Method of producing an insulated exhaust device

A method is provided for producing an exhaust gas aftertreatment or acoustic device (20) having a maximum operating temperature TMAX. The method includes the steps of providing a blanket (40) of silica fiber or alumina insulation material having a weight percentage of SiO2 or Al2O3 of greater than 65%; calcining the insulating material by heating the blanket (40) so that all of silica fiber insulation material is raised to a temperature T greater than TMAX; and securing the blanket (40) on the device (20) after the calcining step. The blanket is encapsulated in a covering prior to the securing step, and before or after the calcining step, with the covering between the blanket and the device being a selected one of foil, wire mesh, or siliconized fiber glass.