Electric machine windings produced by wrapping a composite insulating material around an electrical conductor, said composite insulating material having been prepared by bonding two or more insulating materials with a silicone resin containing hydroxyl groups in the molecular structure, impregnating said composite insulating material with an insulating varnish comprising 1 equivalent of a polyfunctional epoxy compound and 2.5 to 25 equivalents of a polyfunctional isocyanate compound and curing said varnish, have excellent insulation properties under high temperatures and high humidity and thermal resistance of class H or more.