A thermal insulation product based on mineral wool, characterized in that the fibers have a micronaire of less than 10 l/min, preferably less than 7 l/min and especially between 3 and 6 l/min, and in that the material has a thermal conductivity of less than 31 mW/m·K, especially less than 30 mW/m·K. The parameters for obtaining this product are in particular the pressure of the burner, the rotation speed of the fiberizing spinner and the daily fiber output per spinner orifice.