will ever occur in any void spaces present in this layer.