of phenol formaldehyde and had a specific weight of about 30 kg m 3 and a thickness of about 100 mm the webs had a surface temperature of about 20 u00b0 c the polymer starting material used was a polyester in the form of a granulate marketed under the name ems g760 the polyester was melted in an extruder and subsequently the melt obtained was extruded through the nozzles in the dispensing chamber and the extruded polymer strings were elongated by means of two gas streams and broken up to form separate fibres which were deposited on the upper surface and the side edge surfaces of the rock fibre webs in the tests a surface coating having a surface weight of partly 10 g m 2 and partly 15 g m 2 was formed the surface coating had the appearance of a non woven material the coated rock fibre webs produced had a tactility corresponding completely to the tactility of the prior art mineral fibre elements having a surface coating consisting of polyester the polyester fibres applied had an average diameter of about 5 u03bcm the surface coating having a surface weight of 10 g m 2 had a thermal value of 0 3 mj m 2 whereas the coating having a surface weight of 15 g m 2 had a thermal value of 0 45 mj m.