contributing to an even transfer of heat throughout the screed plate 24 an insulating plate or insulation retainer 48 is substantially parallel to the screed plate 24 and forms a space 50 therebetween the insulating plate 48 performs two functions initially the heated gas passing through the orifices 44 46 will remain close to the screed plate 24 instead of rising away from the screed plate the width 55 of the space 50 see fig5 is selected to ensure that the heated gas will pass through the entire space 50 the second function of the insulation plate or retainer 48 is to retain an insulating material 54 in position the insulating material is placed in the parts of the screed removed from the space 50 the insulating material 54 has to withstand the temperatures of the heated gas 52 and 53 which passes through the tunnel 36 the insulating material prevents heat loss not only from the tunnel 36 but also from the insulating plate 48 the overall purpose of the insulating material 54 and the insulating plate 48 is to maximize the heat transfer from the burner unit 28 directly to the screed plate 24 since the insulating plate 48 is insulated on one side by an insulating material 54 the insulating plate 48 maintains most of the heat applied to it whatever heat.